Compressed file formats have become one of the most effective delivery mechanisms for malicious payloads in modern cybersecurity threats, accounting for approximately 39 percent of all malware delivery methods in recent threat research data. Archive files such as ZIP, RAR, 7-Zip, and TAR formats are simultaneously ubiquitous in legitimate business operations and increasingly weaponized by threat actors who exploit fundamental weaknesses in how these files are created, processed, and verified by both antivirus solutions and end users. The sophistication of archive-based attacks has evolved dramatically, with adversaries now combining multiple evasion techniques including file concatenation, steganography, polyglot file construction, password protection, and manipulation of archive headers to create what security researchers term “highly evasive adaptive threats” that bypass traditional signature-based detection systems, secure email gateways, and even modern endpoint detection and response platforms. Understanding the technical mechanisms behind these attacks, the vulnerabilities inherent in archive file specifications, and the comprehensive protection strategies necessary to defend against them represents a critical knowledge domain for organizations seeking to maintain robust cybersecurity postures in an environment where compressed files serve as both essential business tools and primary infection vectors.
The Evolution and Prevalence of Compressed Files as Attack Vectors
Compressed file formats have maintained their position as primary attack vectors for several decades, but their role in cybersecurity threats has fundamentally transformed alongside the evolution of threat actor capabilities and defensive technologies. Archive files occupy a unique position in the threat landscape because they combine legitimate, widespread business utility with powerful obfuscation capabilities that allow malicious actors to conceal payloads from detection systems. According to threat intelligence data from security vendors, archive files have consistently represented a significant portion of delivered malware throughout 2024 and into 2025, with ZIP files alone accounting for just over fifty percent of all archived file attachments observed in secure email gateway protected environments. This prevalence stems from multiple convergent factors: ZIP files are supported natively across all major operating systems including Windows, macOS, and Linux; they form the underlying structure of numerous critical file formats including Microsoft Office documents (.docx, .xlsx, .pptx), Java Archives (JAR), Android Packages (APK), and Electronic Publication (EPUB) files; and they can be easily encrypted and password-protected with minimal technical sophistication, allowing threat actors to bypass many secure email gateway detection mechanisms.
The widespread adoption of archive formats by both legitimate users and malicious actors has created a fundamental asymmetry in security defenses. Organizations recognize that completely blocking compressed file attachments would severely impact productivity, as employees regularly exchange compressed files for data backup, large file transfers, and collaborative work. Simultaneously, security teams struggle to inspect the contents of deeply nested, password-protected, or structurally corrupted archives in real-time without significantly impacting email gateway performance and throughput. This tension between operational necessity and security risk has created an environment where archive files frequently bypass perimeter defenses, reaching end user inboxes where social engineering and user interaction become the final determinant of successful infection. The integration of archive support into Windows native functionality since November 2023, which added support for .rar, .7z, .tar, and .gz formats in addition to legacy .zip support, has further expanded the attack surface by providing users with built-in decompression capabilities that may lack the security awareness and validation checks implemented in specialized archiving tools.
The historical development of archive-based attacks demonstrates an ongoing arms race between security researchers identifying vulnerabilities and adversaries developing increasingly sophisticated exploitation techniques. Early archive-based attacks relied primarily on simple compression and obfuscation, with threat actors relying on user interaction and convincing social engineering lures to persuade victims into opening malicious attachments. However, as antivirus signatures improved and email security systems became more sophisticated, threat actors progressively developed more advanced techniques including the exploitation of archive format specifications themselves, the manipulation of how different archive parsers interpret ambiguous file structures, and the use of archive files as containers for other advanced evasion techniques such as polyglot files and steganographic encoding. The sophistication reached by 2024 and continuing into 2025 demonstrates that archive-based attacks have transcended simple file delivery and evolved into a comprehensive threat methodology that systematically exploits the fundamental differences between how different archive parsing tools interpret file specifications and how antivirus engines attempt to validate archive contents.
Technical Mechanisms of Archive-Based Malware Delivery and Exploitation
The technical foundations underlying archive-based attacks stem from inherent ambiguities and design redundancies in archive file specifications that, while providing flexibility and interoperability benefits, simultaneously create opportunities for adversarial exploitation. The ZIP format specification, despite being one of the oldest and most widely implemented file formats in computing, contains multiple structural features that allow for variable interpretation depending on which parsing tool processes the archive. Unlike executable file formats which typically have rigid specifications with clearly defined structures and validation requirements, ZIP files incorporate significant flexibility through features such as optional Data Descriptors that can be placed after file data instead of in centralized file headers, ZIP64 extensions for files exceeding 4GB, and multiple methods for specifying file sizes and compression information that can be placed in different structural locations. This flexibility was originally designed to accommodate diverse use cases and ensure interoperability across different platforms and archiving applications, but the principle of being “liberal in what you accept” when parsing ZIP files, known as Postel’s Law, has created a situation where different ZIP parsing tools may produce substantially different outputs when processing the same archive file.
