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HIGH severity

tornado AsyncHTTPClient accumulates decompressed chunks without size limit (gzip bomb)GHSA-mgf9-4vpg-hj56

HIGH

GHSA-mgf9-4vpg-hj56 is a high-severity (CVSS 7.5) CWE-409 vulnerability in tornado. A fix is available for tornado — see the affected versions and patch details below.

Also known asCVE-2026-49855PYSEC-2026-3389
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Oct 15, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for GHSA-mgf9-4vpg-hj56.

EPSS Exploitation Probability

via FIRST.org ↗
0.6%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs48th percentile — riskier than 48% of all scored CVEsHighest risk
0.07%0.42%0.76%1.11%0.6%0.6%0.6%0.6%Aug 26Oct 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

GHSA-mgf9-4vpg-hj56 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 390,400 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected
🐍tornado

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

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.

This 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.

Prior to upgrading, this issue can be mitigated by setting decompress_response=False or using CurlAsyncHTTPClient.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPItornadoall versions6.5.6pip install --upgrade 'tornado==6.5.6'

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for tornado, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update tornado to 6.5.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-mgf9-4vpg-hj56 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatImportant

This Important impact flaw in the Tornado Python web framework allows a malicious server to cause uncontrolled memory consumption by sending highly compressed gzip responses, since the decompression routines do not enforce an overall size limit. This can lead to a denial of service in applications using…

Workaround published by Red Hat
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.
Source: Red Hat security advisory for GHSA-mgf9-4vpg-hj56 (CC BY 4.0)
ProductFixed inAdvisory
Red Hat Enterprise Linux 10python-tornado-0:6.5.8-0.el10_2.1RHSA-2026:67147
Red Hat Enterprise Linux 10.0 Extended Update Supportpython-tornado-0:6.5.8-0.el10_0.1RHSA-2026:80741
Red Hat Enterprise Linux 9python-tornado-0:6.5.8-0.el9_8.1RHSA-2026:67146
Red Hat OpenShift AI 3.3rhoai/odh-llama-stack-core-rhel9:1789121286RHSA-2026:73987

Frequently Asked Questions

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. This bug is fixed in Tornado 6.5.6. `max_body_size` is now checked both for the compressed and cumulative decom
O3 Security · Impact-Aware SCA

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