GHSA-R4J5-9XPP-3985
Vulnerability from github – Published: 2026-10-28 22:43 – Updated: 2026-10-28 22:43Summary
When 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.
Bodies 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.
Impact
All 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.
SimpleAsyncHTTPClient is affected in the same way when it reads a response from a malicious server.
Patches
Fixed 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.
Applications using @stream_request_body should note that the pieces passed to data_received no longer correspond to HTTP chunk boundaries.
Workarounds
- Run Tornado behind a reverse proxy that buffers request bodies and forwards them with a
Content-Lengthheader instead of chunked encoding. - Lower
max_body_size(for the whole server or for individual handlers) to limit the impact of each request.
Credit
Reported by uttej-badwane.
{
"affected": [
{
"package": {
"ecosystem": "PyPI",
"name": "tornado"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "6.6"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-115927"
],
"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"
},
"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).",
"id": "GHSA-r4j5-9xpp-3985",
"modified": "2026-10-28T22:43:50Z",
"published": "2026-10-28T22:43:50Z",
"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"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
"type": "CVSS_V3"
}
],
"summary": "Tornado: Event loop blocking and excess memory use from chunked bodies with many small chunks"
}
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