The most fundamental vulnerability in archive parsing results from the fact that a single ZIP file can contain multiple interpretations of its contents depending on which parsing tool processes it and how that tool chooses to resolve ambiguities in the archive structure. Archive files can contain duplicate file entries with identical names, allowing one archiving application to extract one file while another extracts a completely different file when both have the same apparent name. File size disagreements between the Local File Header, the Central Directory Entry, and optional Data Descriptor structures can cause different archive parsers to extract different portions of data from the same physical location in the file. Compression method variations, where files indicate they are compressed using the DEFLATE algorithm in one structure but LZMA in another, can result in different extraction outcomes. These technical peculiarities were relatively inconsequential when archive files were primarily used for data compression and storage, but when adversaries weaponized them as delivery mechanisms for malicious payloads, these ambiguities became critical security vulnerabilities that could be systematically exploited to evade detection.
The WinRAR zero-day vulnerability discovered in July 2025 and assigned CVE-2025-8088 exemplifies how sophisticated adversaries can exploit archive file structures to achieve code execution while appearing to contain only benign files. This vulnerability operates through the abuse of alternate data streams (ADSes), a Windows NTFS feature that allows multiple data streams to be associated with a single file entry, to achieve path traversal during archive extraction. When attackers craft a specially constructed RAR archive, they can embed malicious files in alternate data streams with carefully constructed paths containing directory traversal sequences such as “../” repeated multiple times, causing WinRAR to extract files to system-critical locations such as the Windows startup directory while displaying to the user an interface showing only a seemingly benign CV document. The exploit is particularly insidious because even when WinRAR displays warning messages about suspicious paths containing parent directory references, attackers can include multiple dummy ADSes with invalid paths that the user must scroll past to see the actual malicious file paths, effectively hiding the suspicious behavior within a flood of false warnings. The RomCom group leveraged this vulnerability to deliver various backdoors including SnipBot variants, RustyClaw, and Mythic agents to targeted organizations in the financial, manufacturing, defense, and logistics sectors, demonstrating that even trusted archiving tools with extensive security histories can be weaponized through subtle exploitation of their handling of archive specifications.
Archive File Concatenation and Format Specification Ambiguities
Among the most sophisticated and effective archive-based evasion techniques identified by security researchers is archive concatenation, wherein multiple archive files are combined into a single file that appears as one cohesive archive to some tools while presenting as multiple separate archives to others, depending on implementation details of the parsing tool. Concatenated archives exploit a fundamental characteristic of ZIP file structure: the archive’s “Central Directory” located at the end of the file contains pointers to where each file’s data is stored. A standard ZIP archive contains a single Central Directory that defines one logical archive. However, a concatenated ZIP contains multiple Central Directories, with data sections between them, effectively creating multiple logical archives within a single physical file. Different archive parsing tools handle this ambiguity differently: 7-Zip processes only the first Central Directory, WinRAR displays the contents from the last Central Directory, and Windows File Explorer refuses to open concatenated ZIP files entirely (though it may open them if renamed to a .rar extension and the second archive becomes accessible). These differing behaviors create what security researchers term a “ZIP confusion” vulnerability where attackers can place malicious files in the second logical archive while placing benign content in the first archive, knowing that users who employ 7-Zip (the archiving tool of choice for most security professionals and developers) will see only the benign files and extract only the safe content, while less technically sophisticated users employing Windows File Explorer or WinRAR may encounter the malicious payload.
Research conducted by Perception Point demonstrated the practical effectiveness of this technique by constructing test samples where malicious payloads were embedded in the hidden archives of concatenated ZIP files. When researchers submitted these files to antivirus scanning engines through standard archive extraction mechanisms, only sixteen of thirty-four available antivirus engines detected the malware in the basic concatenated archive case. When the researchers added another layer of complexity by nesting archives within archives, this detection rate dropped to just eleven of thirty-four engines. By the time the researchers created a third layer of nested concatenation, only seven of thirty-four antivirus engines could identify the malware, and notably, four engines that had previously detected the threat had now failed to do so, demonstrating that the concatenation technique actively degrades detection capabilities across multiple security tools. The same research team tested the files using standard external archive extraction tools, but these tools often only examined the first and last portions of the archive file, missing entirely the malicious payloads stored in intermediate sections.
The practical implications of concatenation attacks became evident in real-world deployments of the OPSWAT MetaDefender platform, which incorporated an Archive Extraction Engine capable of fully extracting concatenated archives and presenting all hidden file variations for scanning. When subjected to the three-layer concatenated archive attack, the OPSWAT platform with Archive Extraction enabled detected malware in thirty-four out of thirty-four antivirus engines by fully extracting all possible files and submitting them all for scanning, whereas standard antivirus approaches had failed almost completely. This demonstrated that modern malware detection requires not merely processing archives as tools encounter them, but rather systematically identifying all possible interpretations of ambiguous archive structures and scanning each interpretation comprehensively. The technique has become sufficiently widespread that security researchers regularly discover evidence of concatenation attacks in the wild, with attackers utilizing it as a reliable method to bypass signature-based detection systems and secure email gateways that lack sophisticated archive parsing capabilities.
