What’s Vulnerable › Vulnerabilities

CVE-2026-49855: tornado AsyncHTTPClient accumulates decompressed chunks without size limit (gzip bomb)

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.6% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-07-14 · updated 2026-10-24
• Weakness: CWE-409

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 products

• Pypi Tornado: before 6.5.6; 0.2, 1.0, 1.1, 1.1.1, 1.2, 1.2.1 (+75) — fixed in 6.5.6
• tornadoweb tornado: before 6.5.6

How to fix

Upgrade to a fixed version: Pypi Tornado 6.5.6. Check the vendor advisory for mitigations.

References

• https://github.com/tornadoweb/tornado/commit/ff808b33adc52d89a549376a5e3628e92abbc8ff
• https://github.com/tornadoweb/tornado/pull/3626
• https://github.com/tornadoweb/tornado/security/advisories/GHSA-mgf9-4vpg-hj56

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