CVE-2026-44950
CriticalCVSS 9.0Summary
In libXfont2's font-server client fs_read_glyphs() (src/fc/fserve.c), each glyph's bitmap is copied into a single buffer, and existing checks only validate that the source slice (position, length) lies within the source bitmap buffer. They do not check whether the running destination cursor has exceeded the allocation. A malicious font server can send overlapping source offsets (e.g. 1000 glyphs each referencing {position:0, length:64} with nbytes=64), resulting in 64000 bytes written into a 64-byte destination buffer — a heap buffer overflow with attacker-controlled content.
Risk Assessment
A malicious font server can trigger a heap buffer overflow, potentially leading to code execution or a crash of the client process. The risk affects systems using libXfont2.
Recommendation
Update libXfont2 to a version containing the fix. Limit trust in remote font servers and use only trusted font sources.
Other vulnerabilities in libXfont2
See all- CVE-2026-59679Critical
In libXfont2's font-server client fs_read_glyphs() (src/fc/fserve.c), the per-character encoding[] array is indexed using num_chars from the FS_QueryXBitmaps16 reply, but that array was allocated with a size derived from num_extents in the separate FS_QueryXExtents16 reply. The two CARD32 fields are never cross-checked. A malicious or compromised font server can send a small num_extents (e.g. 1) followed by a large num_chars (e.g. 100000), causing attacker-controlled out-of-bounds heap reads and writes.
- CVE-2026-56003High
A heap buffer overflow due to missing size checking in the property buffer when parsing PCF files in libXfont2 ComputeScaledProperties() before version 2.0.8 allows authenticated X clients to execute code within the X server.
- CVE-2026-56002High
A heap buffer overflow in pcfReadFont() due to missing glyph bounds checking in libXfont2 before 2.0.8 allows authenticated X clients to execute code within the X server.
- CVE-2026-56001High
A heap buffer overflow in BitmapScaleBitmaps in libXfont2 before 2.0.8 due to an overflowing 32-bit size could be used by attackers able to access the X Server to execute code within the X server context.
Original NVD description (English source)
fs_read_glyphs() in the libXfont2 font-server client (src/fc/fserve.c) copies each glyph's bitmap into a single buffer. Existing checks validates only that the source slice (position, length) lies within the source bitmap buffer. It does not check whether the running destination cursor has exceeded the allocation. A malicious font server can send overlapping source offsets -- for example 1000 glyphs each referencing {position:0, length:64} with nbytes=64. Each individual source range passes the existing validation, but the cumulative writes total 64000 bytes into a 64-byte destination buffer. This is a heap buffer overflow with attacker-controlled content.