CVE-2025-55315 is a critical vulnerability classified under CWE-444 (Inconsistent Interpretation of HTTP Requests), commonly known as HTTP request/response smuggling, affecting Microsoft ASP.NET Core 8.0. This vulnerability stems from discrepancies in how HTTP requests are parsed and interpreted by the ASP.NET Core framework, allowing an attacker with authorized network access to craft specially formed HTTP requests that bypass security controls. The inconsistency can cause the server to misinterpret the boundaries between HTTP requests and responses, enabling attackers to smuggle malicious requests that evade detection by security mechanisms such as firewalls or intrusion detection systems. The vulnerability requires low attack complexity and only privileges equivalent to an authorized user, with no user interaction needed, making it easier to exploit in targeted environments. The impact includes potential unauthorized access to sensitive data (confidentiality), modification of data or application behavior (integrity), and partial denial of service (availability). The CVSS v3.1 score of 9.9 reflects the critical severity, with network attack vector, low complexity, and partial privileges required. While no public exploits are currently known, the vulnerability poses a significant risk due to the widespread adoption of ASP.NET Core 8.0 in enterprise and cloud environments. The lack of available patches at the time of publication necessitates immediate attention to mitigation strategies to reduce exposure.

For European organizations, the impact of CVE-2025-55315 is substantial given the extensive use of Microsoft ASP.NET Core 8.0 in web applications across sectors such as finance, government, healthcare, and critical infrastructure. Exploitation could lead to unauthorized access to sensitive personal and corporate data, undermining GDPR compliance and exposing organizations to regulatory penalties and reputational damage. Integrity violations may allow attackers to alter application logic or inject malicious content, potentially facilitating further compromise or fraud. Availability impacts, though less severe, could disrupt essential services, affecting business continuity. The vulnerability's network-based exploitation vector increases the risk of remote attacks, especially in environments with exposed web services or insufficient network segmentation. European entities with complex supply chains and interconnected systems may face cascading effects, amplifying the threat. The absence of known exploits provides a window for proactive defense, but the critical severity demands urgent mitigation to prevent potential exploitation as threat actors develop attack tools.

1. Apply official security patches from Microsoft immediately once they become available to address the vulnerability directly. 2. Until patches are released, implement strict input validation and normalization on HTTP headers and request bodies to detect and block malformed or suspicious requests that could exploit parsing inconsistencies. 3. Deploy and properly configure Web Application Firewalls (WAFs) with updated rulesets capable of detecting HTTP request/response smuggling patterns, including anomalies in Content-Length and Transfer-Encoding headers. 4. Conduct thorough network segmentation to limit exposure of critical web applications to untrusted networks, reducing the attack surface. 5. Monitor network traffic and server logs for unusual HTTP request patterns or errors indicative of smuggling attempts. 6. Educate development and security teams about the risks of HTTP request smuggling and encourage secure coding practices that avoid ambiguous HTTP parsing. 7. Review and harden reverse proxy and load balancer configurations to ensure consistent HTTP request parsing across all components. 8. Perform penetration testing and vulnerability scanning focused on HTTP request smuggling to identify and remediate weaknesses proactively.