Vulnerability · Published Jul 14, 2026 · Assigned by GitHub_M
CVE-2026-49855tornado AsyncHTTPClient accumulates decompressed chunks without size limit (gzip bomb)
High 7.5
In plain English
A high-severity security flaw in tornado from tornadoweb. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 47% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| AlmaLinux:10 | python3-tornado | 6.5.8-0.el10_2.1 | ALSA-2026:67147 |
| AlmaLinux:9 | python3-tornado | 6.5.8-0.el9_8.1 | ALSA-2026:67146 |
| Debian:12 | python-tornado | no fix listed | DEBIAN-CVE-2026-49855 |
| Debian:13 | python-tornado | no fix listed | DEBIAN-CVE-2026-49855 |
| Debian:14 | python-tornado | no fix listed | DEBIAN-CVE-2026-49855 |
| PyPI | tornado | 6.5.6 | GHSA-mgf9-4vpg-hj56 |
| PyPI | tornado | 6.5.6 | PYSEC-2026-3389 |
| Rocky Linux:10 | python-tornado | 0:6.5.8-0.el10_2.1 | RLSA-2026:67147 |
| Rocky Linux:9 | python-tornado | 0:6.5.8-0.el9_8.1 | RLSA-2026:67146 |
| Ubuntu:24.04:LTS | python-tornado | no fix listed | UBUNTU-CVE-2026-49855 |
| Ubuntu:26.04:LTS | python-tornado | no fix listed | UBUNTU-CVE-2026-49855 |
| Ubuntu:26.10 | python-tornado | no fix listed | UBUNTU-CVE-2026-49855 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| tornadoweb | tornado | < 6.5.6 |
Weakness: CWE-409 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
References
- https://github.com/tornadoweb/tornado/security/advisories/GHSA-mgf9-4vpg-hj56 x_refsource_CONFIRM
- https://github.com/tornadoweb/tornado/pull/3626 x_refsource_MISC
- https://github.com/tornadoweb/tornado/commit/ff808b33adc52d89a549376a5e3628e92abbc8ff x_refsource_MISC