{"id":"CVE-2026-49855","aliases":["GHSA-mgf9-4vpg-hj56","PYSEC-2026-3389"],"url":"https://o3.security/vulnerability/CVE-2026-49855","summary":"tornado AsyncHTTPClient accumulates decompressed chunks without size limit (gzip bomb)","details":"Tornado's gzip decompression routines work in limited-size chunks, but have no overall limit for the total size of decompressed chunks that they will accumulate (There has always been a limit for the total *compressed* size). This allows a malicious server to consume effectively unlimited amounts of memory if it is accessed via SimpleAsyncHTTPClient in its default configuration. `HTTPServer` is not affected in its default configuration, but it is if `decompress_request=True` is set.\n\nThis bug is fixed in Tornado 6.5.6. `max_body_size` is now checked both for the compressed and cumulative decompressed size of the response.\n\nPrior to upgrading, this issue can be mitigated by setting `decompress_response=False` or using `CurlAsyncHTTPClient`.","published":"2026-07-14T20:45:02.982Z","modified":"2026-10-08T19:26:47.814242601Z","cvss":{"score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"},"epss":{"score":0.00572,"percentile":0.44996,"asOf":"2026-09-01"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"PyPI","name":"tornado","fixedVersion":"6.5.6"}],"fix":{"url":"https://github.com/tornadoweb/tornado/commit/ff808b33adc52d89a549376a5e3628e92abbc8ff","label":"tornadoweb/tornado@ff808b3"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/49xxx/CVE-2026-49855.json"},{"type":"ADVISORY","url":"https://github.com/tornadoweb/tornado/security/advisories/GHSA-mgf9-4vpg-hj56"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-49855"},{"type":"FIX","url":"https://github.com/tornadoweb/tornado/commit/ff808b33adc52d89a549376a5e3628e92abbc8ff"},{"type":"FIX","url":"https://github.com/tornadoweb/tornado/pull/3626"},{"type":"PACKAGE","url":"https://github.com/tornadoweb/tornado"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-08T19:26:47.814242601Z"}}