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

Source: https://junglewise.ai/threats/cve-2026-49855-tornado-memory-exhaustion-via-gzip-decompression-amplification  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-14. Updated: 2026-07-14  
License: CC BY 4.0 with a link back

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)](https://junglewise.ai/threats/technologies/tornado). Vendors: [PyPI](https://junglewise.ai/threats/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

  ```sh
  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

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

## Related threats

- [Tornado open redirect in RedirectHandler URL templates](https://junglewise.ai/threats/cve-2026-116578-tornado-is-a-python-web-framework-and-asynchronous-networking)
- [Tornado HTTP header parser quadratic CPU consumption via line folding](https://junglewise.ai/threats/cve-2026-116577-tornado-is-a-python-web-framework-and-asynchronous-networking)
- [Tornado HTTP/1.1 chunked encoding denial of service](https://junglewise.ai/threats/cve-2026-115927-tornado-is-a-python-web-framework-and-asynchronous-networking)
- [Tornado StaticFileHandler path traversal via symlinks](https://junglewise.ai/threats/cve-2026-103263-tornado-staticfilehandler-follows-symlinks-outside-static-root)
- [Tornado CurlAsyncHTTPClient unbounded memory accumulation in decompression](https://junglewise.ai/threats/cve-2026-103262-tornado-curlasynchttpclient-enforces-no-response-size-limit)