What’s Vulnerable › Vulnerabilities

CVE-2026-115927: Tornado: Event loop blocking and excess memory use from chunked bodies with many small chunks

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-10-26 · updated 2026-10-27
• Weakness: CWE-400

Description

Tornado is a Python web framework and asynchronous networking library. Prior to 6.6.0, Tornado decodes HTTP/1.1 chunked transfer encoding by processing body data that has already arrived without yielding to the event loop. A remote unauthenticated peer can send millions of one-byte chunks to monopolize the process event loop and stall every other connection served by that process, while lowering max_body_size only limits the duration of each request. For handlers that do not use @stream_request_body, retaining each chunk as a separate object can consume roughly twenty times the received body size, and SimpleAsyncHTTPClient has the same event-loop behavior when reading a response from a malicious server. This issue is fixed in version 6.6.0.

Affected products

• tornadoweb tornado: before 6.6.0

References

• https://github.com/tornadoweb/tornado/commit/ceab33581489751a66fba8566ad6d5960d4b01c8
• https://github.com/tornadoweb/tornado/pull/3773
• https://github.com/tornadoweb/tornado/releases/tag/v6.6.0
• https://github.com/tornadoweb/tornado/security/advisories/GHSA-r4j5-9xpp-3985

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