CVE-2025-55315 is a critical vulnerability classified under CWE-444, involving inconsistent interpretation of HTTP requests, commonly known as HTTP request/response smuggling, in Microsoft ASP.NET Core 8.0. This vulnerability allows an authorized attacker—meaning one with some level of privileges but no user interaction required—to craft specially formed HTTP requests that are interpreted differently by front-end proxies and the ASP.NET Core backend. This discrepancy enables the attacker to bypass security controls, potentially injecting malicious requests or responses that can manipulate session handling, authentication, or caching mechanisms. The vulnerability is exploitable remotely over the network with low complexity, and it affects confidentiality, integrity, and availability of the web applications running on ASP.NET Core 8.0. The CVSS v3.1 score of 9.9 reflects the critical nature of this flaw, with high impact on confidentiality and integrity, low attack complexity, and no user interaction needed. Although no public exploits have been observed yet, the vulnerability's characteristics make it a prime target for attackers once exploit code becomes available. The lack of patch links suggests that a fix may be pending or in development, emphasizing the need for vigilance. This vulnerability is particularly concerning for organizations relying on ASP.NET Core 8.0 for web services, as it can be leveraged to bypass security features and potentially gain unauthorized access or disrupt services.

The impact on European organizations is significant due to the widespread use of Microsoft ASP.NET Core 8.0 in enterprise web applications and services. Exploitation can lead to unauthorized access to sensitive data, session hijacking, bypass of authentication and authorization controls, and potential service disruption. This can result in data breaches, compliance violations (e.g., GDPR), reputational damage, and operational downtime. Given the critical severity and network exploitability, attackers can remotely compromise systems without user interaction, increasing the risk of large-scale attacks. Organizations in sectors such as finance, government, healthcare, and critical infrastructure that rely heavily on ASP.NET Core 8.0 are particularly vulnerable. The ability to bypass security features can also facilitate further lateral movement and privilege escalation within networks, compounding the threat. The absence of known exploits in the wild currently provides a window for proactive defense, but the high severity demands immediate attention.

1. Monitor Microsoft security advisories closely and apply official patches immediately once released to address CVE-2025-55315. 2. Until patches are available, implement strict HTTP request validation at the network edge, including web application firewalls (WAFs) configured to detect and block malformed or suspicious HTTP requests indicative of request smuggling attempts. 3. Employ layered security controls such as reverse proxies and API gateways that correctly parse and normalize HTTP requests to prevent inconsistent interpretation. 4. Conduct thorough code reviews and testing of custom middleware or proxy configurations that handle HTTP requests to identify and remediate potential parsing inconsistencies. 5. Enable detailed logging and monitoring of HTTP traffic to detect anomalies or patterns consistent with request smuggling attacks. 6. Limit privileges of accounts that can send HTTP requests to sensitive endpoints to reduce the risk posed by authorized attackers. 7. Educate development and security teams about HTTP request smuggling risks and mitigation techniques specific to ASP.NET Core environments. 8. Consider network segmentation to isolate critical web services and reduce the blast radius of potential exploitation.