{"id":"CVE-2026-96637","aliases":["GHSA-g842-9rc4-69pw"],"url":"https://o3.security/vulnerability/CVE-2026-96637","summary":"Pillow: Decompression bomb check bypass in SpiderImagePlugin.seek() leads to unbounded allocation DoS","details":"## Summary\nSpiderImagePlugin's seek() bypasses the decompression bomb check, allowing a tiny (<1KB) crafted SPIDER stack file to trigger a multi-gigabyte allocation on im.load().\n\n## Affected versions\nAll versions up to and including current HEAD (v13.0.0.dev0, commit e7a7ea84)\n\n## Root cause\nImage.open() runs Image._decompression_bomb_check(im.size) exactly once, in _open_core (src/PIL/Image.py:3668), right after the initial _open(). \n\nSPIDER stack files are multi-frame: SpiderImageFile.seek(frame) (SpiderImagePlugin.py:179-190) computes an attacker-influenced stkoffset and calls self._open() AGAIN directly (line 190) — NOT through Image.open(). _open() re-parses a fresh 27-float header at that offset and reassigns:\n\n    self._size = int(h[12]), int(h[2])      # SpiderImagePlugin.py:131\n\nstraight from unvalidated header floats (isSpiderHeader only checks internal labrec*lenbyt==labbyt consistency, never bounds nsam/nrow). No bomb check runs on this new size.\n\nOn the subsequent load(), ImageFile.load_prepare() (ImageFile.py:436-437) executes `self.im = Image.core.new(self.mode, self.size)` with the unchecked, attacker-chosen size. Mode is always \"F\" (4 bytes/pixel), so e.g. 40000x40000 = 6.4 GB.\n\n## Why this is a real bug\nSibling multi-frame plugins DO re-check on their alternate paths:\n- GifImagePlugin.py:286 - Image._decompression_bomb_check(self._size) (in seek)\n- TiffImagePlugin.py:1309 - Image._decompression_bomb_check(self._tile_size)\n- IcnsImagePlugin.py:121/138, IcoImagePlugin.py:236/240 (sub-image selection)\n\nSpiderImagePlugin has NO bomb check anywhere. This is the same class of bug just fixed for GdImageFile (GHSA-phj9-mv4w-65pm) and the FontFile classes (GHSA-5x94/8v84/45hq): an allocation reached on a non-Image.open() path sized by an attacker header field, independent of the one-time pixel bomb check.\n\n## Reproduction\nPython 3.13, Pillow HEAD e7a7ea84\n\nAttached: pillow_poc_spider_bomb.py\n\nSets Image.MAX_IMAGE_PIXELS=5000, then:\n- CONTROL: Image.open(standalone 100x100 SPIDER) -> DecompressionBombWarning fires correctly.\n- BYPASS: Image.open(stack, frame0=1x1) passes; im.seek(1) sets size (100,100) with NO DecompressionBombWarning/Error -> the check is bypassed.\n\nReal-world impact: a large frame-1 nsam/nrow (e.g. 40000x40000) causes im.load() to allocate ~6.4 GB (mode F) with the bomb check fully bypassed, from a file under 1 KB.\n\n## Suggested fix\nCall Image._decompression_bomb_check(self.size) at the end of SpiderImageFile._open() (or in seek() after _open()), mirroring GifImagePlugin / TiffImagePlugin. Optionally validate nsam/nrow are positive and sane in _open().\n\nCredit: Yazan Balawneh - CyStack Security Team \n[pillow_poc_spider_bomb.py](https://github.com/user-attachments/files/30244916/pillow_poc_spider_bomb.py)","published":"2026-10-21T21:01:20.249Z","modified":"2026-10-23T02:46:08.322101919Z","cvss":null,"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"PyPI","name":"pillow","fixedVersion":"13.0.0"}],"fix":{"url":"https://github.com/python-pillow/Pillow/commit/7cb0e02389d8aac2a60896a490f572db0b07b841","label":"python-pillow/Pillow@7cb0e02"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/96xxx/CVE-2026-96637.json"},{"type":"FIX","url":"https://github.com/python-pillow/Pillow/commit/7cb0e02389d8aac2a60896a490f572db0b07b841"},{"type":"FIX","url":"https://github.com/python-pillow/Pillow/pull/10076"},{"type":"WEB","url":"https://github.com/python-pillow/Pillow/releases/tag/13.0.0"},{"type":"ADVISORY","url":"https://github.com/python-pillow/Pillow/security/advisories/GHSA-g842-9rc4-69pw"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-96637"},{"type":"PACKAGE","url":"https://github.com/python-pillow/Pillow"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-23T02:46:08.322101919Z"}}