CVE-2026-56001

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.

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.

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.

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.