PYSEC-2026-4209

Docling: METS-GBS archive member limit enforced after full member enumeration (memory exhaustion during format detection)

### Summary When docling detects the input format of a gzip-compressed tar archive (METS-GBS), and later when the METS-GBS backend opens it, it calls `tarfile.TarFile.getmembers()`. That builds the full member list in memory before the `max_member_count` limit is checked. A small archive with a very large number of empty members therefore makes docling allocate memory in proportion to the member count, and the limit has no effect. ### Details `docling/datamodel/document.py` (format detection) and `docling/backend/mets_gbs_backend.py` both iterate `tar.getmembers()` and count members inside the loop. `getmembers()` reads every header up front. Measured on Python 3.12: an archive of 1,000,000 empty members compresses to about 6.2 MB and makes `getmembers()` hold about 408 MB (about 66 times the input size). Format detection runs for any `application/gzip` input before `allowed_formats` is applied, so the allocation happens even when METS-GBS is not an allowed format. This check was introduced by the fix for CVE-2026-44018 (GHSA-r3xg-rg9j-67fv) in 2.91.0. The eager enumeration it relies on has been there since METS-GBS detection was added in 2.45.0. ### Impact Memory exhaustion of the converting process, proportional to the size of the input archive. Confidentiality and integrity are not affected. ### Patches Fixed in docling 2.131.0 by [#4412](https://github.com/docling-project/docling/pull/4412). Format detection and the METS-GBS backend now read archive members one at a time and stop as soon as `max_member_count` is exceeded, including when looking up page files. ### Workarounds Upgrade to 2.131.0. For older versions: Reject gzip or tar inputs before they reach docling when METS-GBS support is not needed, and run conversions of untrusted input with memory limits.