Microsoft Details ASP.NET Vulnerability That Enables Attackers To Smuggle HTTP Requests
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Microsoft has issued a critical security update for ASP.NET Core to address CVE-2025-55315, a high-severity flaw that enables HTTP request smuggling and could allow attackers to bypass key security controls.<\/p>\n
Disclosed on October 14, 2025, this vulnerability has a CVSS v3.1 score of 9.9, making it one of the most severe issues ever reported in the ASP.NET ecosystem.<\/p>\n
The flaw stems from inconsistent handling of HTTP requests in the Kestrel web server component, which could let authenticated attackers inject hidden requests to escalate privileges or access sensitive data.<\/p>\n
While HTTP request smuggling is a well-known attack vector, this specific implementation in ASP.NET Core amplifies risks for web applications relying on the framework for authentication<\/a> and authorization.<\/p>\n Attackers exploit discrepancies between how proxies and servers parse headers like Content-Length and Transfer-Encoding, smuggling malicious payloads that evade normal processing. <\/p>\n For instance, a crafted POST request might embed a concealed GET to an admin endpoint, tricking the system into executing unauthorized actions without detection.<\/p>\n At its core, HTTP request smuggling leverages parsing inconsistencies across network components, such as front-end proxies and back-end servers. <\/p>\n An attacker sends a request with ambiguous headers, like combining Content-Length and Transfer-Encoding, causing the proxy to interpret it one way while the server sees the smuggled content differently. <\/p>\n This can result in the second request bypassing rate limits, CSRF protections, or even authentication checks, leading to severe outcomes in multi-tiered environments.<\/p>\n In the CVE-2025-55315<\/a> case, the Kestrel server\u2019s failure to validate request boundaries under certain conditions allows smuggled requests to reach application logic intact.<\/p>\n This affects all supported ASP.NET Core versions, including 8.0, 9.0, and 10.0 previews, particularly in setups with reverse proxies like NGINX or Azure Front Door. <\/p>\n Exploitation requires network access and often low privileges, but the scope can extend to confidential data exposure or server crashes in worst-case scenarios.<\/p>\n The vulnerability\u2019s high score underscores its potential for chained attacks, from session hijacking to server-side request forgery. <\/p>\n Not all applications are equally exposed; risks heighten if custom request parsing, header-based decisions, or skipped validations are in play. <\/p>\n For regulated sectors handling sensitive data, unpatched systems could face compliance violations alongside direct threats like privilege escalation.<\/p>\nUnderstanding HTTP Request Smuggling<\/strong><\/h2>\n