Email rules have emerged as a sophisticated post-compromise persistence mechanism that allows attackers to maintain stealthy access to compromised mailboxes while facilitating data exfiltration and operational continuity. This comprehensive analysis examines how malicious email rules function within the broader context of email security, the critical timelines involved in detecting these attacks, and the audit methodologies organizations must employ to identify and remediate compromised accounts before significant damage occurs. The implementation of email rules represents a particularly insidious attack vector because it enables adversaries to operate undetected within legitimate user accounts, forwarding sensitive communications to external addresses, hiding security alerts, and deleting evidence of their presence—all while the account appears to function normally to its legitimate owner. Understanding the audit timeline and detection capabilities is essential for security teams to transform their defensive posture from purely reactive to proactive threat hunting.
The Evolution of Email Rules as a Post-Compromise Attack Vector
Email rules have become a cornerstone technique in the attacker’s toolkit for maintaining persistence following account compromise. The fundamental appeal of this technique lies in its elegance and simplicity: once an attacker gains access to a victim’s email account—whether through phishing, credential stuffing, or credential compromise from previous breaches—they can immediately implement automated rules that operate silently and continuously. Unlike malware that can be detected by endpoint security solutions or suspicious login patterns that might trigger behavioral anomaly detection, email rules execute entirely within the email platform itself, leveraging legitimate functionality that users and administrators have come to expect as part of normal email operations.
The sophistication of using email rules as a persistence mechanism lies in how well it aligns with the attacker’s operational objectives throughout the post-compromise phase. According to Barracuda research, a significant percentage of organizations worldwide—approximately 75 percent—experienced at least one email security breach in 2022, with many of these breaches involving the establishment of malicious email rules. These rules serve multiple purposes in a comprehensive attack campaign: they enable real-time monitoring of victim communications, facilitate data exfiltration by automatically forwarding sensitive messages, hide incriminating evidence by deleting security alerts and system notifications, and maintain access even if the original compromise vector is discovered.
The critical distinction in the attack lifecycle is that email rule creation represents a post-compromise activity, meaning the attacker has already successfully penetrated the organization’s defenses and obtained legitimate credentials or system access. This temporal relationship is crucial for understanding the audit timeline because it means detection of malicious email rules often occurs much later in the attack chain—potentially days, weeks, or even months after the initial account compromise. The average time between network breach and containment was documented at approximately 279 days in 2019, and this figure had increased from 266 days in 2018, indicating a troubling trend of extended dwell times before detection. This extended timeline creates a significant window of opportunity for adversaries to collect intelligence, exfiltrate data, and establish additional persistence mechanisms before remediation efforts begin.
Attack Mechanisms: How Adversaries Create and Deploy Malicious Email Rules
The technical mechanics of malicious email rule creation reveal why this technique has proven so effective and persistent in the threat landscape. Once an attacker has compromised a user account—typically through spear-phishing emails that coerce credential disclosure, malware that captures credentials from the victim’s machine, or by leveraging previously compromised credentials from other breach incidents—they gain the ability to access the victim’s email account just as a legitimate user would. At this point, the attacker can create email rules using standard email client interfaces or programmatic access through PowerShell cmdlets such as `New-InboxRule`, `Set-InboxRule`, or by modifying mailbox settings through `Set-Mailbox` commands.
The sophistication of rule design varies depending on the attacker’s objectives and level of operational discipline. Some attackers create rules that forward all incoming emails to an external address controlled by the attacker, providing a constant stream of victim communications. Others implement more surgical rules that selectively forward emails containing specific keywords that indicate high-value communications—such as “invoice,” “payroll,” “wire transfer,” “direct deposit,” “password reset,” or other financially or operationally sensitive terms. This keyword-based approach represents a more refined attack methodology, as it reduces the volume of forwarded emails, making the compromise less likely to be detected through unusual outbound email traffic patterns, while simultaneously ensuring the attacker captures the most valuable communications.
Red Canary’s threat analysis revealed that adversaries frequently employ deliberately obscure and minimal rule names to avoid drawing attention during routine administrative review or user inspection of their mailbox rules. Common rule naming patterns observed in actual attacks include simple punctuation marks such as a single period (`.`), double period (`..`), semicolon (`;`), single letters, or repetitive characters like `aaaa` or `………` This naming strategy relies on the principle that users typically do not thoroughly review their mailbox rules and that such minimal naming conventions may fail to trigger pattern recognition in automated detection systems that look for suspicious-sounding rule names.
