CVE-2025-67725 is a vulnerability classified under CWE-400 (Uncontrolled Resource Consumption) affecting Tornado, a popular Python web framework and asynchronous networking library. In Tornado versions 6.5.2 and earlier, the HTTPHeaders.add method concatenates header values as strings when the same header name appears multiple times in a request. Due to Python strings being immutable, each concatenation operation creates a new string copy, resulting in quadratic time complexity (O(n²)) relative to the number of repeated headers. An attacker can exploit this by sending a single HTTP request with a large number of repeated headers, causing the server’s event loop to be blocked for an extended period. This effectively leads to a Denial of Service (DoS) by exhausting CPU resources and preventing the server from processing legitimate requests. The severity of the impact varies with the max_header_size configuration parameter; increasing this value above the default 64KB allows larger headers and thus amplifies the resource consumption. The vulnerability requires no authentication or user interaction and can be exploited remotely over the network. Although no known exploits are reported in the wild yet, the vulnerability is rated with a CVSS v3.1 score of 7.5 (high severity). The issue is resolved in Tornado version 6.5.3, where the header concatenation logic has been optimized or safeguarded to prevent excessive resource consumption.

For European organizations, this vulnerability poses a significant risk of service disruption, especially for those relying on Tornado-based web applications or APIs. A successful exploit can cause server unresponsiveness, leading to downtime and degraded user experience. Critical services such as e-government portals, financial platforms, healthcare systems, and e-commerce sites using Tornado could face operational interruptions. The impact on confidentiality and integrity is minimal since the vulnerability does not allow data leakage or modification, but availability is severely affected. Organizations with customized max_header_size settings above the default are at higher risk due to increased potential for resource exhaustion. The disruption could also have cascading effects on dependent services and compliance with service-level agreements (SLAs). Given the ease of exploitation (no authentication or user interaction required) and network accessibility, attackers can launch DoS attacks remotely, potentially causing widespread outages in affected infrastructures.

The primary mitigation is to upgrade all Tornado instances to version 6.5.3 or later, where the vulnerability is fixed. Organizations should audit their environments to identify Tornado versions in use, including in containerized and serverless deployments. Until upgrades are applied, administrators should enforce strict limits on max_header_size, ideally retaining the default 64KB or lower, to reduce the attack surface. Implementing web application firewalls (WAFs) or reverse proxies that detect and block requests with excessive repeated headers can provide an additional layer of defense. Monitoring server performance and unusual spikes in CPU usage or request patterns can help detect exploitation attempts early. Network-level rate limiting and IP reputation filtering may also mitigate attack volume. Finally, incorporating this vulnerability into incident response and patch management workflows ensures timely remediation and reduces exposure.