CVE-2026-103261 Tornado before 6.5.9 Denial of Service via Query String
Remote attackers can cause denial of service in Tornado web servers by sending requests with excessive query parameters, stalling the event loop.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-103261?
CVE-2026-103261 is a medium-severity denial of service vulnerability in tornadoweb tornado, published on 1 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-770
Which versions are affected by CVE-2026-103261?
2 open source packages ship an affected version, including tornado.
| Package | Affected | Fixed in |
|---|---|---|
tornado PyPI | All versions | 6.5.9 |
tornado PyPI | All versions | 6.5.9 |
How do I fix CVE-2026-103261?
Upgrade tornado to 6.5.9 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-103261 being exploited?
There is no known exploitation of CVE-2026-103261 in the wild and no public exploit code.
Sources
Also known as AZL-105372, BELL-CVE-2026-103261, CLEANSTART-2026-IM25560, DEBIAN-CVE-2026-103261, ECHO-5be6-c7b0-c3f1, GHSA-2v9w-f3h6-74rj and 23 more.