tornado AsyncHTTPClient accumulates decompressed chunks without size limit (gzip bomb)

Description

Tornado is a Python web framework and asynchronous networking library. Prior to 6.5.6, Tornado gzip decompression routines processed limited-size chunks but did not enforce an overall limit on accumulated decompressed chunks, allowing a malicious server accessed by SimpleAsyncHTTPClient or an HTTPServer configured with decompress_request=True to consume effectively unlimited memory. This issue is fixed in version 6.5.6.

Scores

EPSS (exploitation in 30 days)

0.57% · top 53% of all CVEs (6 Nov 2026)

CISA SSVC

Exploitation: none · Automatable: yes · Technical impact: partial

The list uses the assigner's score when there is one, then CISA's, then NVD's: High 7.5.

Weakness

• CWE-409: Improper Handling of Highly Compressed Data (Data Amplification) (opens in a new tab)All about CWE-409 and its CVEs

Affected products

• tornadoweb · tornado

  • affected < 6.5.6

Affected packages

From reviewed advisories of the GitHub Advisory Database.

• PyPItornadoGHSA-mgf9-4vpg-hj56 (opens in a new tab)

References (3)

• https://github.com/tornadoweb/tornado/security/advisories/GHSA-mgf9-4vpg-hj56 (opens in a new tab)x_refsource_CONFIRM
• https://github.com/tornadoweb/tornado/pull/3626 (opens in a new tab)x_refsource_MISC
• https://github.com/tornadoweb/tornado/commit/ff808b33adc52d89a549376a5e3628e92abbc8ff (opens in a new tab)x_refsource_MISC

Sources

The same CVE in the primary databases and aggregators, to check against.

• CVE.org record (opens in a new tab)
• CVE List JSON (GitHub) (opens in a new tab)
• NVD (opens in a new tab)
• FIRST EPSS (opens in a new tab)
• OSV.dev (opens in a new tab)
• GitHub Advisory GHSA-mgf9-4vpg-hj56 (opens in a new tab)
• CIRCL Vulnerability-Lookup (opens in a new tab)

Badge for a README or a ticket

[![CVE-2026-49855](https://trackcve.app/badge/CVE-2026-49855.svg)](https://trackcve.app/cve/CVE-2026-49855)

Record last updated 16 Jul 2026, 14:56 UTC.