CVE-2017-2624 in X.org
Summary
by MITRE
It was found that xorg-x11-server before 1.19.0 including uses memcmp() to check the received MIT cookie against a series of valid cookies. If the cookie is correct, it is allowed to attach to the Xorg session. Since most memcmp() implementations return after an invalid byte is seen, this causes a time difference between a valid and invalid byte, which could allow an efficient brute force attack.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 08/29/2025
The vulnerability identified as CVE-2017-2624 resides within the xorg-x11-server component of X Window System implementations, specifically affecting versions prior to 1.19.0. This flaw represents a timing side-channel attack vector that exploits the behavior of the memcmp() function during authentication processes. The vulnerability occurs during the MIT cookie validation process where the server compares received authentication credentials against a series of valid cookies to establish session access. The implementation uses a standard memory comparison function that terminates execution as soon as a mismatch is detected, creating measurable time differences between valid and invalid authentication attempts.
The technical exploitation of this vulnerability stems from the predictable timing behavior of the memcmp() function implementation. When comparing authentication cookies, the system processes each byte sequentially until a mismatch occurs, causing the comparison to terminate prematurely. This differential execution time between successful and failed comparisons creates a timing leak that can be measured and exploited by attackers. The vulnerability specifically affects the X11 authentication mechanism where MIT cookies are used to validate session access, making it possible for malicious actors to determine valid cookie values through statistical analysis of timing variations.
This timing side-channel vulnerability directly impacts the security of X Window System implementations by enabling efficient brute force attacks against authentication mechanisms. Attackers can systematically measure response times for authentication attempts and use this information to deduce valid cookie values without requiring direct access to the system or knowledge of the underlying authentication protocol. The operational impact extends beyond simple session hijacking, potentially allowing unauthorized access to graphical user interfaces and system resources that depend on X11 for display management. The vulnerability affects systems where X11 server implementations are used for remote access, kiosk deployments, and any environment where authentication timing differences could be measured and exploited.
The vulnerability maps to CWE-203: "Observable Timing Discrepancy" and aligns with ATT&CK technique T1212: "Exploitation for Credential Access" within the credential access category. Organizations implementing X11 server solutions should immediately upgrade to version 1.19.0 or later to address this timing side-channel vulnerability. Mitigation strategies include implementing constant-time comparison functions during authentication processes, which prevent timing variations that could reveal information about authentication credentials. Additionally, network segmentation and access controls should be enforced to limit exposure of X11 services to untrusted networks, while monitoring systems should be configured to detect unusual authentication patterns that might indicate timing attack attempts. The fix implemented in xorg-x11-server version 1.19.0 addresses this issue by replacing the vulnerable memcmp() usage with constant-time comparison mechanisms that do not expose timing information during authentication validation processes.