Python Pillow EUVD-2026-104648

Exploitation requires that the target application use Pillow to append an image to an attacker-supplied PDF, which invokes PdfParser.rewrite_pages(); the attacker must craft a PDF whose page-tree nodes form a cycle via mutual Parent references, which the pre-13.0.0 code follows without cycle detection. Limiting factors: the vulnerable code path is only reached by applications that specifically append to/rewrite externally supplied PDFs, so deployments that merely open or convert images are not exposed; impact is confined to availability (CPU exhaustion of the processing worker) with no data exposure or code execution. Fixed in Pillow 13.0.0.

This is a denial-of-service issue (CWE-835 uncontrolled infinite loop) with no confidentiality or integrity impact - the CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H) correctly reflects availability-only impact scored 8.7 (High). The network vector and PR:N assume a scenario where an application accepts attacker-supplied PDFs over the network and appends an image to them, so real-world exploitability is entirely conditional on the consuming application exposing Pillow's PDF-append path to untrusted input - many Pillow deployments never touch PdfParser.rewrite_pages() at all, which materially lowers the practical blast radius versus the raw score. No EPSS score was provided in the input, so exploitation probability cannot be quantified. KEV status is not listed, so there is no confirmed active exploitation; no public exploit identified at time of analysis, though the fixing commit (b3ebf20c) ships a regression test (test_write_catalog_loop) that demonstrates the cyclic page-tree condition, making reproduction trivial for a motivated attacker. Net: a genuine but bounded availability risk - high-CVSS is justified for exposed PDF-processing workers, but for the large population of Pillow users who do not rewrite/append PDFs from untrusted sources, real priority is lower.

An attacker uploads a malformed PDF whose page-tree nodes reference each other cyclically through Parent relationships to a service that uses Pillow to append an image to the uploaded PDF. When PdfParser.rewrite_pages() walks the page tree it never detects the cycle and loops forever, pinning the worker at 100% CPU and denying service to other requests. No authentication or user interaction is required by the CVSS vector (PR:N/UI:N), and no public exploit is identified, but the fix commit includes a test that reproduces the loop.