CVE-2026-103261
Deferred Deferred - Pending Action

Tornado Event-Loop Stalling via Unbounded Query Parameters

Publication date: 2026-10-01

Last updated on: 2026-10-01

Assigner: VulnCheck

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.

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Meta Information

Published
2026-10-01
Last Modified
2026-10-01
Generated
2026-10-01
AI Q&A
2026-10-01
EPSS Evaluated
N/A
NVD
CVE-2026-103261
EUVD
EUVD-2026-90655

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
tornadoweb tornado to 6.5.9 (exc)

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-770 The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.

Attack-Flow Graph

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Executive Summary

Tornado before 6.5.9 fails to limit the number of query string fields in HTTP requests. Attackers can send unauthenticated GET requests with thousands of query parameters, causing the event loop to stall and degrading response times for all users sharing the same IOLoop.

Detection Guidance

Monitor for unusually high CPU usage during HTTP requests with excessive query parameters. Check Tornado server logs for GET requests with thousands of query fields. Use network tools like tcpdump or Wireshark to capture and analyze query strings for abnormal parameter counts.

Impact Analysis

This vulnerability can cause Denial of Service (DoS) by stalling the event loop, leading to degraded performance for all users sharing the same IOLoop. Attackers can exploit it with minimal bandwidth to make the service unavailable.

Mitigation Strategies

Upgrade Tornado to version 6.5.9 or later to apply the fix. If upgrading is not immediately possible, implement rate limiting or request size limits on your web server or proxy to block excessively large query strings.

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