# CVE-2026-82397: Tornado urlencoded body parsing DoS via unbounded fields

Source: https://junglewise.ai/threats/cve-2026-82397-tornado-urlencoded-body-parsing-dos-via-unbounded-fields  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-31. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-82397 · Severity: high · CVSS 7.5 · Published 2026-08-31

Tornado, a popular Python web application framework, fails to limit the number of form fields parsed from HTTP POST requests with urlencoded bodies. An attacker can craft a malicious POST request with a body consisting almost entirely of field separators, creating tens of millions of fields. Because parsing occurs on the main event loop before request handling, a single request stalls the entire server and blocks all other clients from being served.

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

## Quick answers

**Is CVE-2026-82397 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-82397 being exploited in the wild.

**How severe is CVE-2026-82397?** High severity, with a CVSS score of 7.5, published 31 August 2026.

**Which products and versions are affected?** tornado (PyPI) <= 6.5.7, ECOSYSTEM:introduced=0,fixed=6.5.8.

**Is there a fix for CVE-2026-82397?** Yes. tornado (PyPI): upgrade to 6.5.8 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-82397 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-82397?** 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 6.5.8 or later

  ```sh
  pip install "tornado>=6.5.8"
  ```


## Technical details

The vulnerability exists in tornado/escape.py, where urllib.parse.parse_qs() is called without the max_num_fields parameter introduced in CPython to prevent ReDoS-style attacks. A malicious client sends an HTTP POST request with Content-Type: application/x-www-form-urlencoded and a body up to 100 MB consisting primarily of field separators ('&' characters). This generates approximately 50 million fields. The parsing occurs synchronously in tornado/web.py during RequestHandler._execute before user code runs, blocking the single-threaded event loop. All concurrent connections wait indefinitely, creating a complete denial of service. The attack requires no authentication if any route accepts form data (standard for web apps). The fix is to pass max_num_fields to parse_qs with a conservative default, allowing applications to raise the limit if needed. Patched in version 6.5.8.

## Affected products

- tornadoweb tornado <= 6.5.7

## Identifiers

- CVE-2026-82397
- GHSA-mpf4-983q-p7j4
- PYSEC-2026-3928

## Timeline

- 2026-08-07: disclosed: Security advisory published to GitHub Advisory Database
- 2026-08-07: patched: Fix released in version 6.5.8

## References

- [https://api.github.com/users/arpitjain099](https://api.github.com/users/arpitjain099)
- [https://github.com/arpitjain099](https://github.com/arpitjain099)
- [https://api.github.com/users/arpitjain099/gists%7B/gist_id%7D](https://api.github.com/users/arpitjain099/gists%7B/gist_id%7D)
- [https://api.github.com/users/arpitjain099/repos](https://api.github.com/users/arpitjain099/repos)
- [https://avatars.githubusercontent.com/u/3242828?v=4](https://avatars.githubusercontent.com/u/3242828?v=4)
- [https://api.github.com/users/arpitjain099/events%7B/privacy%7D](https://api.github.com/users/arpitjain099/events%7B/privacy%7D)

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- [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)
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- [Tornado CurlAsyncHTTPClient unbounded memory accumulation in decompression](https://junglewise.ai/threats/cve-2026-103262-tornado-curlasynchttpclient-enforces-no-response-size-limit)