CVE-2019-3842 in systemd
Summary
by MITRE
In systemd before v242-rc4, it was discovered that pam_systemd does not properly sanitize the environment before using the XDG_SEAT variable. It is possible for an attacker, in some particular configurations, to set a XDG_SEAT environment variable which allows for commands to be checked against polkit policies using the "allow_active" element rather than "allow_any".
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Analysis
by VulDB Data Team • 05/21/2025
The vulnerability identified as CVE-2019-3842 resides within the systemd service management framework, specifically affecting versions prior to v242-rc4. This flaw manifests in the pam_systemd module which serves as a Pluggable Authentication Module component responsible for integrating systemd with the authentication subsystem. The issue stems from inadequate environment sanitization practices during the processing of the XDG_SEAT variable, creating a potential privilege escalation vector that could be exploited by malicious actors. The vulnerability is particularly concerning because it operates at the intersection of authentication and authorization mechanisms, where improper handling of environment variables can lead to unexpected behavior in policy enforcement.
The technical flaw occurs when the pam_systemd module processes the XDG_SEAT environment variable without sufficient validation or sanitization. This variable is typically used to identify the seat in graphical environments, but in vulnerable configurations, attackers can manipulate this variable to influence how polkit policies are evaluated. Specifically, the flaw allows an attacker to cause commands to be evaluated against polkit policies using the "allow_active" element instead of the more restrictive "allow_any" element. This change in policy evaluation can result in reduced security controls and potentially enable unauthorized actions that would normally be restricted. The vulnerability operates under CWE-20, which encompasses improper input validation, and specifically relates to CWE-78, which deals with command injection vulnerabilities.
The operational impact of this vulnerability extends beyond simple privilege escalation to encompass broader authorization bypass scenarios. When an attacker successfully manipulates the XDG_SEAT variable, they can effectively weaken the security posture of systems running vulnerable systemd versions. This weakness is particularly dangerous in multi-user environments where different users have varying levels of access rights, as it could allow a less privileged user to execute commands that should only be available to administrators or users with active sessions. The vulnerability's exploitation requires specific environmental conditions, making it more targeted but no less dangerous in environments where such configurations exist.
Mitigation strategies for CVE-2019-3842 should focus on upgrading to systemd version v242-rc4 or later, which contains the necessary patches to properly sanitize the XDG_SEAT environment variable. System administrators should also implement additional monitoring of environment variable handling within authentication modules and consider implementing stricter access controls for session management components. The vulnerability's classification under ATT&CK technique T1068, which covers 'Exploitation for Privilege Escalation', underscores the need for comprehensive security monitoring and incident response procedures. Organizations should also review their current configurations to ensure that environment variables are properly validated and sanitized, particularly in components that interface with authorization systems like polkit. Additionally, implementing principle of least privilege controls and regular security audits can help detect and prevent similar vulnerabilities from being exploited in other system components.