Pillow JPEG2000 tiled decode retains a growing scratch buffer and can be used for denial of service
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: exercise_j2k_tile_realloc.zip Generate the testcase: ``bash 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: `bash python exercise_j2k_tile_realloc.py load poc_3664_rgba_tile1832.jp2 ` Load it again under a 160 MB address-space cap: `bash 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.