Tornado is a Python web framework and asynchronous networking library.
CVE-2026-116577
Summary Tornado's HTTP header parser takes quadratic time (O(n²)) when a header value is continued across many "obsolete line folding" (obs-fold) lines. An unauthenticated attacker can send a request whose header block is mostly short continuation lines. Parsing it uses a large amount of CPU on the IOLoop thread and blocks every other connection that process is serving. This issue is closely related to CVE-2025-67725 (GHSA-c98p-7wgm-6p64). The fix for that advisory covered repeated headers but not line-folded continuations. Impact All Tornado HTTP servers are affected. Header parsing runs before any application code, so no authentication or particular handler is required. How bad it gets depends on maxheadersize: Default (64 KB): each malicious request blocks the event loop for tens of milliseconds. Repeated requests can degrade service. Raised maxheadersize: the cost grows with the square of the header size. A single 1 MB request can block the process for several seconds. Only availability is affected. Confidentiality and integrity are not. Patches Fixed in Tornado 6.6 by removing support for obsolete line folding completely; header lines that begin with whitespace are now…
Assessment
- CVSS
- 5.3 (v3.1)
- Severity
- Medium
- EPSS
- 0.00384% chance of exploitation in the next 30 days
- Known exploited
- Not listed in CISA KEV
- Exploit availability
- Nothing found in Exploit-DB, Metasploit, Nuclei or public PoC repositories
- Published
- 26 October 2026
- Last updated
- 31 October 2026
- Affected
- pip · tornado
- Weakness
- CWE-407, CWE-770
- Assigner
- security-advisories@github.com
CVSS vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
At the source
References
- https://github.com/tornadoweb/tornado/commit/1b7a1dc9f340f52de929852e8125aa12f25297c9
- https://github.com/tornadoweb/tornado/pull/3765
- https://github.com/tornadoweb/tornado/releases/tag/v6.6.0
- https://github.com/tornadoweb/tornado/security/advisories/GHSA-pcg2-g6cj-q92c
- https://nvd.nist.gov/vuln/detail/CVE-2026-116577
- https://github.com/advisories/GHSA-pcg2-g6cj-q92c
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