Beyond simple email forwarding, attackers leverage rules to manipulate email visibility and create confusion in the victim’s mailbox. Malicious rules can automatically move emails containing sensitive keywords to obscure folders such as RSS feeds, archived folders, or other rarely accessed locations, making it appear to the legitimate user that these emails were never received. Additionally, attackers can configure rules to delete emails entirely or mark them as read before moving them, further obscuring the presence of communications the attacker is monitoring or intercepting. Some advanced implementations involve rules that delete security alerts and system notifications from the organization’s security infrastructure, preventing the user from becoming aware that suspicious activities have been detected on their account.
The creation mechanism itself varies depending on the email platform and the compromise method. In Microsoft 365 environments, compromised accounts allow attackers to create rules through multiple pathways: the Outlook desktop client, Outlook on the Web (OWA), or directly through Exchange Online PowerShell with appropriate privileges. In Google Workspace environments, the technical constraints are different, as Google does not provide the same granular rule-creation capabilities, though attackers can still configure forwarding settings. Each creation method leaves different audit trails and evidence, which has significant implications for the detection and investigation timeline.
The Account Compromise Progression and Timeline to Rule Implementation
Understanding the temporal progression from initial account compromise to malicious rule creation is essential for audit planning and detection strategy. Research by Microsoft Incident Response has documented that attackers frequently follow a predictable operational pattern following successful credential acquisition. In real-world incidents investigated by Arctic Wolf’s security teams, the timeline from initial compromise to active rule manipulation can be remarkably compressed—in one case, malicious inbox rules were created and modified within approximately 19 minutes of the attacker gaining initial access to the compromised account.
The typical progression involves several distinct phases. First, the attacker obtains valid credentials through one of several vectors: phishing emails that convince users to enter their credentials on fake login pages, malware that captures credentials from the victim’s machine, leaked credentials from previous breaches that are tested against target systems using credential stuffing techniques, or compromised passwords obtained through social engineering. Once credentials are validated, the attacker may initially perform reconnaissance activities—checking the contents of the mailbox, examining the organizational structure through the Global Address List, and identifying high-value targets or sensitive communications.
Following this reconnaissance phase, attackers often implement their persistence mechanisms before conducting more active malicious operations. This timing makes strategic sense from an operational security perspective: by establishing rules that automatically forward emails and hide evidence of compromise, the attacker can ensure continuous access to valuable communications even if the compromise is later discovered and the original credentials are invalidated. In sophisticated campaigns, attackers may also establish multiple layers of persistence, including not only email rules but also mailbox forwarding configurations, custom Outlook forms that execute code, and OAuth tokens or application permissions that grant persistent access to cloud services.
The delay between credential compromise and actual rule implementation can vary significantly based on attacker sophistication and objectives. In some cases, especially with automated attack tools, credentials may be used immediately upon capture, resulting in rule implementation within minutes. In other scenarios, particularly in targeted advanced persistent threat campaigns, attackers may deliberately wait days or weeks before implementing rules, conducting extensive reconnaissance to identify the highest-value targets and timing their actions to maximize operational advantage.
Detection Methodologies: The Critical Audit Timeline
The timeline for detecting malicious email rules represents one of the most significant variables in determining whether organizations can contain a compromise before substantial damage occurs. Unlike many security threats that require sophisticated detection algorithms or behavioral anomaly analysis, malicious email rules can be detected through straightforward examination of mailbox configurations—if that examination is performed regularly and thoroughly.
Microsoft provides multiple pathways for auditing email rules, each with different implications for audit timeline and scope. The most comprehensive approach involves running the `Get-AllTenantRulesAndForms.ps1` PowerShell script, which automatically dumps all mail forwarding rules and custom forms for all users in an organization. This script generates output files in comma-separated values (CSV) format that can be systematically reviewed for indicators of compromise. The advantage of this script-based approach is that it can audit the entire tenant’s worth of mailboxes relatively quickly, providing broad visibility across potentially thousands of user accounts.
