{"id":"CVE-2026-96644","aliases":["GHSA-9v9v-9qvf-h2hj"],"url":"https://o3.security/vulnerability/CVE-2026-96644","summary":"Pillow: Heap over-read in WMF/EMF rendering via 16-bit dimension truncation (Windows builds)","details":"### Summary\nOn Windows builds, Pillow stores EMF and WMF image dimensions without a range check and passes them to the native `drawwmf` handler. That code sizes its allocation from dimensions truncated into 16-bit header fields but copies the result using the full 32-bit dimensions. A crafted metafile that is wide and short passes the decompression-bomb pixel check yet causes a heap over-read.\n\n### Details\nThe EMF branch computes the size straight from signed 32-bit header fields and stores it with no positivity or range check:\n\n- `src/PIL/WmfImagePlugin.py:121-129` reads the bounding box; `:148` stores it; the size reaches the native handler through `src/PIL/WmfImagePlugin.py:56`.\n\nIn the native handler the DIB header fields `bcWidth` and `bcHeight` are 16-bit and truncate the allocation, while the copy length uses the untruncated ints:\n\n- `src/display.c:793-794` assign the 16-bit fields\n- `src/display.c:800` sizes the allocation from them\n- `src/display.c:827` copies `height * ((width * 3 + 3) & -4)` bytes using the full values\n\nThe whole handler is compiled only on Windows (`#ifdef _WIN32`, `src/display.c:33-841`).\n\n### PoC\n```python\nimport struct\nfrom PIL import Image\n\ndef L(v):\n    return struct.pack(\"<i\", v)\n\n# Minimal EMF header. Bounding box is 70000 x 1, so the pixel count (70000) passes the\n# decompression-bomb check, but the width truncates to 16 bits for the allocation while\n# the copy uses 70000.\nemf  = b\"\\x01\\x00\\x00\\x00\" + L(88)          # record type + size\nemf += L(0) + L(0) + L(70000) + L(1)        # rclBounds: x0, y0, x1, y1\nemf += L(0) + L(0) + L(100) + L(100)        # rclFrame (non-zero, used for the dpi calc)\nemf += b\" EMF\"                              # signature at offset 40\nemf += bytes(44)\nopen(\"poc.emf\", \"wb\").write(emf)\n\nim = Image.open(\"poc.emf\")\nprint(\"size:\", im.size, \"pixels:\", im.size[0] * im.size[1])\nim.load()                                   # over-read happens here on a Windows build\n```\n\nOn Linux the first part is observable: the 88-byte file opens, `size` is `(70000, 1)`, and the bomb check passes. The native over-read only exists on Windows because `Image.core.drawwmf` is compiled out elsewhere (on Linux `im.load()` raises `OSError` instead).\n\n### Impact\nCWE-125, out-of-bounds read in a C extension on Windows builds. A crafted EMF or WMF that passes the pixel check can read past a heap buffer, leaking memory or crashing the process.","published":"2026-10-21T20:54:43.537Z","modified":"2026-10-24T07:25:10.376484060Z","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/4cf46bee338304c5c8968fd7f132ba0d40c428b7","label":"python-pillow/Pillow@4cf46be"},"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-2026-96644.json"},{"type":"ADVISORY","url":"https://github.com/python-pillow/Pillow/security/advisories/GHSA-9v9v-9qvf-h2hj"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-96644"},{"type":"FIX","url":"https://github.com/python-pillow/Pillow/commit/4cf46bee338304c5c8968fd7f132ba0d40c428b7"},{"type":"FIX","url":"https://github.com/python-pillow/Pillow/pull/10120"},{"type":"PACKAGE","url":"https://github.com/python-pillow/Pillow"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-24T07:25:10.376484060Z"}}