CVE-2026-49855: Tornado memory exhaustion via gzip decompression amplification

CVE-2026-49855 · Severity: high · CVSS 7.5 · Published 2026-07-14

Tornado, a popular Python web framework and networking library, is vulnerable to a 'gzip bomb' attack that can crash applications by exhausting their memory. A malicious server or client can send highly compressed data that expands to an enormous size when processed, leading to a denial-of-service (DoS) condition. This affects services using Tornado's asynchronous HTTP client or servers configured to automatically decompress incoming requests.

Technologies: tornado (PyPI). Vendors: PyPI.

Quick answers

Is CVE-2026-49855 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-49855 being exploited in the wild.
How severe is CVE-2026-49855?
High severity, with a CVSS score of 7.5, published 14 July 2026.
Which products and versions are affected?
tornado (PyPI) < 6.5.6, ECOSYSTEM:introduced=0,fixed=6.5.6.
Is there a fix for CVE-2026-49855?
Yes. tornado (PyPI): upgrade to 6.5.6 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-49855 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-49855?
Check whether you run tornado (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • tornado (PyPI): upgrade to 6.5.6 or later
    pip install "tornado>=6.5.6"

Technical details

A data amplification vulnerability (CWE-409) exists in Tornado's gzip decompression routines prior to version 6.5.6. While the library enforced limits on the compressed size of incoming data (via Content-Length), it failed to enforce a cumulative limit on the total size of the decompressed output. An attacker can exploit this by providing a 'gzip bomb'—a small compressed payload that expands into a massive amount of data—to a SimpleAsyncHTTPClient or an HTTPServer with decompress_request=True. This leads to uncontrolled memory consumption and a process crash. The fix in 6.5.6 ensures max_body_size is enforced for both compressed and decompressed data.

Affected products

  • tornadoweb Tornado < 6.5.6

Identifiers

  • CVE-2026-49855
  • GHSA-mgf9-4vpg-hj56
  • PYSEC-2026-3389

Timeline

  • 2026-05-21: patched: Fixes committed to repository
  • 2026-05-27: advisory: GitHub Security Advisory published
  • 2026-07-14: disclosed: CVE published to NVD

References

This page as Markdown. Free to reuse under CC BY 4.0 with a link back.