CVE-2011-4613 in X.Org
Summary
by MITRE
The X.Org X wrapper (xserver-wrapper.c) in Debian GNU/Linux and Ubuntu Linux does not properly verify the TTY of a user who is starting X, which allows local users to bypass intended access restrictions by associating stdin with a file that is misinterpreted as the console TTY.
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Analysis
by VulDB Data Team • 01/05/2025
The vulnerability described in CVE-2011-4613 represents a critical access control flaw within the X.Org X server wrapper implementation on Debian and Ubuntu systems. This issue stems from improper verification of terminal tty associations during X server initialization, creating a pathway for local privilege escalation and unauthorized access to graphical sessions. The flaw specifically affects the xserver-wrapper.c component that manages the execution context for X server processes, where the system fails to validate the authenticity of the controlling terminal associated with user sessions.
The technical implementation of this vulnerability resides in the wrapper's failure to properly authenticate the TTY connection before granting access to the X server. When a user attempts to start an X session, the wrapper should verify that stdin is connected to a legitimate console TTY device rather than allowing arbitrary file descriptors to be interpreted as valid terminals. This misinterpretation occurs because the wrapper does not perform adequate checks on the file descriptor associated with stdin, enabling malicious users to manipulate the terminal association through file descriptor redirection or symbolic link manipulation. The vulnerability essentially allows an attacker to bypass the intended security boundaries that normally restrict X server access to authenticated console sessions.
From an operational perspective, this vulnerability presents significant security implications for systems running affected Debian or Ubuntu distributions. Local users who can access the system can exploit this weakness to gain unauthorized access to graphical sessions, potentially escalating privileges or bypassing additional security controls that depend on proper TTY verification. The impact extends beyond simple access bypass, as it can enable attackers to establish persistent access points within the graphical environment, potentially leading to broader system compromise. This flaw particularly affects multi-user systems where console access controls are critical for maintaining security boundaries between different user sessions.
The vulnerability aligns with CWE-284, which addresses improper access control issues, specifically focusing on inadequate verification of terminal associations in Unix-like systems. It also maps to ATT&CK technique T1068, which covers the exploitation of local privileges through improper access control mechanisms. The attack vector requires local system access but does not necessitate network connectivity, making it particularly concerning for environments where physical access or shell access is possible. Organizations should consider implementing immediate mitigations including system updates, proper file descriptor validation, and monitoring for unauthorized TTY manipulations. The recommended solution involves patching the xserver-wrapper.c implementation to enforce strict TTY verification, ensuring that only legitimate console TTY devices can initiate X server sessions and preventing file descriptor manipulation attacks that exploit this access control bypass.