Server-wide denial of service via unbounded form parsing
Published Aug 31, 2026 · Updated Sep 2, 2026
Denial of service in Tornado before 6.5.8 allows remote attackers to stall the server through a crafted urlencoded request body. The parse_qs_bytes path calls urllib.parse.parse_qs without max_num_fields before handler dispatch, allowing millions of fields to monopolize the event loop. Any unauthenticated route accepting a form POST exposes the synchronous, single-threaded parser, delaying every connection in the process.
Summary
What happened
Denial of service in Tornado before 6.5.8 allows remote attackers to stall the server through a crafted urlencoded request body. The parse_qs_bytes path calls urllib.parse.parse_qs without max_num_fields before handler dispatch, allowing millions of fields to monopolize the event loop. Any unauthenticated route accepting a form POST exposes the synchronous, single-threaded parser, delaying every connection in the process.
The record
- CVE
- CVE-2026-82397
- Published
- Aug 31, 2026
- Updated
- Sep 2, 2026
- Vendor
- tornadoweb
- Product
- Tornado
- Classifications
- CWE-400, CWE-1284, T1499.003
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Aug 31, 2026CVE publishedPublication date reported by the CVE source.
- Sep 2, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=< 6.5.8
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
No public exploit references are available in this record.
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Technologies
Your stack
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Your stack
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