However, organizations should recognize that executing this comprehensive script requires proper planning, as it generates significant data and necessitates having appropriate Microsoft 365 permissions and PowerShell module installations. The alternative approach involves manual examination of individual mailboxes using the Outlook client interface, navigating through each user’s mailbox to inspect rules visually. While this manual approach provides direct visibility and requires no additional tools, it is prohibitively time-consuming for organizations with large user populations and becomes impractical as an organization-wide audit strategy.
The audit log search capabilities within Microsoft Purview represent another critical detection pathway with direct implications for the audit timeline. Organizations can search for specific audit events related to inbox rule creation and modification by querying for activities such as `New-InboxRule`, `Set-InboxRule`, `Remove-InboxRule`, and `UpdateInboxRules`. These audit log searches provide historical visibility into when rules were created, by whom, and what parameters were configured—critical forensic evidence for investigation and attribution. The audit log functionality allows security teams to retrospectively identify when a compromise likely occurred by correlating the timing of rule creation with other suspicious activities such as unusual login patterns or impossible travel scenarios.
The Critical Limitation: Audit Log Retention and Timeline Constraints
One of the most consequential factors affecting the audit timeline for detecting malicious email rules is the limited retention period for audit logs in Microsoft 365 environments. By default, audit records in Microsoft 365 are retained for only 90 days for organizations using standard licensing. This means that audit evidence of rule creation disappears automatically after 90 days, creating a critical window within which detection and investigation must occur. For organizations that have not implemented comprehensive audit log retention policies or that lack Microsoft 365 E5 licensing, this 90-day limitation represents a significant constraint on the ability to conduct retrospective investigations into account compromises that occurred more than three months in the past.
Organizations with Microsoft 365 E5 licensing or E5 Compliance/E5 eDiscovery and Audit add-on licenses receive extended retention of one year for Exchange, Azure Active Directory, and SharePoint audit records, providing substantially more time for investigations and forensic analysis. For organizations requiring retention beyond one year—particularly those in heavily regulated industries or subject to specific legal holds—Microsoft 365 offers a 10-year audit log retention add-on license that extends retention to a full decade, though this represents an additional cost.
This retention timeline creates a critical operational requirement: organizations must implement efficient detection and alerting mechanisms that trigger alerts within the 90-day window (or extended window for E5-licensed organizations), because investigation of incidents discovered after audit logs have expired becomes significantly more challenging. The audit timeline therefore dictates that organizations either implement continuous or frequent periodic auditing of email rules, or risk losing the audit evidence necessary to determine when and how a compromise occurred.
Investigation and Audit Procedures: Step-by-Step Timeline
When organizations detect suspicious email rules or suspect that an account may have been compromised, a structured investigation timeline becomes critical for effective remediation and evidence preservation. The recommended approach, as detailed in Microsoft’s incident response guidance and industry best practices, follows a four-phase framework that should be executed in sequence to minimize further damage and preserve evidence.
The first phase—limiting damage—should be executed immediately upon identification of a potentially malicious rule. This phase involves disabling or deleting the suspicious rule to prevent further exfiltration of emails through that pathway, and resetting the compromised user’s password as a precautionary measure, even if the investigative evidence is incomplete. Disabling can be accomplished through the Push Security platform’s administrative console or directly through Outlook by toggling the rule to disabled status. The critical principle in this phase is speed: every hour the rule remains active represents additional emails that may be forwarded to attacker-controlled addresses or deleted before the legitimate user can see them.
The second phase—understanding the root cause—requires detailed investigation of how the account became compromised in the first place. This investigation should examine multiple vectors: suspicious login activity from unusual IP addresses or locations inconsistent with the user’s normal work patterns, OAuth applications that were granted inappropriate permissions to the mailbox through consent phishing attacks, phishing emails sent to the user that may have contained credential-stealing links or malware attachments, or compromised credentials obtained from previous breach incidents. Each investigation pathway leaves different evidence and requires different analytical approaches.
Analyzing login history provides crucial evidence about potential compromise vectors. Security teams should examine Exchange Online signin logs for logons from IP addresses associated with known VPN services, anonymizing proxies, or geographic locations inconsistent with the user’s expected location. Comparing the timestamps of suspicious logons with the timestamps of rule creation can establish whether the user account was actively being used by an attacker at the time the malicious rule was implemented. Services like haveibeenpwned.com can reveal whether the user’s credentials were exposed in previous breach incidents, potentially explaining how credentials were obtained even if they were not captured through current-period phishing or malware.
