{"id":"CVE-2025-67726","aliases":["PYSEC-2025-267","GHSA-jhmp-mqwm-3gq8"],"title":"Tornado is a Python web framework and asynchronous networking library. Versions 6.5.2 and below use an inefficient algorithm when parsing…","summary":"Tornado is a Python web framework and asynchronous networking library. Versions 6.5.2 and below use an inefficient algorithm when parsing parameters for HTTP header values, potentially causing a DoS. The _parseparam function in httputil.…","severity":"high","cvss":7.5,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","vendor":"tornado","product":"tornado","ecosystem":"pip","affected":["tornado < 6.5.3"],"patched":["tornado 6.5.3"],"published":"2025-12-12","updated":"2026-07-13","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/PYSEC-2025-267","references":[{"url":"https://github.com/tornadoweb/tornado/releases/tag/v6.5.3"},{"url":"https://github.com/tornadoweb/tornado/security/advisories/GHSA-jhmp-mqwm-3gq8"},{"url":"https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd"}],"tags":["osv","pip"],"epss":0.00535,"epssPercentile":0.44111,"ingestedAt":"2026-07-13T18:58:06.943Z","slug":"CVE-2025-67726","body":"## Overview\n\nTornado is a Python web framework and asynchronous networking library. Versions 6.5.2 and below use an inefficient algorithm when parsing parameters for HTTP header values, potentially causing a DoS. The _parseparam function in httputil.py is used to parse specific HTTP header values, such as those in multipart/form-data and repeatedly calls string.count() within a nested loop while processing quoted semicolons. If an attacker sends a request with a large number of maliciously crafted parameters in a Content-Disposition header, the server's CPU usage increases quadratically (O(n²)) during parsing. Due to Tornado's single event loop architecture, a single malicious request can cause the entire server to become unresponsive for an extended period. This issue is fixed in version 6.5.3.\n\n## Affected packages\n\n- `tornado < 6.5.3`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `tornado 6.5.3`","depth":"twilight","depthScore":41,"depthScoreParts":{"impact":41.3,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}