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 occurs when the server and intermediary components interpret the boundaries of HTTP requests differently, allowing an attacker to craft specially formed HTTP requests that bypass security controls. An authorized attacker with network access can exploit this flaw to smuggle malicious requests past security mechanisms such as firewalls, reverse proxies, or application gateways. The attacker can manipulate HTTP headers to desynchronize the request parsing between front-end and back-end servers, potentially leading to unauthorized access, data leakage, request hijacking, or injection of malicious payloads. The CVSS v3.1 score of 9.9 indicates critical severity, with attack vector being network-based, low attack complexity, requiring privileges but no user interaction, and resulting in high confidentiality and integrity impact, and low availability impact. The vulnerability affects only ASP.NET Core 8.0, a widely used framework for building web applications and APIs. No patches or exploits are currently publicly available, but the potential for severe impact is high given the nature of the flaw and the criticality of the affected platform.

For European organizations, the impact of CVE-2025-55315 could be severe. ASP.NET Core 8.0 is commonly used in enterprise web applications, government portals, and critical infrastructure systems across Europe. Exploitation could allow attackers to bypass security controls, leading to unauthorized data access, manipulation of sensitive information, and potential disruption of services. Confidentiality breaches could expose personal data protected under GDPR, resulting in regulatory penalties and reputational damage. Integrity compromises could allow attackers to alter application behavior or inject malicious content, facilitating further attacks such as phishing or malware distribution. Although availability impact is rated low, targeted attacks could still disrupt critical services. The requirement for an authorized attacker implies that insider threats or compromised credentials could be leveraged, increasing risk in environments with complex access controls. The absence of known exploits currently provides a window for proactive defense, but the critical severity demands urgent attention.

1. Immediate application of official patches from Microsoft once released is paramount; monitor Microsoft security advisories closely. 2. Until patches are available, implement strict HTTP request validation and normalization at all network boundaries, including web application firewalls (WAFs) and reverse proxies, to detect and block malformed or suspicious HTTP headers indicative of request smuggling attempts. 3. Employ network segmentation and zero-trust principles to limit the ability of an attacker with limited privileges to reach vulnerable ASP.NET Core 8.0 instances. 4. Conduct thorough code reviews and security testing of applications built on ASP.NET Core 8.0 to identify and remediate any additional request parsing inconsistencies. 5. Monitor logs and network traffic for anomalies such as unexpected HTTP header sequences or desynchronized request/response patterns. 6. Enforce strict authentication and authorization controls to reduce the risk posed by authorized attackers. 7. Educate development and security teams about HTTP request smuggling risks and detection techniques. 8. Consider deploying runtime application self-protection (RASP) solutions that can detect and block suspicious HTTP request manipulations in real time.