Self-Extracting Archives and Hidden Functionality Exploitation
Self-extracting archive (SFX) files represent a particularly dangerous category of archive-based threats because they combine the obfuscation capabilities of compressed formats with the execution capabilities of binary executables, creating files that appear as one type to analysis tools while functioning as another in practice. Self-extracting archives are hybrid files where the first portion of the file is an executable stub (the decompression code) and the latter portion is the actual compressed data, allowing users to decompress archive contents without requiring separate archiving software installed on their systems. This hybrid nature creates fundamental detection challenges because security tools must make decisions about whether to analyze the file as an executable or as an archive, and different tools may reach different conclusions. More critically, WinRAR and other advanced SFX implementations support extended SFX commands through archive comments, wherein specialized commands embedded in the archive metadata specify additional executable files that should be run after successful decompression, command-line arguments to pass to those executables, and various operational parameters controlling the extraction process.
CrowdStrike Falcon OverWatch researchers discovered advanced abuse of WinRAR SFX archives wherein threat actors created password-protected SFX archives that contained only benign decoy files visible in the file metadata, while the archive comments contained setup commands specifying that PowerShell, command prompt, and task manager should be executed with NT AUTHORITY\SYSTEM privileges immediately after successful decompression. Because the archive could be run from the logon screen and provided persistent backdoor access to system command execution utilities with elevated privileges, attackers effectively created a mechanism to maintain access to compromised systems while evading traditional antivirus detection that focused on identifying malware within the archive contents rather than on analyzing the behavior of the SFX archive decompressor stub. Analysis of publicly available malware repositories revealed that SFX archives have been weaponized in numerous ways including as download cradles that retrieve and invoke remote payloads in memory, as containers for scripts designed to launch malware, and as vehicles for displaying decoy documents to users while silently launching malicious code in the background. Notably, password-protected SFX archives or those containing only benign files but utilizing malicious WinRAR setup parameters had relatively low detection rates both at initial submission to antivirus engines and even after being publicly available for multiple years, indicating that this exploitation technique has successfully remained effective for extended periods despite being well-documented in security research.
Polyglot Files and Multi-Format Masquerading Attacks
Polyglot files represent a sophisticated evolution in archive-based attack methodology wherein a single file is constructed to be simultaneously valid in multiple different file formats, appearing to antivirus tools, file upload restrictions, and unsuspecting users as one type of file while actually containing executable code or malware when processed by different applications. A polyglot PDF-ZIP file, for instance, is simultaneously a valid PDF document that opens in Adobe Reader and a valid ZIP archive that extracts when processed by archiving software. A polyglot JPEG-PHP file is simultaneously viewable as a valid image file in image viewers and executable as PHP code when processed by web servers. The power of polyglot techniques in malware delivery stems from the fact that file classification tools including those embedded in antivirus engines, secure email gateways, and file upload systems typically make decisions about file type and appropriate processing based on file extension analysis, magic number signatures, or initial header bytes. When a file is polyglot, different classification systems may reach different conclusions about its type, causing some systems to treat it as a harmless image file while others process it as an executable archive.
The PhantomPyramid backdoor attack attributed to the Head Mare group and documented by Kaspersky researchers provides a concrete example of polyglot exploitation in targeted cyberattacks. The initial malicious payload arrived as an email attachment with a .zip extension, appearing to be a standard ZIP archive to email recipients and email security systems that performed basic file type validation. However, the file was actually constructed as a binary executable with a small ZIP archive appended to the end, creating a file that was simultaneously valid both as an executable file and as an archive. Inside the apparently benign archive, attackers had embedded a shortcut file with a double extension (.pdf.lnk), masquerading as a PDF file to any user viewing the archive contents. When the victim clicked on the file believing they were opening a PDF document, the shortcut executed a PowerShell script that launched the binary executable portion of the polyglot file, which then deployed the actual PhantomPyramid backdoor while simultaneously displaying a decoy PDF document in the user’s temporary directory to maintain the illusion that they had merely opened a document.
Researchers studying polyglot file abuse across thousands of samples found evidence of polyglot-based attack chains used by well-known advanced persistent threat groups. The study identified thirty distinct polyglot samples and fifteen complete attack chains leveraging polyglot files, representing the first comprehensive survey of polyglot usage by malicious actors in the wild. The most common polyglot combinations involved image-based polyglots (JPEG, PNG, BMP images combined with other formats), office document polyglots (Word, Excel, PowerPoint documents combined with archives or executables), and HTML-based polyglots combining web pages with archives or executables. These researchers developed PolyConv, a machine learning-based detection solution specifically trained on polyglot files discovered in real-world attacks, which achieved a precision-recall area-under-curve score of 0.999 and an F1 score of 99.20 percent for polyglot detection, significantly outperforming all other tested tools. Notably, existing file format detection tools including those specifically developed for polyglot file identification failed to reliably detect polyglot files used in actual cyberattacks, leaving organizations that relied on these tools vulnerable to exploitation.
