{"id":"GHSA-654x-cwwv-5j9c","summary":"Pillow: Arrow export leaks stale heap memory when PILLOW_ALIGNMENT \u003e 1","details":"### Summary\n\nWhen [`PILLOW_ALIGNMENT`](https://pillow.readthedocs.io/en/stable/reference/block_allocator.html#memory-pools) is set above 1, exporting an image to Arrow returns uninitialized heap memory mixed in with the pixel data. That memory can include pixels from images the process freed earlier. \n\n### Details\n\nWith an alignment above 1, the default allocator pads each row up to `aligned_linesize` and starts the first row at an aligned offset into the block. It then exports `im-\u003eblocks[0].ptr` as `xsize * ysize` tightly packed pixels, so exported data includes the padding bytes, and can include a few bytes from before the first row.\n\nTested vulnerable on 11.2.1, 12.3.0, 4c1ed27a5.\n\n### PoC\n\nWhether things leak may depend on allocator configuration. This does show a leak on my Mac.\n\n```python\nimport os\n\nos.environ[\"PILLOW_ALIGNMENT\"] = \"64\"\n\nfrom PIL import Image\n\n# Image A: 64 px wide, so its rows fill the whole aligned stride\na = Image.new(\"L\", (64, 40), 0x41).transpose(Image.Transpose.FLIP_LEFT_RIGHT)\ndel a\n\n# Image B: 61 px wide, so each row has 3 bytes of padding; its pixels are all 0\nb = Image.new(\"L\", (61, 40), 0).transpose(Image.Transpose.FLIP_LEFT_RIGHT)\n\ntry:\n    exported = Image.fromarrow(b, b.mode, b.size).tobytes()\nexcept ValueError as e:\n    print(\"Not vulnerable:\", e)\nelse:\n    if leaked := len(exported) - exported.count(0):\n        raise RuntimeError(f\"{leaked} of {len(exported)} exported bytes are not B's pixels\")\n```\n\n### Impact\n\nInformation disclosure for applications that set `PILLOW_ALIGNMENT` and export images to Arrow. Likely any image whose row length isn't a multiple of the alignment will do. In a service handling several users' images, one user's exported data can contain pixels from another user's image, or other stale heap contents.","aliases":["CVE-2026-114501"],"modified":"2026-10-23T22:00:03.744757414Z","published":"2026-10-23T21:53:21Z","database_specific":{"cwe_ids":["CWE-226","CWE-908"],"severity":"MODERATE","github_reviewed":true,"github_reviewed_at":"2026-10-23T21:53:21Z","nvd_published_at":"2026-10-21T22:16:49Z"},"references":[{"type":"WEB","url":"https://github.com/python-pillow/Pillow/security/advisories/GHSA-654x-cwwv-5j9c"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-114501"},{"type":"WEB","url":"https://github.com/python-pillow/Pillow/pull/10172"},{"type":"WEB","url":"https://github.com/python-pillow/Pillow/commit/19f64946eb247b973dd4560be7dea3bf979d2e6a"},{"type":"PACKAGE","url":"https://github.com/python-pillow/Pillow"},{"type":"WEB","url":"https://github.com/python-pillow/Pillow/releases/tag/13.0.0"}],"affected":[{"package":{"name":"pillow","ecosystem":"PyPI","purl":"pkg:pypi/pillow"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"11.2.1"},{"fixed":"13.0.0"}]}],"versions":["11.2.1","11.3.0","12.0.0","12.1.0","12.1.1","12.2.0","12.3.0"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/10/GHSA-654x-cwwv-5j9c/GHSA-654x-cwwv-5j9c.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N"}]}