What’s Vulnerable › Vulnerabilities

CVE-2026-103261: Tornado before 6.5.9 Denial of Service via Query String

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-10-01 · updated 2026-10-01
• Weakness: CWE-770

Description

Tornado before 6.5.9 fails to limit the number of query string fields in HTTPServerRequest.__init__, allowing remote attackers to cause event-loop stalling by sending requests with thousands of query parameters. Attackers can send unauthenticated GET requests with unbounded query-string field counts to degrade response times for all clients sharing the same IOLoop.

Affected products

• Pypi Tornado: before 6.5.9; 0.2, 1.0, 1.1, 1.1.1, 1.2, 1.2.1 (+78) — fixed in 6.5.9
• tornadoweb tornado: before 6.5.9 — fixed in 6.5.9

How to fix

Upgrade to a fixed version: Pypi Tornado 6.5.9; tornadoweb tornado 6.5.9. Check the vendor advisory for mitigations.

References

• https://github.com/tornadoweb/tornado/commit/03945136ea9746eccf61caf88edae39642e59c93
• https://github.com/tornadoweb/tornado/security/advisories/GHSA-3hv7-mjh2-fv65
• https://www.vulncheck.com/advisories/tornado-before-6.5.9-denial-of-service-via-query-string

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