Steganography and Hidden Payload Encoding Within Archive Contents
While not strictly archive-specific, the combination of steganographic encoding techniques with compressed file delivery represents a particularly insidious threat methodology wherein malicious code is hidden not merely compressed and archived, but rather concealed within the pixel data of image files, rendering it virtually invisible to both casual inspection and many automated analysis tools. Steganography, the technique of hiding data within other data to avoid detection, has been employed by threat actors for data exfiltration and command-and-control communications for over a decade, with historical examples including the Duqu malware that transmitted encrypted data appended to JPEG files and the Zeus banking malware that similarly appended encrypted configuration files to image files. However, more advanced steganographic implementations completely embed malicious code within the image pixel data itself, making the distinction between clean and malicious images nearly imperceptible without specialized analysis tools or extraction of the hidden data.
The Gatak/Stegoloader malware identified in 2015 pioneered advanced steganographic techniques wherein malicious code was completely hidden within PNG image files by encoding the encrypted code into the pixel color values of the image. When examined visually or through standard image analysis tools, the infected images appeared as normal photographs or images with perhaps minimal visible quality degradation. The malware’s extraction process involved downloading seemingly innocuous image files from attacker-controlled servers, then using Windows GDI APIs to retrieve the pixel data from each pixel location in the image, utilizing that pixel color information as a stream generator to reconstruct the hidden encrypted malicious code, and finally decrypting that code using an RC4 algorithm with a hard-coded key to obtain the final shellcode payload. The sophistication of this approach lay partly in the encryption and key management, but more critically in the exploitation of image format specifications that allow for visual imperceptibility while maintaining perfectly valid image format compliance. Later research by ESET discovered that the Worok cyberespionage group employed similar steganographic techniques, hiding malicious code within image files and extracting only specific pixel information to reconstruct the payload, particularly in scenarios where systems had already been compromised and the attackers required methods to evade detection of their communications.
The least significant bit (LSB) steganographic technique, wherein the lowest-order bit of each RGB (red, green, blue) or RGBA (red, green, blue, alpha) color value is replaced with message data, represents one of the most effective implementations because modifications to the least significant bit produce virtually undetectable changes to image appearance while still permitting reliable data embedding and extraction. Alternatively, adversaries can embed payloads into an image’s alpha channel (which defines color opacity), utilizing only an insignificant portion of the opacity information to hide complete malicious code. These techniques render the visual difference between clean and malicious images nearly impossible for humans to detect and extremely difficult for automated systems to identify without deliberately checking for steganographic encoding. The key vulnerability exploited by attackers employing steganographic encoding is that image files compressed and transmitted through social media platforms, cloud storage services, or email systems typically undergo re-compression and quality degradation that would completely destroy embedded steganographic data. Consequently, steganography is most commonly employed for data exfiltration from already-compromised systems where attackers have achieved code execution and are attempting to move stolen information to external servers while evading network-based detection systems, rather than as an initial infection vector.
Vulnerabilities in Archive Processing Tools and Zero-Day Exploitation
Archive processing tools themselves have become targets for vulnerability research and exploitation, with critical security flaws discovered in widely-deployed tools including 7-Zip, WinRAR, and Windows native archive handling. The CVE-2024-11477 vulnerability in 7-Zip versions prior to 24.07 represents a critical remote code execution flaw stemming from improper validation of user-supplied data during Zstandard decompression. The vulnerability arises from an integer underflow that occurs before writing to memory when processing specially crafted archives containing malicious data, allowing remote attackers to execute arbitrary code with the privileges of the user running 7-Zip. Classified as a high-severity issue with a CVSS score of 7.8, this vulnerability exemplifies the critical risk posed when attackers can exploit archive processing tools directly. The ease of exploitation compounds the severity, as attackers can readily create malicious archives specifically crafted to trigger the vulnerability and distribute them through standard channels including email attachments or cloud storage services, requiring only that victims possess a vulnerable version of 7-Zip and interact with the malicious archive.
The WinRAR CVE-2025-8088 vulnerability discovered in July 2025 affected then-current versions of WinRAR including version 7.12, with the developer releasing a patch within one day upon notification. Prior to this, WinRAR had been the target of another path traversal vulnerability CVE-2025-6218 approximately one month earlier, indicating an active campaign of vulnerability discovery or exploitation targeting the widely-deployed archiving tool. Additional historical WinRAR vulnerabilities including CVE-2022-29072 and CVE-2023-31102 demonstrate a recurring pattern wherein sophisticated threat actors identify and exploit vulnerabilities in archive processing tools, then leverage these vulnerabilities in targeted campaigns against specific sectors or organizations. The significance of these vulnerabilities extends beyond immediate technical impact, as the widespread deployment of these tools means that a single zero-day vulnerability can potentially affect millions of systems globally, particularly in environments where automatic updates are not configured or where legacy versions remain in operation.
