CVE-2024-21647 is a medium-severity vulnerability affecting the Puma web server, a popular server for Ruby/Rack applications designed for parallel request handling. The vulnerability arises from an inconsistent interpretation of HTTP requests, specifically related to the parsing of chunked transfer encoding bodies. Prior to Puma versions 6.4.2 and 5.6.8, the server did not properly limit the size of chunk extensions in HTTP requests. This flaw enables HTTP request smuggling attacks, where an attacker crafts malicious HTTP requests that are interpreted differently by intermediary devices and the backend server. Such discrepancies can lead to request desynchronization, allowing attackers to bypass security controls, poison web caches, or conduct cross-user attacks. Additionally, the lack of limits on chunk extension sizes can be exploited to cause unbounded resource consumption, including excessive CPU and network bandwidth usage, potentially leading to denial-of-service (DoS) conditions. The vulnerability does not impact confidentiality or integrity directly but severely affects availability. The issue has been addressed in Puma versions 6.4.2 and 5.6.8 by enforcing limits on chunk extension sizes, mitigating the risk of resource exhaustion and request smuggling. There are no known exploits in the wild at this time, but the vulnerability's nature and medium CVSS score (5.9) indicate a tangible risk if left unpatched, especially in high-traffic or exposed environments.

For European organizations, the impact of this vulnerability can be significant, particularly for those relying on Puma to serve Ruby/Rack web applications. Exploitation could lead to denial-of-service attacks that degrade or disrupt service availability, affecting customer-facing applications, internal portals, or APIs. This disruption can result in operational downtime, loss of user trust, and potential financial losses. While the vulnerability does not directly compromise data confidentiality or integrity, the ability to smuggle HTTP requests could be leveraged in complex attack chains to bypass security controls or poison caches, indirectly facilitating further attacks. Organizations in sectors with high web traffic or critical online services—such as e-commerce, finance, healthcare, and government—are at heightened risk. The resource exhaustion aspect could also increase infrastructure costs due to unexpected load spikes. Given the widespread use of Puma in Ruby on Rails deployments, many European enterprises and service providers could be affected if they have not updated to the fixed versions.

European organizations should immediately audit their Puma server versions and upgrade to at least version 6.4.2 or 5.6.8 to ensure the vulnerability is patched. Beyond patching, organizations should implement strict input validation and limit HTTP request sizes at the web application firewall (WAF) or reverse proxy level to mitigate malformed requests. Deploying robust rate limiting and anomaly detection can help identify and block unusual request patterns indicative of smuggling attempts or resource exhaustion attacks. Network segmentation and isolating critical web services can reduce the blast radius of potential exploitation. Logging and monitoring HTTP request headers and chunk extensions can provide early indicators of attack attempts. Security teams should also review and update incident response plans to include scenarios involving HTTP request smuggling and DoS conditions. Finally, organizations should maintain an up-to-date inventory of Ruby/Rack applications using Puma to ensure comprehensive coverage of affected assets.