The third phase of the investigation timeline involves determining whether the compromise is isolated to a single user account or whether it represents a broader organizational compromise affecting multiple accounts. This phase requires checking whether other users showed similar suspicious rule creation patterns during the same time window, whether the attacker established persistence mechanisms on other systems or accounts, whether similar phishing emails were sent to other users, or whether a shared vulnerability or supply chain compromise may have affected multiple accounts. In the incident response timeline documented by Arctic Wolf, investigators discovered that while the immediate compromise affected a single user, the attacker had taken actions—such as uploading phishing PDF files to OneDrive—that posed risk to multiple potential victim users.
The fourth and final phase—recovery and cleanup—should only be initiated once the root cause has been identified and the investigative team has reasonable confidence that the attacker’s access has been revoked and no additional persistence mechanisms remain in place. This phase involves resetting the user’s password to a strong, complex value, enabling multi-factor authentication if not previously enabled, examining other devices on which the user has accessed email to check for malware or additional compromises, and conducting a comprehensive audit of the mailbox for any additional suspicious rules or forwarding configurations that may have been implemented but not yet detected. In some cases, particularly where malware is suspected, organizations may decide to rebuild affected devices completely, though with modern patched Outlook clients, the risk of re-infection through compromised Outlook files is lower than in legacy versions.
Comprehensive Audit Procedures and Search Methodologies
Organizations implementing comprehensive audit procedures for detecting malicious email rules must employ multiple complementary search strategies, as each approach reveals different aspects of potential compromises. The audit procedures detailed in Microsoft’s Purview documentation and security guidance outline specific search parameters that should be configured when searching the unified audit log.
For detecting inbox rule creation, auditors should leave the Activities field blank in the audit log search tool to ensure all activities are captured, then search across an appropriate date range—at minimum monthly, but preferably weekly or continuously for high-risk organizations. Crucially, after running the initial broad search, results should be filtered by searching specifically for `New-InboxRule` and `Updated inbox rules` operations, as these represent the activities most likely to be associated with rule creation by attackers versus routine administrative operations. When reviewing the search results for suspicious rules, investigators should examine the Parameters field within each audit record, looking for properties such as `ForwardTo`, `RedirectTo`, `MoveToFolder`, and `MarkAsRead`, which are commonly used in malicious rules.
For detecting mailbox-level forwarding configurations, auditors should search for `Set-Mailbox` operations specifically looking for parameters related to `ForwardingAddress`, `ForwardingSmtpAddress`, and `DeliverToMailboxAndForward`. This distinction is important because mailbox-level forwarding represents a different attack vector than inbox rules—it forwards all emails rather than emails matching specific conditions, and it is typically configured by administrators rather than individual users, making it an interesting target for attackers who have obtained administrative credentials.
Analyzing the audit log output requires understanding the specific indicators of compromise embedded in the rule parameters. Red Canary’s detection research identified that suspicious inbox rules typically contain one or more of the following characteristics: rules that forward messages to external SMTP addresses not matching the organization’s domain, rules that move emails to uncommon or rarely-used folders such as RSS folders or archive folders, rules that mark emails as read or delete them entirely, rules that apply conditions based on suspicious keywords such as “phish,” “invoice,” “payroll,” “do not reply,” or “suspicious email,” and rules with minimal or obscure names such as single punctuation marks or single letters.
Advanced Detection: PowerShell-Based Audit Techniques
For organizations requiring more sophisticated audit capabilities than what the standard Purview portal provides, PowerShell-based detection offers granular control over audit query parameters and output formatting. The administrative audit log can be queried using the `Search-AdminAuditLog` cmdlet with specific parameters targeting mailbox forwarding configuration changes: `Search-AdminAuditLog -Cmdlets Set-Mailbox -Parameters ForwardingAddress,ForwardingSmtpAddress -StartDate (Get-Date).AddDays(-90) -EndDate (Get-Date).AddDays(1)` This query searches the administrative audit log for the past 90 days—matching the default retention window—for any `Set-Mailbox` operations involving forwarding parameters.
