Tornado<6.5.3 httputil._parseparam O(n2) DoS
CVE-2025-67726 Published on December 12, 2025
Tornado is Vulnerable to Quadratic DoS via Crafted Multipart Parameters
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.
Vulnerability Analysis
CVE-2025-67726 is exploitable with network access, and does not require authorization privileges or user interaction. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality and integrity, and a high impact on availability.
Weakness Types
Excessive Iteration
The software performs an iteration or loop without sufficiently limiting the number of times that the loop is executed. If the iteration can be influenced by an attacker, this weakness could allow attackers to consume excessive resources such as CPU or memory. In many cases, a loop does not need to be infinite in order to cause enough resource consumption to adversely affect the software or its host system; it depends on the amount of resources consumed per iteration.
What is a Resource Exhaustion Vulnerability?
The software does not properly control the allocation and maintenance of a limited resource, thereby enabling an actor to influence the amount of resources consumed, eventually leading to the exhaustion of available resources.
CVE-2025-67726 has been classified to as a Resource Exhaustion vulnerability or weakness.
Products Associated with CVE-2025-67726
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Affected Versions
tornadoweb tornado Version < 6.5.3 is affected by CVE-2025-67726Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.