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PYSEC-2026-3496

HighCVSS 8.7 / 10
Published Jul 23, 2026·Last modified Jul 23, 2026
Affected Components(0)

No affected components available

Description

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:

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.

Risk Scores
Base Score
8.7

The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability.

Threat Intelligence
6.6

Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.

EPSS
0.39%

The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.

Exploit
Not available

We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.

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High severity
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