However, security teams should note an important limitation of this approach: it only captures changes made through the `Set-Mailbox` PowerShell cmdlet, not forwarding rules created through the Outlook desktop client or Outlook on the Web interface. To capture the broader range of rule creation activities, organizations should use the unified audit log search capabilities with PowerShell queries such as: `Search-UnifiedAuditLog -StartDate (Get-Date).AddDays(-90) -EndDate (Get-Date) -Operations New-InboxRule,Set-InboxRule,UpdateInboxRules -ResultSize 5000` This query captures all three primary operations associated with rule creation across all access methods within a specified time window and returns results in a format suitable for programmatic analysis and alerting.
The mailbox-specific audit log can also be queried to detect when email messages are accessed through unusual protocols or in unusual patterns that may indicate an attacker reading through the compromised mailbox. The `MailItemsAccessed` mailbox audit action captures accesses of email messages through mail clients and protocols, providing forensic evidence of which messages an attacker may have viewed. Organizations can search for MailItemsAccessed records using the command: `Search-UnifiedAuditLog -StartDate 01/06/2020 -EndDate 02/20/2020 -UserIds [email protected] -Operations MailItemsAccessed -ResultSize 1000` This audit record type is particularly valuable for determining the scope of a compromise, as it reveals which specific messages were accessed by the attacker, helping organizations assess the sensitivity of information that may have been exposed.
Timeline Considerations in Business Email Compromise Investigations
When malicious email rules are discovered in the context of business email compromise investigations, the audit timeline becomes critical for establishing the attacker’s objectives and operational methodology. Business email compromise typically involves attackers impersonating trusted individuals to manipulate employees into transferring funds or disclosing sensitive information. The creation of malicious email rules fits strategically into this attack pattern: by forwarding payment-related emails and other financial communications to attacker-controlled addresses, the attacker can intercept legitimate business communications and respond to payment requests before the intended recipient sees them, effectively redirecting funds to attacker accounts.
In the real-world investigation timeline documented by Arctic Wolf, which identified the compromise within 19 minutes of malicious rule creation, the detailed analysis of the attack sequence revealed a sophisticated progression. The attacker, having gained initial access to the compromised account through unknown means (but discovered through Duo authentication logs showing suspicious login attempts), immediately began manipulating the mailbox environment. The attack timeline proceeded as follows: at 12:57 PM, the initial access was gained and Duo recorded the suspicious logon; at 1:16 PM, the attacker began creating and deleting forwarding rules while simultaneously modifying calendar entries; at 1:16 PM, the security monitoring system detected these rule manipulations and escalated the incident; and at 1:18 PM, the incident investigation team began their investigation. This compressed timeline illustrates why continuous monitoring of email rule changes is essential—the window between compromise and remediation may be measured in minutes rather than hours or days.
Remediation Timeline and Post-Incident Procedures
Once malicious email rules have been identified through audit procedures, the remediation timeline must balance the need for immediate containment against the investigative imperative of preserving evidence for forensic analysis and incident response. The recommended remediation procedures, as documented in Microsoft security guidance and industry incident response frameworks, should be executed in a specific sequence to minimize further damage while ensuring complete elimination of the compromise.
The immediate remediation step—disabling the malicious rule—should be completed within minutes of detection, as every passing moment allows the rule to continue forwarding emails or deleting communications. The rule can be disabled through the Outlook client by accessing the rule properties and toggling the rule to disabled status, or through PowerShell using commands such as `Disable-InboxRule -Identity “RuleName”` The advantage of disabling rather than deleting is that it preserves the rule configuration for forensic examination while immediately stopping its operational impact.
Following immediate rule disablement, the user’s password must be reset to a strong, complex value that differs substantially from any previously used passwords. This password reset should be enforced by the administrator rather than permitting the user to set their own password, ensuring that the attacker cannot retain access through password knowledge. Simultaneously, if the user account does not have multi-factor authentication (MFA) enabled, this should be implemented as part of the remediation process to prevent future compromise through credential attacks.
For users with multiple devices that access email through Outlook clients, each device should be examined and potentially rebuilt to eliminate any persistent malware that may have been installed alongside the email rule compromise. While current versions of Outlook include protections against the rule and custom form injection attacks that previously allowed code execution, older or unpatched versions may remain vulnerable, and the presence of a rule compromise may indicate other malware infections are present.
