CVE-2026-56001 in libXfont2
Summary
by MITRE • 07/08/2026
A heap buffer overflow in BitmapScaleBitmaps in libXfont2 before 2.0.8 due to an overflowing 32bit size could be used by attackers able to access the X Server to execute code within the X server cont
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Analysis
by VulDB Data Team • 07/08/2026
The vulnerability under discussion represents a critical heap buffer overflow flaw存在于libXfont2 library version prior to 2008, specifically within the BitmapScaleBitmaps function. This security weakness arises from improper handling of 32-bit size parameters that can lead to memory corruption when processing bitmap scaling operations. The flaw exists in the X Window System font rendering subsystem, making it particularly dangerous as it operates within the privileged context of the X server itself.
The technical implementation of this vulnerability stems from inadequate bounds checking during bitmap scaling operations where a 32-bit integer value is used to determine buffer allocation sizes. When an attacker can send maliciously crafted font data to an X server running the vulnerable libXfont2 version, they can cause the size parameter to overflow and exceed the allocated heap memory boundaries. This allows for arbitrary memory corruption that can be leveraged to overwrite critical data structures or function pointers, ultimately enabling code execution within the X server process context.
The operational impact of this vulnerability is severe as it requires only local access to the X server to exploit, making it particularly dangerous in multi-user environments where attackers might gain access through legitimate user sessions. The vulnerability creates a privilege escalation path since code execution occurs within the context of the X server process, which typically runs with elevated privileges and has extensive system access capabilities. This aligns with CWE-121, heap-based buffer overflow conditions, and represents a classic example of how improper integer handling can lead to memory corruption vulnerabilities.
Attackers exploiting this vulnerability can leverage it through the X11 protocol by sending specially crafted font requests that trigger the vulnerable code path during bitmap scaling operations. The attack vector specifically targets the X server's font processing capabilities, making it difficult to detect and prevent without proper input validation. This vulnerability directly maps to ATT&CK technique T1068, 'Exploitation for Privilege Escalation', as exploitation can lead to elevated privileges within the X server environment. The memory corruption can be used to overwrite return addresses or function pointers, enabling attackers to redirect execution flow and potentially execute arbitrary code with the privileges of the X server process.
Mitigation strategies include immediate patching of libXfont2 to version 2.0.8 or later where the buffer overflow has been corrected through proper bounds checking and integer overflow protection mechanisms. System administrators should also implement network segmentation to limit access to X servers from untrusted networks, as well as employ input validation controls that can detect and reject malformed font data before it reaches the vulnerable code path. Additionally, privilege separation techniques such as running X servers with reduced privileges or implementing mandatory access controls can help limit the potential impact of successful exploitation attempts. Organizations should also consider monitoring X server processes for unusual memory allocation patterns or unexpected execution flows that might indicate exploitation attempts against this vulnerability.