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

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). 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
    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

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