The vulnerability landscape surrounding archive tools demonstrates why organizations cannot rely solely on keeping archive processing tools updated but must implement defense-in-depth strategies incorporating multiple protective layers. Organizations may lack comprehensive visibility into which systems have which versions of archive tools installed, particularly in heterogeneous environments with distributed endpoints, legacy systems, and bring-your-own-device policies. As a result, even when patches are released promptly, deployment across entire organizations takes time, during which systems remain vulnerable to exploitation. Furthermore, the critical nature of archive processing functionality means that many organizations cannot uninstall or disable archiving tools entirely, as these tools are essential for legitimate business operations including file backup, data transfer, and software distribution.
Password Protection and Archive Encryption as Evasion Mechanisms
Password-protected and encrypted archive files represent one of the simplest yet most effective methods for threat actors to evade detection by secure email gateways and automated scanning systems, because encrypted content cannot be analyzed by antivirus engines or content filters without first being decrypted. Approximately forty-two percent of all malware is delivered as archive files, and a significant portion of these archives are password-protected, leveraging encryption to bypass email security systems. The evasion effectiveness of password protection stems from fundamental technical limitations in secure email gateway implementations: most email security systems cannot efficiently decrypt password-protected archives to inspect contents, as doing so would require either receiving the password directly from users, employing brute-force attacks that would be computationally expensive and time-consuming, or maintaining encrypted content in quarantine indefinitely. Many organizations configured their secure email gateways to simply accept password-protected files because the perceived productivity loss from blocking all encrypted archives would be unacceptable, creating a security gap that threat actors have readily exploited.
The psychological engineering accompanying password-protected archive delivery compounds the technical evasion provided by encryption. Threat actors typically send the archive attachment along with an email claiming the attachment contains sensitive information, important documents, or other content designed to create urgency and curiosity. The password for the archive is included directly in the email body, with the attacker’s social engineering lure providing a plausible justification for why the archive requires a password (such as “Password-protected for security purposes” or “Password provided in email body for verification”). Most secure email gateways lack the sophisticated natural language processing and semantic analysis required to distinguish the password hidden within legitimate-sounding lure text from other email body content, causing the gateway to fail to correlate the password in the email body with the encrypted attachment and therefore fail to flag the combination as suspicious. Even advanced secure email gateways employing machine learning algorithms struggle with this distinction, as the classification task requires understanding the semantic relationship between email body content and attachment properties, rather than merely identifying malicious keywords or file signatures.
Research conducted by Cofense Intelligence examining email-based threats between May 2023 and May 2024 revealed that password-protected archives, while representing only approximately five percent of all attached archive files, occurred predominantly when delivered via embedded URLs rather than direct email attachments. This suggests that threat actors have adapted their delivery methods in response to security improvements, potentially moving password-protected archive delivery to alternate channels such as cloud storage links, collaboration tools, or direct messaging platforms where secure email gateway protections provide no defense. The psychological effectiveness of this approach lies in the fact that users receiving links to cloud storage services containing password-protected archives have generally been trained to consider such links legitimate, as organizations frequently use cloud storage for secure file sharing. Users may be less inclined to scrutinize a file downloaded from their organization’s officially-sanctioned cloud storage service than they would be an attachment directly from an external email sender, creating an additional psychological manipulation layer in the attack methodology.
Detection Challenges and Limitations of Traditional Security Solutions
Traditional antivirus and malware detection systems face fundamental technical challenges when attempting to protect against archive-based threats, challenges that stem from the combination of archive format complexity, nested compression, content encryption, and the design limitations of signature-based detection approaches. Most antivirus solutions employ signature-based detection methodologies wherein known malware samples are represented as a specific series of bytes, file hash, or behavioral pattern, and the antivirus engine scans files against this signature database to identify threats. However, this detection methodology faces critical limitations when confronted with archives because the malicious content is compressed and potentially encrypted, making direct signature comparison impossible without first extracting and decompressing the archive contents. Some antivirus solutions possess the capability to temporarily decompress archive files and scan the extracted contents, but many antivirus solutions can only perform full scanning after the archive has been completely extracted by the user, meaning the antivirus protection is essentially “unscannable” for archived content.