The full forensic investigation timeline—establishing root cause, examining mailbox access logs, and checking for additional persistence mechanisms—should proceed in parallel with these immediate remediation steps but should not delay the containment actions. Once the attacker’s access has been revoked through password reset and MFA implementation, investigators can conduct more thorough analysis to determine the full scope of the compromise, examine which emails may have been accessed or forwarded, and assess the organizational impact of the incident.
Preventive Measures and Continuous Audit Practices
Beyond reactive detection and investigation of existing malicious email rules, organizations should implement preventive strategies and continuous audit practices designed to minimize the likelihood of successful rule-based attacks and to detect compromises as quickly as possible. One of the most effective preventive strategies is to disable external email auto-forwarding at the organization level through outbound spam filter policies, though this approach must be balanced against legitimate business use cases where external forwarding serves genuine operational purposes.
For organizations using Microsoft 365, the outbound spam filter policy can be configured with the setting for automatic external forwarding to “Off,” which disables automatic forwarding rules and results in non-delivery reports to senders attempting to send emails to auto-forwarded external addresses. This setting prevents both user-created inbox rules and administrator-configured mailbox forwarding from forwarding emails externally. However, organizations should recognize that this setting affects the ability of users to forward emails to external recipients, which may impact legitimate business processes such as forwarding emails to external contractors, clients, or partner organizations.
An alternative approach that balances security with operational flexibility is to permit the creation of forwarding rules while using outbound spam filter policies to block the actual forwarding of emails to external recipients. In this configuration, if an attacker creates a malicious forwarding rule, the rule will be created successfully (and detected through audit), but the actual forwarding of emails will be blocked, preventing data exfiltration while maintaining high-fidelity detection of compromise attempts. This approach ensures that security teams receive alerts about compromise attempts (through detection of rule creation) while minimizing the actual damage from those attempts (through blocking of email delivery).
Continuous audit practices should include regular—ideally monthly or more frequently—execution of PowerShell-based audit queries to identify any new inbox rules or forwarding configurations. Organizations implementing security information and event management (SIEM) systems should integrate email rule creation events into their SIEM platforms and implement correlation rules that alert security teams to suspicious rule creation patterns, such as rules created outside of normal business hours, rules created from unusual IP addresses, or multiple rules created in rapid succession from a single user account.
Additionally, organizations should implement alert policies within Microsoft 365 to notify administrators when new forwarding rules are created. The “Creation of forwarding/redirect rule” informational alert policy can be enabled to send alerts to tenant administrators when forwarding rules are created, providing real-time notification of potential compromises. However, security teams should recognize that these alerts will fire for both benign user-created rules and malicious rules, requiring tuning and investigation of each alert to distinguish legitimate administrative activity from attack activity.
Organizational Compliance and Audit Retention Strategies
From an organizational governance and compliance perspective, the audit timeline for email rules intersects with broader email security audit and compliance obligations. Regular email security audits should encompass evaluation of email rule configurations, authentication and access controls, email filtering and spam security, encryption standards, user training and awareness, and incident response preparedness. These comprehensive audits should be conducted at least annually for most organizations, with higher-risk or more heavily regulated organizations potentially requiring quarterly or even continuous audits.
The audit log retention timeline becomes particularly significant for organizations subject to regulatory requirements such as the Health Insurance Portability and Accountability Act (HIPAA), the Gramm-Leach-Bliley Act (GLBA), or the Securities and Exchange Commission (SEC) Rule 17a-4. These regulations typically impose specific requirements for preserving audit records and communications as evidence of compliance with security controls and incident response procedures. Microsoft 365 organizations should ensure that audit log retention policies are configured to meet regulatory requirements, recognizing that default retention periods of 90 or 180 days may be insufficient for compliance purposes.
For organizations implementing Preservation Lock—Microsoft’s mechanism for locking retention policies to prevent modification—the audit timeline becomes immutable once the lock is applied. Preservation Lock ensures that retention policies cannot be turned off or made less restrictive after being implemented, providing organizations with a technical guarantee of audit evidence preservation that can be demonstrated to regulators or auditors. This is particularly valuable for organizations in regulated industries where it must be demonstrated that audit records are protected from tampering or deletion by administrators.
