CVE-2025-55315: ASP.NET Core Auth Bypass Vulnerability

CVE-2025-55315 Overview

CVE-2025-55315 is a critical HTTP Request Smuggling vulnerability affecting Microsoft ASP.NET Core applications. This vulnerability stems from inconsistent interpretation of HTTP requests between front-end proxies and back-end ASP.NET Core servers, allowing an authorized attacker to bypass security features over a network. HTTP Request Smuggling attacks exploit discrepancies in how intermediary systems and servers parse HTTP request boundaries, enabling attackers to inject malicious requests that bypass security controls.

Critical Impact

Authenticated attackers can exploit this vulnerability to bypass security features, potentially gaining unauthorized access to protected resources, hijacking user sessions, or performing actions on behalf of other users across network boundaries.

Affected Products

• Microsoft ASP.NET Core (multiple versions)
• Microsoft Visual Studio 2022 (multiple versions)
• Applications built on vulnerable ASP.NET Core frameworks

Discovery Timeline

• 2025-10-14 - CVE-2025-55315 published to NVD
• 2025-10-28 - Last updated in NVD database

Technical Details for CVE-2025-55315

Vulnerability Analysis

This vulnerability is classified under CWE-444 (Inconsistent Interpretation of HTTP Requests). HTTP Request Smuggling occurs when front-end servers and back-end servers disagree on where one HTTP request ends and another begins. In the case of CVE-2025-55315, ASP.NET Core's HTTP request parsing logic can be exploited to smuggle malicious requests through security controls.

The vulnerability allows an authenticated attacker to craft specially formed HTTP requests that are interpreted differently by load balancers, reverse proxies, or web application firewalls compared to the ASP.NET Core backend. This inconsistency enables the attacker to "smuggle" a second request inside the first, bypassing authentication checks, access controls, and other security mechanisms.

The scope change indicated in the vulnerability assessment means successful exploitation can affect resources beyond the vulnerable component's security scope, potentially compromising other users' sessions or accessing protected endpoints.

Root Cause

The root cause lies in inconsistent parsing of HTTP request boundaries within ASP.NET Core's request handling pipeline. Specifically, the vulnerability emerges from differences in how Content-Length and Transfer-Encoding headers are processed, or how chunked encoding boundaries are interpreted. When ASP.NET Core and upstream proxies disagree on request demarcation, attackers can inject additional HTTP requests that bypass security validation performed by the proxy layer.

Attack Vector

The attack vector is network-based and requires low privileges (authenticated access) with no user interaction needed. An attacker would:

1. Identify an ASP.NET Core application behind a reverse proxy or load balancer
2. Craft a malformed HTTP request with ambiguous boundaries
3. Send the request through the proxy, which interprets it as a single request
4. The ASP.NET Core backend interprets the payload as two requests
5. The "smuggled" second request bypasses proxy-level security controls

For detailed technical analysis of the exploitation mechanics, refer to Andrew Lock's analysis which provides an in-depth breakdown of the vulnerability. A proof-of-concept demonstrating the attack is also available in the GitHub Gist by N3mes1s.

Detection Methods for CVE-2025-55315

Indicators of Compromise

• Unusual HTTP request patterns with conflicting Content-Length and Transfer-Encoding headers
• Backend logs showing requests that did not appear in proxy access logs
• User complaints of session hijacking or unexpected authentication failures
• Anomalous request sequences in web server logs with malformed or duplicate headers

Detection Strategies

• Deploy web application firewall rules to detect and block requests with ambiguous HTTP boundaries
• Enable verbose logging on both proxy and ASP.NET Core backend to identify request parsing discrepancies
• Monitor for HTTP requests containing both Content-Length and Transfer-Encoding: chunked headers
• Implement intrusion detection signatures for known HTTP smuggling patterns

Monitoring Recommendations

• Compare request counts between front-end proxies and backend servers to identify smuggled requests
• Set up alerts for HTTP 400 errors that may indicate malformed smuggling attempts
• Monitor authentication logs for sessions being accessed from unexpected contexts
• Review application logs for actions performed without corresponding proxy-level authentication

How to Mitigate CVE-2025-55315

Immediate Actions Required

• Apply the latest Microsoft security updates for ASP.NET Core immediately
• Update Visual Studio 2022 to the latest patched version
• Review proxy and load balancer configurations to normalize HTTP requests before forwarding to backends
• Temporarily disable chunked transfer encoding if feasible until patches are applied

Patch Information

Microsoft has released security updates to address this vulnerability. Organizations should apply the official patches available through the Microsoft Security Response Center (MSRC) advisory. Ensure all ASP.NET Core runtimes and Visual Studio 2022 installations are updated to versions that include the fix.

Workarounds

• Configure reverse proxies to strictly normalize HTTP requests, rejecting ambiguous boundary conditions
• Implement WAF rules to block requests with conflicting Content-Length and Transfer-Encoding headers
• Use HTTP/2 end-to-end where possible, as it is not susceptible to traditional HTTP smuggling attacks
• Ensure front-end servers and backends use consistent HTTP parsing configurations

# Example: Configure nginx to reject ambiguous requests
# Add to nginx.conf server block to help mitigate smuggling attempts

# Reject requests with both Content-Length and Transfer-Encoding
if ($http_transfer_encoding ~* "chunked") {
    set $smuggle_check "chunked";
}
if ($http_content_length) {
    set $smuggle_check "${smuggle_check}+length";
}
if ($smuggle_check = "chunked+length") {
    return 400;
}