CVE-2026-96640

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
• Likely affected range: >= 5.1.0
• Patched versions: none known

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

4. linearize_page_tree() recursively follows every /Kids entry and extends the page list. It does not track page-tree node identity, duplicate references, or active traversal references:

   def linearize_page_tree(
       self, node: PdfDict | None = None
   ) -> list[IndirectReference]:
       page_node = node if node is not None else self.page_tree_root
      …
   

Python is an interpreted, interactive, object-oriented programming language, which includes modules, classes, exceptions, very high level dynamic data types and dynamic typing. Python supports interfaces to many system calls and libraries, as well as to various windowing systems.

Security Fix(es):

• pillow: Pillow: Denial of Service via unbounded recursion in PDF parser (CVE-2026-112391)

• pillow: pillow: Denial of service via double free during image transformation (CVE-2026-112402)

• pillow: pillow: Denial of Service via zero-size images in rank filters (CVE-2026-112392)

• pillow: pillow: Denial of Service via out-of-bounds memory access on zero-width images (CVE-2026-112397)

• pillow: Pillow: Denial of Service via null pointer dereference in Image.merge() (CVE-2026-112405)

• pillow: pillow: Denial of Service via crafted multi-frame image conversion (CVE-2026-112894)

• pillow: pillow: Denial of service via crafted PDF during append operations (CVE-2026-112895)

• pillow: pillow: Denial of Service via infinite loop during PDF page rewriting (CVE-2026-112406)

• pillow: pillow: Denial of Service via duplicate page references in PDF files (CVE-2026-96640)

• pillow: pillow: Denial of Service via unbounded memory expansion in MSP image decoder (CVE-2026-112892)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

Pillow is a Python imaging library. From 5.1.0 until 13.0.0, PdfParser.linearize_page_tree() in src/PIL/PdfParser.py treats a PDF /Pages graph as a tree without tracking duplicate indirect references. When Image.save(..., format="PDF", append=True) processes an attacker-controlled existing PDF whose /Kids arrays repeatedly reference the same child /Pages node, Pillow expands the shared subtree as distinct pages. A compact malformed PDF can therefore create exponentially many page references and consume disproportionate CPU before the append completes. This issue is fixed in version 13.0.0.

Python is an interpreted, interactive, object-oriented programming language, which includes modules, classes, exceptions, very high level dynamic data types and dynamic typing. Python supports interfaces to many system calls and libraries, as well as to various windowing systems.

Security Fix(es):

• pillow: Pillow: Denial of Service via unbounded recursion in PDF parser (CVE-2026-112391)
• pillow: pillow: Denial of service via double free during image transformation (CVE-2026-112402)
• pillow: pillow: Denial of Service via zero-size images in rank filters (CVE-2026-112392)
• pillow: pillow: Denial of Service via out-of-bounds memory access on zero-width images (CVE-2026-112397)
• pillow: Pillow: Denial of Service via null pointer dereference in Image.merge() (CVE-2026-112405)
• pillow: pillow: Denial of Service via crafted multi-frame image conversion (CVE-2026-112894)
• pillow: pillow: Denial of service via crafted PDF during append operations (CVE-2026-112895)
• pillow: pillow: Denial of Service via infinite loop during PDF page rewriting (CVE-2026-112406)
• pillow: pillow: Denial of Service via duplicate page references in PDF files (CVE-2026-96640)
• pillow: pillow: Denial of Service via unbounded memory expansion in MSP image decoder (CVE-2026-112892)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

Pillow is a Python imaging library. From 5.1.0 until 13.0.0, PdfParser.linearize_page_tree() in src/PIL/PdfParser.py treats a PDF /Pages graph as a tree without tracking duplicate indirect references. When Image.save(..., format="PDF", append=True) processes an attacker-controlled existing PDF whose /Kids arrays repeatedly reference the same child /Pages node, Pillow expands the shared subtree as distinct pages. A compact malformed PDF can therefore create exponentially many page references and consume disproportionate CPU before the append completes. This issue is fixed in version 13.0.0.

TuxCare rebuilt pillow to address 11 CVEs. Fixed in 11.3.0.post7+tuxcare.

