Heap Buffer Overflow in libXfont2

Executive Summary

CVE-2026-56001 is a heap buffer overflow vulnerability in the libXfont2 library before version 2.0.8. It occurs in the BitmapScaleBitmaps() function, where a 32-bit variable used to track the number of bytes to allocate can overflow. This overflow causes the program to allocate a buffer that is too small.

An attacker can exploit this by loading a specially crafted PCF font with a scale factor that causes the per-glyph byte counts to become excessively large, triggering the overflow. This can lead to memory corruption within the X server.

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Impact Analysis

This vulnerability allows attackers who have access to the X Server to execute arbitrary code within the X server process. Because the X server often runs with elevated privileges, successful exploitation can lead to full compromise of the system's graphical environment.

The CVSS score of 8.5 indicates a high severity, with impacts including high confidentiality, integrity, and availability loss.

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Mitigation Strategies

To mitigate this vulnerability, you should update libXfont2 to version 2.0.8 or later, where the issue has been fixed.

Additionally, restrict or monitor access to the X Server to prevent attackers from loading specially crafted PCF fonts that exploit this vulnerability.

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Detection Guidance

This vulnerability involves a heap buffer overflow in the BitmapScaleBitmaps() function of libXfont2 prior to version 2.0.8, triggered by loading specially crafted PCF fonts with inflated per-glyph byte counts.

To detect this vulnerability on your system, you should first check the installed version of libXfont2 to see if it is older than 2.0.8.

• On Linux systems, run: rpm -q libXfont2
• Or, if using a Debian-based system: dpkg -l | grep libxfont2

If the version is older than 2.0.8, your system is vulnerable.

Detecting exploitation attempts on the network or system would involve monitoring for unusual or malformed PCF font files being loaded via the X Server's SetFontPath and OpenFont requests, but no specific detection commands or signatures are provided.

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