CVE-2026-66141 in Exim
Summary
by MITRE • 07/24/2026
Exim before 4.99.5 allows .forward privilege escalation because force_command for a pipe transport is mishandled.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 07/24/2026
This vulnerability exists in Exim versions prior to 4.99.5 and represents a critical privilege escalation flaw that stems from improper handling of the force_command directive within pipe transport configurations. The technical implementation fails to properly validate or sanitize user-provided command strings when they are processed through .forward files, creating an avenue for malicious actors to execute arbitrary commands with elevated privileges. This issue directly relates to CWE-78 which addresses improper neutralization of special elements used in OS commands, and CWE-20 which covers input validation weaknesses that can lead to command injection scenarios.
The operational impact of this vulnerability is severe as it allows attackers to escalate their privileges from regular user accounts to system-level access through the manipulation of .forward files. When a user's mail is configured to be forwarded through a pipe transport with force_command set, an attacker can craft malicious forward entries that bypass normal security controls and execute commands with the privileges of the Exim process itself. This creates a persistent backdoor mechanism that can be exploited across multiple user accounts on the system. The vulnerability operates at the intersection of privilege management and input sanitization, making it particularly dangerous in multi-user environments where different users may have varying levels of access to mail services.
Attackers can exploit this weakness by creating specially crafted .forward files that contain malicious command sequences which are then processed by Exim's pipe transport mechanism. The force_command directive is designed to enforce specific command execution patterns but fails to properly validate input, allowing attackers to inject additional commands or modify existing ones. This vulnerability aligns with ATT&CK technique T1059.001 for command and scripting interpreter usage, and T1548.001 for abuse of privilege escalation techniques. The exploitation requires minimal privileges initially since users can typically create .forward files in their home directories, but the ultimate impact extends to full system compromise.
Mitigation strategies focus on immediate patching of Exim installations to version 4.99.5 or later where this vulnerability has been addressed through proper input validation and sanitization of force_command parameters. System administrators should also implement strict access controls on .forward files and monitor mail server configurations for unauthorized modifications. Network segmentation and privilege separation measures can help limit the potential impact if exploitation occurs, while regular security audits of mail server configurations should be conducted to identify any remaining instances of vulnerable pipe transport setups. Additional defensive measures include implementing proper logging of mail delivery operations and monitoring for suspicious command execution patterns that may indicate exploitation attempts.