The detection limitation extends to the fundamental architectural challenge that antivirus signatures represent only known threats, while zero-day exploits and newly discovered malware variants remain undetectable until they are analyzed, a signature is created, and that signature is distributed to all endpoint systems. Researchers discovered new malware samples at a rate measured in hundreds or thousands daily, making mathematically impossible for antivirus signature databases to remain current with the constantly evolving threat landscape. Archive-based delivery mechanisms provide particular advantages to attackers attempting to evade signature-based detection because the mere act of re-compressing malware with slightly different compression parameters, encrypting with a different password, or packaging within a different archive tool can produce entirely different file hashes and byte signatures despite the actual malware payload being identical. This malleability of archive-based delivery creates what researchers term “polymorphic” characteristics wherein a single malware sample can generate essentially infinite variations through simple repackaging in archives, rendering signature-based detection increasingly ineffective.
Secure email gateways, which represent a critical control point for stopping malware before it reaches user inboxes, face similar limitations when confronted with archive-based threats. Secure email gateways operate by examining email metadata, message content, attachments, and embedded links against known threat signatures, machine learning models trained on malicious email characteristics, and real-time threat intelligence feeds. However, these systems must balance detection effectiveness against email throughput requirements, meaning they cannot perform exhaustive analysis on every attachment. When confronted with deeply nested archives, password-protected files, or archives exploiting file format ambiguities to present different content to different parsing tools, secure email gateways frequently fail to detect the malicious payload entirely. Research by Cofense Intelligence analyzing archive files that successfully bypassed secure email gateways found that many of the bypassing files employed relatively simple evasion techniques, indicating that a significant proportion of organizations deploy secure email gateways with insufficient archive handling capabilities or with configurations that prioritize user experience over security.
The challenge of detecting threats in nested or concatenated archives arises partly from computational considerations: attempting to recursively extract all possible layers of archive nesting could create denial-of-service conditions where attackers craft archives with hundreds of layers of nesting, causing security scanning systems to exceed memory limits or timeout thresholds while attempting to process them. Zip bomb attacks, wherein relatively small archive files decompress into massive volumes of data, represent an extreme example of this threat class, with famous examples such as the “42.zip” file comprising only forty-two kilobytes of compressed data but expanding to approximately four and a half petabytes of uncompressed data. Most modern antivirus programs implement protections against zip bombs by limiting the number of archive layers that will be recursively extracted and by aborting decompression if file sizes exceed predetermined thresholds, but these protections inherently mean that deeply nested malicious archives may escape detection because security systems will abort processing before reaching the malicious payload.
Advanced Detection Technologies and Content Disarm and Reconstruction
Content Disarm and Reconstruction (CDR) technology represents a paradigm shift in how organizations approach file-based threat detection, moving away from the “detect and block” model of traditional antivirus systems toward a “assume all files are malicious and sanitize” model” wherein files are essentially deconstructed, inspected component by component, and then reconstructed with only safe elements preserved. Rather than attempting to identify whether a file contains malware, CDR systems assume that any file could be malicious and systematically strip away elements that could potentially contain threats while preserving file functionality. For archive files specifically, this approach means extracting all archive contents, scanning each extracted file with multiple antivirus engines, removing any files identified as malicious or suspicious, then reconstructing the archive with only the safe files included for delivery to users. This methodology has proven particularly effective against archive-based threats because it does not depend on being able to parse and understand the ambiguous archive file formats that attackers exploit to evade traditional detection.
The OPSWAT MetaDefender platform demonstrates practical implementation of CDR technology for archive-based threat protection, providing multi-layered scanning combined with archive extraction and content reconstruction capabilities. Research testing MetaDefender against standard archive files found detection rates of sixteen out of thirty-four antivirus engines using standard approaches. However, when the same files were submitted to MetaDefender with Archive Extraction Engine enabled, the detection rate improved substantially because the system extracted all possible interpretations of ambiguous archive structures and scanned each interpretation. When Deep CDR was additionally enabled, the system not only detected threats but also generated sanitized versions of files with malicious content removed, creating cleaned archives that users could safely interact with. This multi-layered approach proved particularly effective against evasion techniques like concatenated archives, where standard archive extraction tools might see only the first or last archive section while missing malicious payloads hidden in intermediate sections.
YARA rules represent another advanced detection technology that provides significant value in identifying malicious code patterns within files and backups. YARA rules are signature-based detection patterns that search for specific strings, hex patterns, binary signatures, or behavioral indicators within files or memory samples. Unlike traditional antivirus signatures that typically represent entire malware samples as file hashes, YARA rules can identify specific characteristics or code segments that appear across multiple variants of a malware family, enabling detection of related threats even when the exact binary differs. Organizations employ YARA rules particularly for identifying ransomware signatures within backup environments before attempting recovery, ensuring they restore from clean backup points and avoid reintroducing ransomware during the recovery process. Custom YARA rules can be written to identify specific malware families, suspicious archive characteristics, or behavioral patterns, making them particularly valuable in detecting variants and novel threats that may not yet have commercial antivirus signatures available.
