{"id":"GHSA-8423-8fgw-73vq","title":"tornado: multipart split() creates huge temp list before max_parts check -> memory amplification DoS (httputil.py:34)","summary":"tornado: multipart split() creates huge temp list before max_parts check -> memory amplification DoS (httputil.py:34)","severity":"medium","vendor":"tornado","product":"tornado","ecosystem":"pip","affected":["tornado < 6.5.8"],"patched":["tornado 6.5.8"],"published":"2026-09-01","updated":"2026-09-10","sourceUpdated":"2026-09-10T03:51:15.579433219Z","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/GHSA-8423-8fgw-73vq","references":[{"url":"https://github.com/tornadoweb/tornado/security/advisories/GHSA-8423-8fgw-73vq"},{"url":"https://github.com/tornadoweb/tornado/pull/3704"},{"url":"https://github.com/tornadoweb/tornado/commit/de85b3f87446e323e881bbaa3d5a74f4b76e5f05"},{"url":"https://gist.github.com/afldl/649861f25d39b53b7edbe0298e171617"},{"url":"https://github.com/tornadoweb/tornado"},{"url":"https://github.com/tornadoweb/tornado/releases/tag/v6.5.8"},{"url":"https://github.com/advisories/GHSA-8423-8fgw-73vq"}],"tags":["osv","pip","ghsa"],"cwe":["CWE-770"],"ingestedAt":"2026-09-01T20:32:02.179Z","slug":"GHSA-8423-8fgw-73vq","body":"## Overview\n\n## Description\n\n### Summary\n\n`parse_multipart_form_data` (httputil.py:34) calls\n`data.split(b\"--\"+boundary+b\"\\r\\n\")` **before** the `max_parts` check (:35).\nA 600KB body with 100k parts creates a 100k-element transient list first,\nthen rejects   transient memory amplification (each split element is a copy).\nPre-auth HTTP DoS.\n\n### Root cause\n\n```python\nparts = data[:final_boundary_index].split(b\"--\" + boundary + b\"\\r\\n\")  # :34  huge list first\nif len(parts) > config.max_parts:                                       # :35  check after\n    raise HTTPInputError(\"multipart/form-data has too many parts\")\n```\n\n### PoC\n\ngist: https://gist.github.com/afldl/649861f25d39b53b7edbe0298e171617\n`poc.py` + `output.txt` (100k parts from 600KB   transient list).\n\n### Fix\n\nCount separators without materializing the list (e.g. `data.count(b\"--\"+boundary)` first).\n\n### Credit\n\nReported by afldl, 2026-07.\n\n## Affected packages\n\n- `tornado < 6.5.8`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `tornado 6.5.8`","depth":"sunlit","depthScore":28,"depthScoreParts":{"impact":27.5,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}