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High severity7.5NVD Advisory· Published Dec 12, 2025· Updated Jun 17, 2026

CVE-2025-67726

CVE-2025-67726

Description

Tornado is a Python web framework and asynchronous networking library. Versions 6.5.2 and below use an inefficient algorithm when parsing parameters for HTTP header values, potentially causing a DoS. The _parseparam function in httputil.py is used to parse specific HTTP header values, such as those in multipart/form-data and repeatedly calls string.count() within a nested loop while processing quoted semicolons. If an attacker sends a request with a large number of maliciously crafted parameters in a Content-Disposition header, the server's CPU usage increases quadratically (O(n²)) during parsing. Due to Tornado's single event loop architecture, a single malicious request can cause the entire server to become unresponsive for an extended period. This issue is fixed in version 6.5.3.

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Affected packages

Versions sourced from the GitHub Security Advisory.

PackageAffected versionsPatched versions
tornadoPyPI
< 6.5.36.5.3

Affected products

333

Patches

Vulnerability mechanics

References

6

News mentions

0

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