Endpoint Detection and Response (EDR) solutions provide continuous monitoring of endpoint systems to detect suspicious behaviors even after malware has bypassed perimeter defenses and executed on user systems. EDR systems track hundreds of security-relevant events including process creation, network connections, file modifications, registry changes, and archive file creation operations. This behavioral monitoring approach complements signature-based detection by identifying suspicious activities that may indicate a compromise regardless of whether the initiating malware can be identified through signatures or file hashes. Importantly, EDR systems maintain historical records of endpoint activities in cloud-based databases, allowing security teams to investigate incidents after they occur, understand the full scope of an attack, and identify lateral movement or data exfiltration that may have occurred. For archive-based threats specifically, EDR systems can detect when processes extract archive contents, execute files from temporary directories associated with archive extraction, or perform suspicious file operations following archive opening, providing detection mechanisms that operate independently of the ability to identify malicious archive contents.
Recent Attack Campaigns and Real-World Exploitation Patterns
The operational employment of compressed file-based attacks by state-sponsored and financially-motivated threat actors has demonstrated consistent evolution toward more sophisticated techniques and multi-stage infection chains. In August 2025, CloudSEK researchers discovered an active cyber-espionage campaign attributed to APT36 (also known as Transparent Tribe, active since at least 2013) targeting Indian government and defense entities, wherein the attack chain leveraged ZIP files containing malicious Linux “.desktop” files disguised as document attachments. The infection methodology involved sending phishing emails containing ZIP archives with filenames suggesting legitimate procurement documents such as “PROCUREMENT_OF_MANPORTABLE_&_COMPAC.pdf.zip”. When users extracted the ZIP file, they received a “.desktop” file (PROCUREMENT_OF_MANPORTABLE_&_COMPAC.pdf.desktop), which appeared in file explorers as a PDF document due to display conventions hiding file extensions, but was actually a Linux desktop entry configuration file containing malicious commands. Upon execution, the “.desktop” file downloaded additional payloads from Google Drive where hex-encoded malware was stored disguised as text, deployed the dropper malware, and established persistence through multiple mechanisms to maintain access to compromised systems.
The sophistication of this campaign lay not in technical innovation but rather in the psychological engineering of combining legitimate-appearing file extensions with archive delivery to exploit user expectations and system file display conventions. The attack specifically targeted Linux systems common in technical environments, and analysts observed the campaign was active during August 2025, indicating ongoing evolution of archive-based attack techniques to newer platforms and environments. The use of cloud storage services (Google Drive) for payload hosting represented a secondary sophistication layer, as these services are generally trusted and whitelisted by organizations, making malware downloads from them more likely to succeed than downloads from obviously malicious external servers. This campaign exemplified the ongoing evolution of archive-based attacks toward multi-stage infection chains combining archives with additional obfuscation, social engineering, and infrastructure evasion techniques.
The StrelaStealer phishing campaign observed in early 2024 affected more than one hundred organizations across the European Union and the United States through remarkably simple yet effective methodology. The attackers distributed spam emails with attachments that delivered StrelaStealer malware, with the primary evasion mechanism being simply changing the file format or extension of the attachment to bypass detection systems. Rather than relying on sophisticated archive manipulation or vulnerability exploitation, the attackers achieved successful delivery and infection through low-technical-complexity approaches that nonetheless evaded perimeter defenses across numerous organizations, suggesting that basic detection improvements could prevent the majority of these attacks. The incident highlighted that sophisticated evasion techniques are not always necessary for successful attacks when organizations maintain insufficient security configurations or when security tools fail to properly validate file types and contents.
A March 2024 phishing campaign discovered by Trustwave SpiderLabs delivered Agent Tesla information stealer and keylogger through archive files masquerading as bank payment notices. The attack chain began with a phishing email with a subject line and body text convincingly mimicking legitimate banking communications, containing a request for the recipient to review an attached payment notice. The attachment was an archive file containing a loader malware utilizing obfuscation techniques to evade antivirus detection. Once executed, the loader deployed Agent Tesla, which then proceeded to steal sensitive banking credentials, email information, and keylogging data from the compromised system. The campaign demonstrated how archive-based delivery provided the initial evasion necessary to bypass email security systems, while secondary obfuscation techniques applied to the loader malware provided additional evasion against endpoint antivirus systems.
Protective Measures and Organizational Defense Strategies
Organizations seeking to implement comprehensive protection against archive-based threats must recognize that no single security technology provides complete protection against all attack variations and evasion techniques. Rather, robust protection requires implementation of multiple overlapping defensive layers that collectively close gaps left by any individual defense mechanism. The FBI recommends that organizations employ multi-faceted approaches including keeping operating systems, software, and applications current with security updates; ensuring antivirus and antimalware solutions are configured to automatically update and run regular scans; maintaining regular data backups that are stored offline and verified to be uncorrupted and malware-free; and creating continuity plans for recovery in the event of successful ransomware attacks. These foundational practices address the reality that sophisticated attackers will occasionally succeed in bypassing perimeter defenses, making recovery capability essential for minimizing damage.
