CVE-2026-59198: Pillow TGA RLE encoder can serialize up to ~57 KB of adjacent heap data into generated images

Pillow’s TGA RLE encoder reads past its row buffer when saving a mode "1" image. Adjacent process heap bytes can be copied into the generated TGA file.

The bug is reachable through the public save API:

im.save(out, format="TGA", compression="tga_rle")

Older affected Pillow versions use the equivalent public option rle=True.

For mode "1", Pillow allocates a packed row buffer of ceil(width / 8) bytes, but ImagingTgaRleEncode() treats the row as one full byte per pixel.

The maximum valid TGA width is 65535. At that width:

allocated packed row buffer: 8192 bytes
encoder byte-offset walk:     65535 bytes
maximum OOB window per row:   57343 bytes

On non-ASAN Pillow 12.2.0, the public-only maximum-width PoC below serialized 57297 bytes from distinct out-of-bounds source offsets into one returned TGA, covering 99.92% of the maximum adjacent heap window. No heap grooming, ctypes, private API, or malformed input file was used. The disclosure is emitted across many TGA packet payload copies of at most 128 bytes each, not one large memcpy().