CVE-2026-112406: Pillow: Infinite loop in PdfParser.rewrite_pages on page tree Parent cycle

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