Emerging Threats and Advanced Email Rule Attack Techniques
As security defenses have improved, attackers have evolved their email rule attack techniques to evade detection systems. Advanced attackers now employ multiple layers of rules to obscure the true objective, creating decoy rules that perform benign operations while hidden rules perform the actual malicious forwarding or deletion operations. Additionally, attackers have learned to create multiple overlapping rules with carefully designed conditions to ensure that at least one rule achieves its objectives even if some are disabled or removed by administrators.
Another emerging technique involves the use of Outlook custom forms injections, which provide an alternative persistence mechanism to inbox rules for executing code when specific emails arrive in the compromised mailbox. Custom forms can be designed to launch remote code execution when triggered by specific emails from the attacker, providing a more sophisticated persistence mechanism than simple email forwarding. Detection of custom forms injection requires running the same `Get-AllTenantRulesAndForms.ps1` PowerShell script used for rule detection, examining the output for custom forms marked as IPM.Note.[custom name] that contain executable code.
Additionally, attackers increasingly leverage OAuth application permissions to achieve persistent access to mailboxes without requiring passwords or creating email rules. By obtaining user consent to an application that requests `MailboxSettings.ReadWrite` or other high-privileged permissions, attackers can create rules, forwarding configurations, and other modifications through the OAuth-authenticated application even after the user’s password has been changed. Detection of OAuth-based compromises requires examining the Enterprise Applications blade in Azure Active Directory for suspicious applications that have been granted permissions to user mailboxes, a process that is distinct from the email rule audit timeline but equally important for comprehensive account compromise investigations.
Reclaiming Your Inbox: The Final Rule Review
The audit timeline for detecting and responding to malicious email rules represents a critical component of comprehensive email security strategies and post-compromise incident response procedures. Organizations must recognize that email rules, while appearing as innocuous mailbox configuration options from a user perspective, represent sophisticated attack mechanisms that enable adversaries to maintain persistent access to compromised accounts, exfiltrate sensitive communications, and conceal evidence of their presence. The compressed timeline from initial account compromise to malicious rule implementation—potentially occurring within minutes—requires that organizations implement continuous or frequent periodic audit procedures capable of detecting rule creation events within the 90-day audit log retention window for standard Microsoft 365 licensing.
Effective email security audit strategies must employ multiple complementary detection methodologies: regular PowerShell-based queries to audit all user mailboxes for suspicious rule configurations, continuous monitoring of audit logs for rule creation events, implementation of alert policies to notify administrators of forwarding rule creation, and comprehensive investigation procedures that examine authentication logs, email client access patterns, and OAuth application permissions to establish the root cause and scope of compromises. The remediation timeline must balance the need for immediate containment—disabling malicious rules within minutes—against the investigative imperative of preserving evidence for forensic analysis and regulatory compliance.
Organizations that implement these audit and detection strategies, combined with preventive measures such as disabling external email forwarding or blocking forwarded emails at the gateway, significantly reduce their vulnerability to email rule-based attacks and improve their ability to detect and respond to account compromises before substantial damage occurs. By understanding the audit timeline, detection methodologies, and remediation procedures detailed in this analysis, security teams can transform their email security posture from purely reactive—detecting compromises only after evidence appears in logs or when users report problems—to proactive, with security teams identifying compromises within hours or minutes of malicious rule implementation and immediately commencing containment and remediation procedures.
References and Recommended Actions
Organizations seeking to improve their email rule audit capabilities should prioritize the following actions: first, implement monthly or more frequent PowerShell-based audit queries across all user mailboxes to detect suspicious inbox rules and forwarding configurations; second, ensure that Microsoft 365 licensing includes E5 or E5 Compliance licenses for extended audit log retention beyond the default 90 days; third, configure alert policies within Microsoft 365 to generate real-time notifications of suspicious rule creation; fourth, integrate email rule creation events into SIEM platforms for correlation with other suspicious activities; fifth, establish incident response procedures specifically addressing the four-phase remediation timeline of limiting damage, understanding root cause, assessing scope, and recovery; and sixth, conduct regular email security audits that encompass rule configurations as part of comprehensive evaluation of organizational email security posture.
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