CVE-2015-3418 in X11R1
Summary
by MITRE
The ProcPutImage function in dix/dispatch.c in X.Org Server (aka xserver and xorg-server) before 1.16.4 allows attackers to cause a denial of service (divide-by-zero and crash) via a zero-height PutImage request.
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Analysis
by VulDB Data Team • 08/29/2025
The vulnerability identified as CVE-2015-3418 resides within the X.Org Server implementation, specifically in the ProcPutImage function located in dix/dispatch.c. This flaw represents a classic denial of service condition that can be exploited through malformed X11 protocol requests. The X.Org Server serves as the foundation for the X Window System across numerous Unix-like operating systems, making this vulnerability particularly concerning as it affects a core component of graphical user interfaces. The vulnerability specifically targets the PutImage request processing mechanism, which is used to transfer image data between client applications and the X server.
The technical flaw manifests when the ProcPutImage function fails to properly validate the height parameter of a PutImage request. When an attacker submits a PutImage request with a zero height value, the server attempts to perform a division operation using this zero value, resulting in a divide-by-zero exception. This mathematical error occurs because the server code does not include proper input validation to check for zero values before performing arithmetic operations. The absence of this validation creates a condition where the server's execution flow becomes unstable, leading to an immediate crash of the X server process. This behavior aligns with CWE-369, which describes the weakness of dividing by zero in software implementations.
The operational impact of this vulnerability extends beyond simple service disruption as it can be exploited by remote attackers to cause persistent denial of service conditions. In environments where graphical interfaces are critical for system operation, such as workstation environments or server management interfaces, this vulnerability can render systems unusable until the X server is manually restarted. The crash occurs during normal protocol processing, meaning that legitimate users may be unable to access graphical applications or desktop environments. This vulnerability is particularly dangerous in multi-user environments where an attacker could repeatedly exploit it to maintain persistent service disruption. The ATT&CK framework categorizes this as a Denial of Service technique under the T1499 category, specifically targeting system services and resources.
Mitigation strategies for CVE-2015-3418 primarily involve updating the X.Org Server to version 1.16.4 or later, where the vulnerability has been patched. The fix implemented in the patched versions includes proper input validation that checks for zero height values before any arithmetic operations are performed. Organizations should also consider implementing network-level protections such as firewall rules that limit access to X11 ports, particularly in environments where X11 forwarding is not required. Additionally, monitoring systems should be configured to detect unusual patterns of X11 protocol requests that might indicate exploitation attempts. Security teams should also review and update their incident response procedures to include rapid recovery protocols for X server crashes, ensuring minimal downtime in critical environments. The vulnerability demonstrates the importance of proper input validation in server-side applications and serves as a reminder that even seemingly benign protocol operations can create critical security weaknesses when proper error handling is not implemented.