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.

How to fix CVE-2025-67726

To remediate CVE-2025-67726, upgrade the affected package to a fixed version below.

• —upgrade to 6.5.3 or later

Is CVE-2025-67726 being exploited?

Low — EPSS is 0.4%, meaning exploitation activity has not been observed at scale.

Affected packages (1)

• from 0, < 6.5.3

CVSS scores

References (6)

• ADVISORYnvd.nist.gov/vuln/detail/CVE-2025-67726
• PATCHgithub.com/tornadoweb/tornado
• WEBgithub.com/pypa/advisory-database/tree/main/vulns/tornado/PYSEC-2025-267.yaml
• WEBgithub.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd