CVE-2026-112896: Pillow: TIFF encoder ColorMap plane-size mismatch causes a heap out-of-bounds read

A small attacker-supplied TIFF can be opened and loaded normally, then reach an out-of-bounds read when an application saves it back to TIFF with retained metadata. The fixed reproducer is a 1×1, 16-bit grayscale, PackBits-compressed TIFF containing tag 320 (ColorMap) with 768 SHORT values: three zero-filled planes of 256 entries each. No Python object is forged and no metadata is injected after loading.

Pillow’s TIFF encoder allocates 768 × 2 = 1536 bytes, then passes three plane pointers separated by 256 SHORT entries to libtiff. With BitsPerSample = 16, libtiff instead expects 65536 SHORT values per plane and attempts a 131072-byte read from a plane pointer. Existing ASAN observations identify a heap-buffer-overflow READ crossing the 1536-byte allocation made in PyImaging_LibTiffEncoderNew.

The malformed ColorMap need not be a valid palette for this grayscale image. The issue is that accepted metadata is forwarded on save without reconciling the fixed plane extent with the effective sample depth. Opening/loading alone is not the demonstrated fault; the encoder path is required.