CVE-1999-0129 in Sendmail
Summary
by MITRE
sendmail allows local users to write to a file and gain group permissions via a .forward or :include: file.
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Analysis
by VulDB Data Team • 10/27/2025
The vulnerability described in CVE-1999-0129 represents a significant privilege escalation flaw in the sendmail mail transfer agent that has persisted as a critical security concern for decades. This vulnerability specifically affects systems where sendmail operates with elevated privileges and allows local users to manipulate file permissions through the use of .forward files or :include: directives. The flaw exploits the inherent trust model of sendmail's configuration handling, where user-controlled input can be interpreted as commands that modify system permissions. The vulnerability is classified under CWE-22 as "Improper Limitation of a Pathname to a Restricted Directory" and also relates to CWE-276 as "Incorrect Permission Assignment for Critical Resources", demonstrating how improper file access controls can lead to privilege escalation.
The technical implementation of this vulnerability relies on sendmail's processing of user mail forwarding configurations. When a user creates a .forward file in their home directory or when sendmail encounters a :include: directive in mail processing, the system can be tricked into creating or modifying files with group permissions that should otherwise be restricted to system administrators. This occurs because sendmail's configuration parsing does not properly validate or sanitize the paths specified in these files, allowing attackers to specify arbitrary file locations that can be modified or created with elevated privileges. The vulnerability is particularly dangerous because it leverages the legitimate mail forwarding functionality of sendmail to achieve unauthorized system access, making it difficult to detect through standard monitoring mechanisms.
The operational impact of this vulnerability extends far beyond simple file modification, as it provides attackers with the ability to escalate privileges and potentially gain control over entire systems. Local users who can create or modify .forward files or manipulate :include: directives can effectively elevate their privileges to group levels, which may include access to sensitive system files, administrative functions, or other users' mail queues. This vulnerability has been classified under the MITRE ATT&CK framework as part of the privilege escalation category, specifically relating to techniques that exploit misconfigurations and improper access controls. The attack vector is particularly insidious because it requires minimal user interaction beyond normal mail system usage, making it a persistent threat in environments where sendmail is widely deployed.
Mitigation strategies for CVE-1999-0129 require a multi-layered approach that addresses both immediate system hardening and long-term architectural changes. System administrators should implement strict file permission controls on sendmail configuration directories and ensure that .forward files are properly validated before processing. The recommended remediation includes disabling the use of :include: directives in sendmail configurations, restricting write access to user home directories, and implementing proper input sanitization for all mail processing functions. Additionally, organizations should consider migrating to more modern mail systems that do not exhibit these legacy permission handling flaws, and implement monitoring solutions that can detect unauthorized file modifications in critical system directories. Regular security audits should verify that sendmail configurations comply with security best practices and that no unauthorized modifications have been made to mail processing components. The vulnerability serves as a classic example of how seemingly benign functionality can be exploited to create serious security breaches, emphasizing the importance of proper access control implementation and security by design principles in all system components.