PYSEC-2026-3496

Summary

src/libImaging/Jpeg2KDecode.c:853 accumulates total_component_width across every tile in a JPEG2000 image instead of recomputing it per tile. That accumulated value is then used in the tile_bytes calculation at src/libImaging/Jpeg2KDecode.c:868, which can make the decoder grow state->buffer via realloc at src/libImaging/Jpeg2KDecode.c:876 up to roughly one full image's decompressed size even when each tile is small. A crafted tiled JPEG2000 file can therefore force substantially higher transient memory usage and trigger out-of-memory failures during decoding. Based on current evidence, the supported impact is denial of service, not memory corruption.

Details

• Location: src/libImaging/Jpeg2KDecode.c:853
• Root cause: total_component_width is initialized only once before the tile loop and keeps growing across tiles. It is then used to derive tile_bytes, so later tiles are treated as if they had the combined component width of all earlier tiles.
• Dangerous operation: tile_bytes is promoted into tile_info.data_size, then state->buffer is grown with realloc at src/libImaging/Jpeg2KDecode.c:876.
• Reachability: any attacker-controlled JPEG2000 image with many tiles reaches this path during normal Image.open(...).load() decoding.

PoC

The attached helper script and testcase were used: exercisej2ktile_realloc.zip

Generate the testcase:

pythonexercise_j2k_tile_realloc.py make poc_3664_rgba_tile1832.jp2 \
  --size 3664 --tile 1832

Expected geometry from the helper:

• image size: 3664 x 3664
• mode: RGBA
• tile size: 1832 x 1832 (2x2 tiles)
• image_bytes=53699584
• uncapped RSS observed:
  • vulnerable build: maxrss_kb=180264
  • fixed comparison build: maxrss_kb=138404

Load it with the current vulnerable build:

python exercise_j2k_tile_realloc.py load poc_3664_rgba_tile1832.jp2

Load it again under a 160 MB address-space cap:

python exercise_j2k_tile_realloc.py load poc_3664_rgba_tile1832.jp2 --limit-mb 160

Impact

Conservative impact: denial of service through memory exhaustion during JPEG2000 decoding.