CVE-2026-112896

## Summary 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. ## Impact and prerequisites An application must accept the input, retain the relevant TIFF metadata, and invoke the libtiff-backed TIFF save path. The observed consequence is an out-of-bounds read and ASAN termination. Process-memory exposure during encoding is a potential consequence, **not demonstrated by extracting arbitrary heap contents in this report**. No arbitrary read primitive, full memory-disclosure chain, code execution, address-space bypass, or cross-process effect is claimed. The 131072-byte attempted copy is not the number of bytes proven disclosed. This report does not run a synthetic-canary serializer or collect an unsanitized encoded output. ## Tested environment and limits of the evidence The existing observations were recorded on August 30, 2026, with **CPython 3.11.8 on macOS ARM64**, using the unmodified official **Pillow 12.3.0*