CVE-2025-55315 is an HTTP request smuggling vulnerability identified in Microsoft's ASP.NET Core framework. HTTP request smuggling occurs when an attacker crafts specially formed HTTP requests that exploit inconsistencies in how front-end proxies and back-end servers parse and process HTTP headers. This can lead to desynchronization between components, allowing attackers to bypass security controls, access unauthorized information, tamper with file contents, or cause denial-of-service conditions by crashing the server. The vulnerability has been assigned a 'highest ever' severity score by Microsoft, indicating its critical nature. Although specific affected versions are not detailed, ASP.NET Core is widely used for building web applications and APIs, making the attack surface substantial. The vulnerability does not require authentication or user interaction, increasing the risk of exploitation. No public exploits are currently known, but the potential for severe impact on confidentiality, integrity, and availability is high. The lack of a CVSS score necessitates an expert severity assessment based on the described impact and exploitability.

For European organizations, the impact of CVE-2025-55315 could be severe. Information leaks may expose sensitive personal data, intellectual property, or confidential business information, leading to regulatory penalties under GDPR and reputational damage. File content tampering could compromise application integrity, enabling attackers to inject malicious code or alter critical files, potentially facilitating further attacks or data corruption. Server crashes caused by exploitation could result in denial-of-service, disrupting business operations and customer services. Sectors such as finance, healthcare, government, and critical infrastructure, which often rely on ASP.NET Core for web services, are particularly vulnerable. The widespread use of Microsoft technologies in Europe amplifies the risk, and failure to mitigate promptly could lead to significant operational and compliance consequences.

Organizations should prioritize the following mitigations: 1) Monitor Microsoft advisories closely and apply security patches for ASP.NET Core as soon as they are released. 2) Implement strict input validation and HTTP header inspection at web application firewalls (WAFs) and reverse proxies to detect and block malformed or suspicious HTTP requests indicative of request smuggling attempts. 3) Employ layered security controls such as network segmentation and least privilege to limit the impact of potential breaches. 4) Conduct thorough security testing, including fuzzing and penetration testing, focusing on HTTP request parsing and handling. 5) Enable detailed logging and monitoring of HTTP traffic to identify anomalies early. 6) Educate development and operations teams about the risks of HTTP request smuggling and secure coding practices. These steps go beyond generic patching by emphasizing proactive detection and defense-in-depth strategies.