TuxCare rebuilt pillow to address 11 CVEs. Fixed in 11.2.1.post9+tuxcare.

TuxCare rebuilt pillow to address 11 CVEs. Fixed in 10.4.0.post11+tuxcare.

TuxCare rebuilt pillow to address 11 CVEs. Fixed in 9.5.0.post9+tuxcare.

TuxCare rebuilt pillow to address 10 CVEs. Fixed in 9.4.0.post10+tuxcare.

TuxCare rebuilt pillow to address 9 CVEs. Fixed in 8.4.0.post9+tuxcare.

These are all security issues fixed in the python313-Pillow-13.0.0-2.1 package on the GA media of openSUSE Tumbleweed.

Root has patched CVE-2026-96640 in the pillow package for Root:PyPI. Multiple fixed versions available.

This update for python-Pillow fixes the following issues:

• CVE-2026-96636: decompression-bomb protection bypass via multi-frame seek() in MPO and DCX (bsc#1286802).
• CVE-2026-96637: decompression bomb check bypass in SpiderImagePlugin.seek() leads to unbounded allocation DoS (bsc#1286801).
• CVE-2026-96640: PDF append accepts malformed shared page-tree DAG, which can cause resource exhaustion (bsc#1286798).
• CVE-2026-96641: reachable undefined behavior in native integer operations can lead to denial of service or logic integrity compromise (bsc#1286797).
• CVE-2026-96642: TarIO negative size field causes infinite CPU-spin loop (bsc#1286796).
• CVE-2026-112390: Denial of Service via infinite loop during crafted MIC image parsing (bsc#1286256).
• CVE-2026-112391: Denial of Service via unbounded recursion in PDF parser (bsc#1286273).
• CVE-2026-112392: Denial of Service via zero-size images in rank filters (bsc#1286287).
• CVE-2026-112393: heap buffer overflow and stale-heap disclosure in _tiffWriteProc when saving compressed TIFF data (bsc#1286325).
• CVE-2026-112394: P;2L and P;4L decoders may read 1 to 3 bytes past the passed buffer size (bsc#1286311).
• CVE-2026-112397: out-of-bounds access in _imagingmorph on a zero-width image (bsc#1286314).
• CVE-2026-112398: heap buffer overflow in FPX JPEG decoding can lead to code execution (bsc#1286406).
• CVE-2026-112399: Denial of service via crafted APNG image (bsc#1286368).
• CVE-2026-112400: Denial of Service via memory exhaustion from crafted GIF image (bsc#1286370).
• CVE-2026-112401: Denial of Service via crafted GIF comments (bsc#1286372).
• CVE-2026-112402: Denial of service via double free during image transformation (bsc#1286381).
• CVE-2026-112403: heap buffer overflow in ImagingJpegDecode allows for code execution (bsc#1286405).
• CVE-2026-112405: NULL pointer dereference in Image.merge via an unvalidated band 0 (bsc#1286404).
• CVE-2026-112406: infinite loop in PdfParser.rewrite_pages on page tree Parent cycle (bsc#1286403).
• CVE-2026-112891: quadratic GIF comment copying during save (bsc#1286402).
• CVE-2026-112892: MSP RLE decoder expands a tiny file into unbounded memory (bsc#1286401).
• CVE-2026-112894: Denial of Service via crafted multi-frame image conversion (bsc#1286408).
• CVE-2026-112895: Denial of service via crafted PDF during append operations (bsc#1286411).
• CVE-2026-112896: TIFF encoder ColorMap plane-size mismatch causes a heap out-of-bounds read (bsc#1286400).
• CVE-2026-112897: TiffDecode returns uninitialized memory as pixels for short JPEG strips (bsc#1286398).
• CVE-2026-112899: EPS load(scale) lets Ghostscript render past the decompression bomb limit (bsc#1286489).
• CVE-2026-112900: argument Injection in Ghostscript call can lead to remote code execution (bsc#1286397).
• CVE-2026-112901: ReDoS in XbmImagePlugin can be triggered by 512-byte file (bsc#1286396).
• CVE-2026-112902: memory disclosure and segfault in libtiff encoder via crafted TIFF tags (bsc#1286804).