CVE-2025-67725 is a vulnerability classified under CWE-400 (Uncontrolled Resource Consumption) affecting Tornado, a popular Python web framework and asynchronous networking library. Versions 6.5.2 and earlier contain a flaw in the HTTPHeaders.add method, which handles HTTP request headers. When a malicious actor sends an HTTP request with repeated header names, the method concatenates header values using Python string concatenation. 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. This inefficiency can cause the Tornado server's event loop to become blocked for an extended period, effectively causing a Denial of Service (DoS). The severity of the impact depends on the max_header_size setting: if it remains at the default 64KB, the impact is lower; if increased, the potential for resource exhaustion and service disruption is higher. The vulnerability requires no privileges or user interaction to exploit, making it accessible to remote unauthenticated attackers. No known exploits are reported in the wild as of publication. The issue is resolved in Tornado version 6.5.3, which optimizes header value handling to prevent excessive resource consumption.

For European organizations, this vulnerability poses a significant risk to the availability of web applications and services built on Tornado versions prior to 6.5.3. Organizations relying on Tornado for asynchronous networking or web server functionality could experience service outages or degraded performance if targeted by attackers sending crafted HTTP requests with repeated headers. This could disrupt business operations, customer-facing services, and internal applications, potentially leading to financial losses and reputational damage. The risk is amplified in environments where max_header_size is configured above the default, increasing the attack surface. Critical infrastructure, financial services, and public sector organizations using Tornado-based applications are particularly vulnerable to DoS attacks exploiting this flaw. Although no known exploits are currently active, the ease of exploitation and lack of required authentication make proactive mitigation essential.

1. Upgrade all Tornado deployments to version 6.5.3 or later immediately to apply the fix addressing this vulnerability. 2. Review and, if possible, revert any increases to the max_header_size configuration to the default 64KB to limit potential resource exhaustion. 3. Implement web application firewalls (WAFs) or reverse proxies capable of detecting and blocking HTTP requests with suspiciously repeated headers or abnormal header sizes. 4. Monitor server logs and network traffic for unusual patterns of repeated HTTP headers or spikes in request processing times indicative of attempted exploitation. 5. Employ rate limiting on incoming HTTP requests to reduce the impact of potential DoS attempts. 6. Conduct regular security audits and vulnerability scans focusing on dependencies such as Tornado to ensure timely patching. 7. Educate development and operations teams about the risks of uncontrolled resource consumption vulnerabilities and best practices for secure configuration.