{"id":"PYSEC-2026-3389","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`.","aliases":["CVE-2026-49855","GHSA-mgf9-4vpg-hj56"],"modified":"2026-07-13T16:33:26.893507727Z","published":"2026-07-13T15:46:17.106405Z","references":[{"type":"WEB","url":"https://github.com/tornadoweb/tornado/security/advisories/GHSA-mgf9-4vpg-hj56"},{"type":"PACKAGE","url":"https://github.com/tornadoweb/tornado"},{"type":"PACKAGE","url":"https://pypi.org/project/tornado"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-mgf9-4vpg-hj56"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-49855"}],"affected":[{"package":{"name":"tornado","ecosystem":"PyPI","purl":"pkg:pypi/tornado"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"6.5.6"}]}],"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","6.5.2","6.5.3","6.5.4","6.5.5","6.5b1"],"database_specific":{"source":"https://github.com/pypa/advisory-database/blob/main/vulns/tornado/PYSEC-2026-3389.yaml"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"}]}