{"schema_version":"1.9.2","id":"GHSA-r4j5-9xpp-3985","published":"2026-10-28T22:43:50Z","modified":"2026-10-28T23:00:04.082411607Z","aliases":["CVE-2026-115927"],"summary":"Tornado: Event loop blocking and excess memory use from chunked bodies with many small chunks","details":"### 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).","affected":[{"package":{"name":"tornado","ecosystem":"PyPI","purl":"pkg:pypi/tornado"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"6.6"}]}],"versions":["0.2","1.0","1.1","1.1.1","1.2","1.2.1","2.0","2.1","2.1.1","2.2","2.2.1","2.3","2.4","2.4.1","3.0","3.0.1","3.0.2","3.1","3.1.1","3.2","3.2.1","3.2.2","4.0","4.0.1","4.0.2","4.1","4.1b2","4.2","4.2.1","4.2b1","4.3","4.3b1","4.3b2","4.4","4.4.1","4.4.2","4.4.3","4.4b1","4.5","4.5.1","4.5.2","4.5.3","4.5b1","4.5b2","5.0","5.0.1","5.0.2","5.0a1","5.0b1","5.1","5.1.1","5.1b1","6.0","6.0.1","6.0.2","6.0.3","6.0.4","6.0a1","6.0b1","6.1","6.1b1","6.1b2","6.2","6.2b1","6.2b2","6.3","6.3.1","6.3.2","6.3.3","6.3b1","6.4","6.4.1","6.4.2","6.4b1","6.5","6.5.1","6.5.10","6.5.2","6.5.3","6.5.4","6.5.5","6.5.6","6.5.7","6.5.8","6.5.9","6.5b1","6.6a1","6.6b1","6.6rc1"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/10/GHSA-r4j5-9xpp-3985/GHSA-r4j5-9xpp-3985.json"}}],"references":[{"type":"WEB","url":"https://github.com/tornadoweb/tornado/security/advisories/GHSA-r4j5-9xpp-3985"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-115927"},{"type":"WEB","url":"https://github.com/tornadoweb/tornado/pull/3773"},{"type":"WEB","url":"https://github.com/tornadoweb/tornado/commit/ceab33581489751a66fba8566ad6d5960d4b01c8"},{"type":"PACKAGE","url":"https://github.com/tornadoweb/tornado"},{"type":"WEB","url":"https://github.com/tornadoweb/tornado/releases/tag/v6.6.0"}],"database_specific":{"cwe_ids":["CWE-400","CWE-770"],"github_reviewed":true,"github_reviewed_at":"2026-10-28T22:43:50Z","nvd_published_at":"2026-10-26T20:17:52Z","severity":"MODERATE"},"severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L"}]}