{"id":"CVE-2026-115927","aliases":["GHSA-r4j5-9xpp-3985"],"url":"https://o3.security/vulnerability/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).","published":"2026-10-26T19:37:40.584Z","modified":"2026-10-28T02:59:29.056316614Z","cvss":{"score":5.3,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L"},"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"PyPI","name":"tornado","fixedVersion":"6.6"}],"fix":{"url":"https://github.com/tornadoweb/tornado/commit/ceab33581489751a66fba8566ad6d5960d4b01c8","label":"tornadoweb/tornado@ceab335"},"references":[{"type":"WEB","url":"https://github.com/tornadoweb/tornado/releases/tag/v6.6.0"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/115xxx/CVE-2026-115927.json"},{"type":"ADVISORY","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":"FIX","url":"https://github.com/tornadoweb/tornado/commit/ceab33581489751a66fba8566ad6d5960d4b01c8"},{"type":"FIX","url":"https://github.com/tornadoweb/tornado/pull/3773"},{"type":"PACKAGE","url":"https://github.com/tornadoweb/tornado"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T02:59:29.056316614Z"}}