At the email gateway level, organizations should implement secure email gateway solutions that employ multiple detection engines working in tandem including signature-based detection, machine learning-based threat modeling, real-time threat intelligence feeds, and sandboxing capabilities that execute suspicious attachments in isolated environments to observe their behavior. However, organizations must recognize that even sophisticated secure email gateways have limitations and should therefore combine gateway-level protection with endpoint-level controls. Importantly, organizations must review their email gateway configurations to ensure they are appropriately scanning archive files and examining archive contents rather than simply passing through encrypted or potentially evasive archives. Some organizations inadvertently relax email gateway security configurations to reduce false positives or improve throughput, inadvertently expanding the attack surface.
At the endpoint level, organizations should deploy endpoint detection and response solutions that provide continuous monitoring and behavioral analysis in addition to signature-based detection. EDR solutions provide visibility into endpoint activities even when traditional antivirus fails to detect malware, enabling incident responders to identify compromises, investigate the full scope of attacks, and understand lateral movement and data exfiltration activities. Organizations should configure EDR to generate alerts when processes execute files from temporary directories commonly associated with archive extraction, when archive files are created or accessed, when suspicious processes spawn from archive extraction utilities, or when unusual network connections are established from recently extracted files.
Additionally, organizations should implement user awareness training focused on the specific risks associated with archive files and email attachments. Despite the sophistication of technical attacks, user behavior remains a critical attack surface, and many successful attacks succeed because users interact with malicious attachments despite various warning signs. Training should specifically address that archive files can conceal malicious content, that password-protected archives do not necessarily indicate legitimacy, that trusted file formats like PDFs can be spoofed or combined with malicious code, and that users should exercise particular caution with archives received from external senders or appearing to contain unusual content. This training is most effective when combined with email authentication technologies such as DMARC, SPF, and DKIM that help verify sender authenticity and reduce the effectiveness of email spoofing attacks commonly used to socially engineer users into opening malicious attachments.
Organizations should implement data loss prevention systems that monitor and restrict the movement of sensitive data, particularly after identifying successful compromises. Even if attackers successfully bypass perimeter defenses and execute malware, data loss prevention systems can detect and block data exfiltration attempts, limiting the damage from compromised environments. These systems should monitor for suspicious data transfer patterns including large compressed archives being created, unusual outbound connections to unknown external servers, and large data transfers occurring at atypical times or from unexpected user accounts.
Finally, organizations should maintain comprehensive asset inventories documenting which systems have which versions of software installed, including archiving utilities. This visibility enables rapid identification of systems vulnerable to specific zero-day vulnerabilities in archiving tools and facilitates targeted patching campaigns prioritizing the highest-risk systems. Organizations should prioritize updating archive processing tools immediately when security patches are released, recognizing that threat actors routinely develop exploits for newly disclosed vulnerabilities in widely-deployed tools.
Final Decompression: Mitigating Hidden Risks
Compressed files and hidden threats represent an evolving attack vector that has maintained prominence across multiple decades of cybersecurity threats while continuously adapting to incorporate new evasion techniques, vulnerability exploitation, and social engineering methodologies. Archive files occupy a unique position in the threat landscape because they simultaneously serve as essential tools for legitimate business operations and as powerful delivery mechanisms for malicious payloads. The technical vulnerabilities inherent in archive file specifications, the ambiguities in how different archive parsing tools interpret these specifications, and the design limitations of traditional antivirus detection systems have collectively created an environment where archive-based attacks remain highly effective despite years of security research focused on these threats. The evolution from simple archive compression as an obfuscation mechanism to sophisticated techniques including file concatenation, polyglot construction, steganographic encoding, and exploitation of zero-day vulnerabilities in archive processing tools demonstrates that threat actors have systematically developed deep understanding of archive formats and continue to identify new attack vectors.
The persistence and evolution of archive-based threats reflects broader challenges in cybersecurity defense wherein perfectly secure systems remain elusive because attack and defense exist in perpetual dynamic tension. As security researchers and vendors develop new detection technologies and protection mechanisms, threat actors identify gaps in these defenses and develop novel evasion techniques. The discovery of zero-day vulnerabilities in widely-deployed archive tools including 7-Zip and WinRAR illustrates that even security-conscious organizations employing updated software may remain vulnerable to novel exploitation techniques. The sophisticated abuse of archive formats documented in this analysis demonstrates that comprehensive threat protection requires technical sophistication, architectural depth, and continuous vigilance to identify and adapt to emerging attack methodologies. Organizations that implement robust protection against archive-based threats recognize that layered defense approaches combining email gateway scanning, endpoint detection and response, content disarm and reconstruction, user awareness training, and data loss prevention provide substantially better protection than any single technology implemented in isolation. As threat actors continue to evolve archive-based attack techniques and discover novel vulnerabilities in archiving tools, organizations must maintain commitment to continuous security improvement and threat monitoring to ensure they remain protected against this persistent and adaptable threat class.
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