CVE-2017-13720 in libXfont
Summary
by MITRE
In the PatternMatch function in fontfile/fontdir.c in libXfont through 1.5.2 and 2.x before 2.0.2, an attacker with access to an X connection can cause a buffer over-read during pattern matching of fonts, leading to information disclosure or a crash (denial of service). This occurs because '\0' characters are incorrectly skipped in situations involving ? characters.
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Analysis
by VulDB Data Team • 01/03/2023
The vulnerability identified as CVE-2017-13720 resides within the libXfont library, specifically in the PatternMatch function located in fontfile/fontdir.c. This library serves as a critical component in X Window System implementations, handling font management and pattern matching operations for graphical user interfaces. The flaw affects versions through 1.5.2 and 2.x before 2.0.2, representing a significant security concern for systems relying on X11 font handling capabilities. The vulnerability manifests as a buffer over-read condition that occurs during font pattern matching operations, creating potential for both information disclosure and denial of service scenarios.
The technical root cause of this vulnerability stems from improper handling of null character sequences within pattern matching logic. Specifically, the implementation fails to correctly process situations where '?' characters are present in font patterns, leading to incorrect skipping of null characters. This improper character handling creates a scenario where memory locations beyond the intended buffer boundaries are accessed during pattern matching operations. The flaw exploits the difference between how null characters and wildcard '?' characters are processed within the same parsing context, resulting in memory access violations that can be triggered through crafted font pattern inputs.
Attackers with access to an X connection can leverage this vulnerability to execute malicious pattern matching operations that cause the application to read beyond allocated memory boundaries. The operational impact encompasses both information disclosure and system availability concerns, as the over-read conditions can expose sensitive memory contents to attackers or cause application crashes that result in denial of service. This vulnerability represents a classic example of improper input validation and memory boundary checking, where the pattern matching algorithm fails to properly account for all character combinations that may occur during font name processing. The security implications are particularly concerning in multi-user environments where X11 connections are accessible to untrusted parties.
The vulnerability maps to CWE-125, which describes out-of-bounds read conditions, and aligns with ATT&CK technique T1059 for execution through command injection or memory corruption. Organizations should prioritize patching affected systems to version 2.0.2 or later of libXfont, as this update resolves the buffer over-read issue through proper character sequence handling and memory boundary validation. Additionally, implementing network segmentation to limit X11 connection access, employing input validation for font pattern matching operations, and monitoring for unusual pattern matching behaviors can help mitigate the risk. System administrators should also consider disabling unnecessary font services and implementing access controls to reduce the attack surface, as the vulnerability requires only X connection access to be exploited effectively.