CVE-2025-55315 is a vulnerability classified as HTTP Request Smuggling affecting Kestrel, the cross-platform web server used by ASP.NET Core applications. The issue arises from improper handling of chunk extensions in HTTP/1.1 chunked transfer encoding. HTTP Request Smuggling exploits discrepancies in how front-end and back-end servers parse HTTP requests, allowing attackers to craft specially malformed requests that are interpreted differently by intermediaries and the target server. In this case, Kestrel's parsing of chunk extensions can be manipulated to smuggle requests, enabling attackers to bypass security controls such as firewalls or intrusion detection systems, poison web caches, hijack user sessions, or perform cross-site scripting and other injection attacks. The vulnerability does not require authentication or user interaction, increasing its risk profile. Although the vulnerability is rated medium severity by the source, the lack of a CVSS score and the nature of HTTP Request Smuggling suggest a potentially higher impact. No public exploits have been reported yet, but the vulnerability's presence in a widely used web server component makes it a critical concern for web-facing applications. The vulnerability was publicly disclosed in November 2025, with technical details initially shared on a Reddit NetSec post linking to a Praetorian blog analysis. No official patches or mitigation guidance have been published at the time of disclosure, emphasizing the need for immediate attention from affected organizations.

For European organizations, the impact of CVE-2025-55315 can be significant due to the widespread use of ASP.NET Core and Kestrel in enterprise web applications and services. Exploitation could lead to unauthorized access to sensitive data, session hijacking, and bypass of security controls, undermining confidentiality and integrity. Attackers could also disrupt service availability by injecting malformed requests or causing application errors. This threat is particularly concerning for industries handling sensitive personal data, such as finance, healthcare, and government sectors, which are heavily regulated under GDPR and other data protection laws. A successful attack could result in data breaches, regulatory fines, reputational damage, and operational downtime. Given the cross-border nature of web applications, the threat could affect multinational corporations and cloud service providers operating in Europe. The absence of known exploits currently provides a window for proactive mitigation, but the potential for rapid weaponization remains high once exploit code becomes available.

Organizations should prepare to apply patches from Microsoft or the ASP.NET Core team as soon as they are released to address this vulnerability in Kestrel. In the interim, network defenders should implement strict input validation and HTTP request normalization at the perimeter to detect and block suspicious chunked transfer encoding patterns. Deploying or tuning Web Application Firewalls (WAFs) to identify anomalies consistent with HTTP Request Smuggling attempts can reduce risk. Monitoring HTTP traffic logs for irregularities in chunk extensions or unexpected request sequences is critical. Organizations should also review their application architecture to minimize reliance on Kestrel as a direct internet-facing server, potentially placing it behind reverse proxies that are known to handle HTTP parsing securely. Security teams should update incident response plans to include scenarios involving HTTP Request Smuggling and conduct penetration testing focused on this vulnerability. Finally, educating developers and operations teams about the risks of HTTP Request Smuggling and secure HTTP parsing practices will help mitigate future risks.