{"id":"CVE-2026-115927","aliases":["GHSA-r4j5-9xpp-3985"],"title":"Tornado: Event loop blocking and excess memory use from chunked bodies with many small chunks","summary":"Tornado: Event loop blocking and excess memory use from chunked bodies with many small chunks","severity":"medium","cvss":5.3,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L","vendor":"tornado","product":"tornado","ecosystem":"pip","affected":["tornado < 6.6"],"patched":["tornado 6.6"],"published":"2026-10-28","updated":"2026-10-28","sourceUpdated":"2026-10-28T23:00:04.082411607Z","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/GHSA-r4j5-9xpp-3985","references":[{"url":"https://github.com/tornadoweb/tornado/security/advisories/GHSA-r4j5-9xpp-3985"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-115927"},{"url":"https://github.com/tornadoweb/tornado/pull/3773"},{"url":"https://github.com/tornadoweb/tornado/commit/ceab33581489751a66fba8566ad6d5960d4b01c8"},{"url":"https://github.com/tornadoweb/tornado"},{"url":"https://github.com/tornadoweb/tornado/releases/tag/v6.6.0"},{"url":"https://github.com/advisories/GHSA-r4j5-9xpp-3985"}],"tags":["osv","pip","ghsa","cve.org"],"epss":0.00384,"epssPercentile":0.32384,"ssvc":{"exploitation":"none","automatable":"yes","technicalImpact":"partial","timestamp":"2026-10-27T13:45:56.157686Z"},"cwe":["CWE-400","CWE-770"],"ingestedAt":"2026-10-26T20:39:42.649Z","slug":"CVE-2026-115927","body":"## Overview\n\n### Summary\n\nWhen Tornado reads an HTTP/1.1 body that uses chunked transfer encoding, it processes all chunks that have already arrived without yielding to the event loop. A client can send a body made of millions of 1-byte chunks, which blocks the event loop (and therefore every other connection served by that process) for the entire time it takes to parse the body. A body near the default `max_body_size` of 100 MB can block the process for close to a minute, and the attacker can repeat or parallelize these requests.\n\nBodies made of tiny chunks also consumed far more memory than their size: a handler that does not use `@stream_request_body` stored each chunk as a separate object, using roughly 20 times as much memory as the bytes received.\n\n### Impact\n\nAll Tornado HTTP servers are affected when they receive a request body for a handler that reads it, which includes any handler that does not reject the request before the body is read. No authentication is required. Lowering `max_body_size` reduces the maximum blocking time and memory use for each request but does not remove the issue.\n\n`SimpleAsyncHTTPClient` is affected in the same way when it reads a response from a malicious server.\n\n### Patches\n\nFixed in Tornado 6.6. Tornado now yields to the event loop periodically while reading any body, combines small chunks before passing them to `data_received`, and stores non-streaming bodies more compactly.\n\nApplications using `@stream_request_body` should note that the pieces passed to `data_received` no longer correspond to HTTP chunk boundaries.\n\n### Workarounds\n\n- Run Tornado behind a reverse proxy that buffers request bodies and forwards them with a `Content-Length` header instead of chunked encoding.\n- Lower `max_body_size` (for the whole server or for individual handlers) to limit the impact of each request.\n\n### Credit\n\nReported by [uttej-badwane](https://github.com/uttej-badwane).\n\n## Affected packages\n\n- `tornado < 6.6`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `tornado 6.6`","depth":"sunlit","depthScore":29,"depthScoreParts":{"impact":29.2,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}