CVE-2025-67726: Tornado is Vulnerable to Quadratic DoS via Crafted Multipart Parameters
Summary
The parseparam function in Tornado's httputil.py is used to parse specific HTTP header values, such as those in multipart/form-data. This function uses an inefficient algorithm that repeatedly calls string.count() within a nested loop while processing quoted semicolons (e.g., param=";").
As a result, 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, leading to a Denial of Service (DoS).
Severity: High
Other sources
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.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
pip/tornadoto a version that resolves this vulnerability.Fixed in 6.5.3 - Upgrade
Upgrade
tornado/httputil.py (_parseparam)to a version that resolves this vulnerability.Fixed in 6.5.3
Event History
Frequently Asked Questions
What is the severity of CVE-2025-67726?
CVE-2025-67726 is classified as a critical vulnerability due to its potential to cause a denial-of-service (DoS) condition.
How do I fix CVE-2025-67726?
To fix CVE-2025-67726, upgrade to Tornado version 6.5.3 or later.
What versions are affected by CVE-2025-67726?
CVE-2025-67726 affects Tornado versions 6.5.2 and below.
What type of vulnerability is CVE-2025-67726?
CVE-2025-67726 is a denial-of-service (DoS) vulnerability stemming from an inefficient HTTP header parsing algorithm.
What component of Tornado is impacted by CVE-2025-67726?
The _parseparam function in httputil.py is the component impacted by CVE-2025-67726.