CVE-2021-41136 is a vulnerability classified under CWE-444, involving inconsistent interpretation of HTTP requests, commonly known as HTTP Request/Response Smuggling. This vulnerability affects Puma, a widely used HTTP 1.1 server for Ruby/Rack applications, specifically versions >= 5.0.0 and < 5.5.1, and versions < 4.3.9. The issue arises when Puma is deployed behind a proxy that forwards HTTP header values containing the line feed (LF) character, particularly when used with Apache Traffic Server, which is currently the only known proxy exhibiting the problematic behavior. The vulnerability exploits the difference in how Puma and the proxy interpret HTTP pipelined requests over persistent connections. The proxy may treat an additional pipelined request as part of the body of the first request, while Puma correctly interprets it as a separate request. Consequently, Puma sends a response to the second request that the proxy does not expect. If the proxy has reused the persistent connection to Puma to serve a different client, this response can be sent to an unintended client, leading to information leakage and potential confusion in request handling. Although the vulnerability does not directly allow remote code execution or denial of service, it compromises confidentiality and integrity by misrouting responses. The issue was patched in Puma versions 5.5.1 and 4.3.9. As a temporary mitigation, users are advised not to use Apache Traffic Server as a proxy with Puma. The CVSS v3.1 base score is 3.7 (low severity), reflecting the limited impact and the requirement for low privileges and user interaction to exploit. No known exploits are currently reported in the wild.

For European organizations, this vulnerability poses a moderate confidentiality risk, especially for those deploying Ruby/Rack applications behind Puma servers proxied by Apache Traffic Server. The unintended disclosure of HTTP responses to unauthorized clients could lead to leakage of sensitive information such as session tokens, personal data, or internal application responses. This could undermine compliance with GDPR and other data protection regulations. The integrity of HTTP communication is also affected, as responses may be delivered to incorrect clients, potentially causing application logic errors or user confusion. However, the vulnerability does not enable denial of service or remote code execution, limiting its impact. Organizations using other proxies or not employing HTTP pipelining are less affected. Since Puma is popular in web application deployments, especially in startups and enterprises using Ruby on Rails frameworks, the risk is notable but constrained by the specific proxy and configuration requirements.

European organizations should immediately upgrade Puma to versions 5.5.1 or 4.3.9 or later to eliminate the vulnerability. If upgrading is not immediately feasible, they should avoid using Apache Traffic Server as a proxy in front of Puma. Instead, alternative proxies that do not exhibit the problematic behavior, such as NGINX or HAProxy, should be used. Additionally, organizations should disable HTTP pipelining on clients and proxies where possible to reduce the risk of request smuggling. Implementing strict input validation and sanitization of HTTP headers to reject or encode LF characters can further mitigate the risk. Monitoring and logging HTTP traffic for anomalies in request and response patterns may help detect exploitation attempts. Finally, reviewing and tightening proxy and server configurations to prevent persistent connection reuse across different clients can reduce exposure.