{"id":"CVE-2026-96638","aliases":["GHSA-24rc-8hj2-v98c"],"url":"https://o3.security/vulnerability/CVE-2026-96638","summary":"Pillow: BMP RLE decoder memory-amplification DoS via unbounded delta escape / absolute mode","details":"## Summary\nBmpRleDecoder can be forced to allocate gigabytes of memory from a kilobyte-sized crafted BMP file via unbounded growth in the delta escape and absolute mode branches.\n\n## Affected versions\nAll versions up to and including current HEAD (v13.0.0.dev0, commit e7a7ea84)\n\n## Root cause\nBmpRleDecoder.decode() (BmpImagePlugin.py:327-395) builds a Python bytearray `data` with dest_length = xsize*ysize, looping `while len(data) < dest_length`. The loop condition is only checked at the TOP; individual branches append attacker-controlled amounts UNBOUNDED by dest_length:\n\n- delta escape (line 370): data += b\"\\x00\" * (right + up * self.state.xsize) with right,up in 0..255 -> up to ~255*xsize bytes in ONE iteration.\n- absolute mode (line 385): data += bytes_read (up to 255).\n\nBecause a single delta can push len(data) to ~256 * (row size), a tiny RLE stream combined with a large-but-legal xsize (xsize*ysize just under MAX_IMAGE_PIXELS, ~89M) yields a multi-GB `data` bytearray (then copied again by bytes(data) and set_as_raw). No heap OOB (bytearray/set_as_raw are bounds-safe), but a severe memory-amplification DoS: the attacker file stays tiny.\n\n## Reproduction\nPython 3.13, Pillow HEAD e7a7ea84\n\nAttached: pillow_poc_bmp_rle_dos.py\n\nA 1084-byte BMP (BI_RLE8, width=3,000,000, height=1 -> 3M px, passes bomb check) with one delta escape (00 02 00 150) produces ~429 MB `data`; measured peak RSS 911 MB. Scaling width toward the bomb limit and up=255 produces tens of GB from a ~1 KB file.\n\n## Suggested fix\nCap growth at dest_length in the delta and absolute branches, e.g. clamp the delta fill to (dest_length - len(data)) and stop appending once len(data) >= dest_length; reject deltas that would move the cursor beyond the image, matching the existing per-row clamp at line 341-343.\n\nCredit: Yazan Balawneh - CyStack Security Team (GitHub: sonicnew)\n\n[pillow_poc_bmp_rle_dos.py](https://github.com/user-attachments/files/30244974/pillow_poc_bmp_rle_dos.py)","published":"2026-10-21T20:47:21.767Z","modified":"2026-10-24T02:50:15.644481535Z","cvss":{"score":6.5,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H"},"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"PyPI","name":"pillow","fixedVersion":"13.0.0"}],"fix":{"url":"https://github.com/python-pillow/Pillow/commit/e7794835e323ff2691d9b3a974cade1a44301b9a","label":"python-pillow/Pillow@e779483"},"references":[{"type":"WEB","url":"https://github.com/python-pillow/Pillow/releases/tag/13.0.0"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/96xxx/CVE-2026-96638.json"},{"type":"ADVISORY","url":"https://github.com/python-pillow/Pillow/security/advisories/GHSA-24rc-8hj2-v98c"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-96638"},{"type":"FIX","url":"https://github.com/python-pillow/Pillow/commit/e7794835e323ff2691d9b3a974cade1a44301b9a"},{"type":"FIX","url":"https://github.com/python-pillow/Pillow/pull/10015"},{"type":"PACKAGE","url":"https://github.com/python-pillow/Pillow"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-24T02:50:15.644481535Z"}}