CVE-2013-6424 in X Server
Summary
by MITRE
Integer underflow in the xTrapezoidValid macro in render/picture.h in X.Org allows context-dependent attackers to cause a denial of service (crash) via a negative bottom value.
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Analysis
by VulDB Data Team • 01/31/2022
The vulnerability identified as CVE-2013-6424 represents a critical integer underflow condition within the X.Org X Window System rendering subsystem. This flaw exists in the xTrapezoidValid macro located in the render/picture.h header file, which is part of the X.Org server implementation responsible for handling graphical rendering operations. The vulnerability manifests when the system processes trapezoid shapes during graphic operations, specifically when a negative bottom value is provided to the rendering pipeline. This condition falls under the CWE-191 Integer Underflow vulnerability class, where an integer value is decremented below its minimum representable value, potentially leading to unexpected behavior in subsequent operations.
The technical implementation of this vulnerability exploits the mathematical properties of signed integer arithmetic within the X.Org rendering engine. When the xTrapezoidValid macro evaluates trapezoid parameters, it performs calculations that assume positive integer values for geometric dimensions. However, when a negative bottom value is passed, the arithmetic operations can result in integer underflow conditions that corrupt memory structures or cause invalid comparisons. The flaw operates within the context of the X Window System's rendering pipeline, specifically affecting the picture rendering extension that handles complex graphical operations including transformations, compositing, and geometric shape rendering. This vulnerability is particularly concerning as it exists in the core rendering components that are fundamental to graphical user interfaces and can be triggered through legitimate rendering operations.
From an operational impact perspective, this vulnerability enables context-dependent attackers to execute denial of service attacks against systems running X.Org servers. The exploitation results in system crashes or application hangs that effectively deny service to legitimate users. The attack requires minimal privileges since it can be triggered through normal rendering operations, making it particularly dangerous in multi-user environments or server configurations where X11 forwarding is enabled. The vulnerability affects systems that utilize the X.Org X Window System for graphical rendering, which includes most Linux desktop environments, server configurations, and any system implementing X11 protocols for graphical user interfaces. The impact extends beyond simple service disruption to potentially affecting graphical applications, remote desktop sessions, and any system components that depend on the X Window System for display operations.
Mitigation strategies for CVE-2013-6424 should focus on immediate patch application from X.Org maintainers and system administrators should prioritize updating their X.Org server installations to versions that address the integer underflow condition in the xTrapezoidValid macro. The fix typically involves implementing proper input validation and boundary checks before performing arithmetic operations on trapezoid parameters, ensuring that negative values are either rejected or properly handled through appropriate conditional logic. Additionally, administrators should consider implementing network segmentation and access controls to limit exposure of X11 services to untrusted networks, as recommended in the ATT&CK framework for mitigating remote exploitation vectors. System hardening measures including disabling unnecessary X11 forwarding, implementing proper firewall rules, and monitoring for unusual X11 traffic patterns can further reduce the attack surface. Organizations should also implement regular vulnerability scanning and patch management processes to identify and remediate similar integer underflow conditions that may exist in other components of their X Window System implementations. The vulnerability demonstrates the importance of proper integer validation in graphics rendering systems and aligns with security best practices outlined in the CWE guidelines for preventing arithmetic overflow and underflow conditions in system code.