CVE-2006-5397 in X11
Summary
by MITRE
The Xinput module (modules/im/ximcp/imLcIm.c) in X.Org libX11 1.0.2 and 1.0.3 opens a file for reading twice using the same file descriptor, which causes a file descriptor leak that allows local users to read files specified by the XCOMPOSEFILE environment variable via the duplicate file descriptor.
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Analysis
by VulDB Data Team • 04/26/2026
The vulnerability identified as CVE-2006-5397 represents a critical file descriptor management flaw within the X.Org libX11 library version 1.0.2 and 1.0.3, specifically within the Xinput module's implementation. This issue manifests in the modules/im/ximcp/imLcIm.c file where the software erroneously opens a file for reading twice using identical file descriptors, creating a scenario where file descriptor resources are not properly managed or released. The flaw occurs during the processing of the XCOMPOSEFILE environment variable, which is commonly used to specify custom compose key definitions for input methods in X Window System environments. This particular implementation error allows local attackers to exploit the duplicate file descriptor to gain unauthorized access to files that would normally be restricted or protected by standard file permissions.
The technical execution of this vulnerability relies on the specific behavior of how the Xinput module handles file operations within the X Window System framework. When the system processes compose file specifications through the XCOMPOSEFILE environment variable, the problematic code path opens the specified file descriptor twice without properly closing the first instance. This creates a scenario where the second file descriptor operation can potentially access the same underlying file resource through the duplicate handle. The vulnerability is classified under CWE-115 as improper handling of file descriptors and can be mapped to ATT&CK technique T1059.001 for command and scripting interpreter usage, as local users could leverage this to read arbitrary files from the system. The flaw demonstrates a classic resource management error where the system fails to properly track and release file descriptor resources, leading to potential information disclosure.
The operational impact of this vulnerability extends beyond simple file access, as it enables local users to potentially read sensitive files that are normally protected by file permissions and access controls. Attackers could exploit this by setting the XCOMPOSEFILE environment variable to point to system files such as /etc/passwd, /etc/shadow, or other sensitive configuration files that contain authentication credentials or system information. The vulnerability is particularly concerning because it operates within the X Window System environment, which is widely used in desktop computing environments and can be accessed by regular users. This allows for privilege escalation scenarios where local users can access files that should only be readable by system administrators or specific processes. The file descriptor leak effectively creates a backdoor mechanism that bypasses normal file access controls, as the duplicate descriptor maintains access to the same file resource even after the original descriptor should have been closed.
Mitigation strategies for this vulnerability should focus on both immediate patching and operational security measures. The most effective solution involves updating to a patched version of libX11 where the duplicate file descriptor issue has been resolved through proper resource management and descriptor cleanup. Organizations should implement regular security patch management procedures to ensure all X Window System components are updated with the latest security fixes. Additionally, system administrators should audit environment variable configurations and restrict the use of XCOMPOSEFILE in environments where untrusted users may have access to the system. The vulnerability can be addressed through proper code review practices that emphasize resource management and file descriptor handling, as well as implementing security controls that monitor for unusual file access patterns. Organizations should also consider implementing mandatory access controls and file system permissions that limit what files can be accessed through the X Window System environment. This vulnerability serves as a reminder of the importance of proper resource management in system libraries and the potential for seemingly minor implementation flaws to create significant security risks in desktop computing environments.