CVE-2026-40372: ASP.NET Core Privilege Escalation Flaw

CVE-2026-40372 Overview

CVE-2026-40372 is a critical cryptographic signature verification vulnerability in ASP.NET Core that enables unauthorized attackers to elevate privileges over a network. The flaw stems from improper verification of cryptographic signatures (CWE-347), allowing malicious actors to bypass authentication controls and gain unauthorized access to protected resources.

Critical Impact

Unauthorized network-based attackers can exploit improper cryptographic signature verification to achieve privilege escalation, potentially compromising confidentiality and integrity of affected systems.

Affected Products

• ASP.NET Core (specific versions not disclosed)

Discovery Timeline

• 2026-04-21 - CVE-2026-40372 published to NVD
• 2026-04-22 - Last updated in NVD database

Technical Details for CVE-2026-40372

Vulnerability Analysis

This vulnerability involves improper verification of cryptographic signatures within ASP.NET Core's authentication mechanisms. When signature validation is bypassed or insufficiently implemented, attackers can forge authentication tokens or certificates that the application incorrectly accepts as legitimate. This allows unauthorized users to impersonate privileged accounts or access restricted functionality without proper credentials.

The network-based attack vector means exploitation requires no prior authentication or user interaction, significantly increasing the risk exposure for internet-facing ASP.NET Core applications. Successful exploitation results in high impact to both confidentiality and integrity, though availability remains unaffected.

Root Cause

The vulnerability is classified as CWE-347: Improper Verification of Cryptographic Signature. This weakness occurs when software does not properly verify that data has been digitally signed by a trusted entity. In the context of ASP.NET Core, the signature verification logic fails to properly validate cryptographic signatures, allowing forged or manipulated signed data to be accepted as authentic.

Attack Vector

The attack is conducted over the network without requiring authentication or user interaction. An attacker can craft malicious requests containing forged cryptographic signatures that exploit the improper verification logic. When the vulnerable ASP.NET Core application processes these requests, it incorrectly accepts the forged signatures as valid, granting the attacker elevated privileges.

The exploitation mechanism typically involves:

1. Analyzing the signature verification implementation in the target application
2. Crafting authentication tokens or signed payloads with manipulated signatures
3. Submitting the forged requests to the vulnerable endpoint
4. Gaining unauthorized access with elevated privileges

For detailed technical information, refer to the Microsoft CVE-2026-40372 Advisory.

Detection Methods for CVE-2026-40372

Indicators of Compromise

• Authentication events with malformed or unusual cryptographic signatures in application logs
• Unexpected privilege escalation events or administrative actions by non-privileged users
• Authentication tokens with invalid or tampered signature components
• Anomalous access patterns to protected resources without proper credential chains

Detection Strategies

• Implement signature validation auditing to log all cryptographic signature verification attempts and failures
• Monitor ASP.NET Core application logs for authentication anomalies and privilege escalation patterns
• Deploy network intrusion detection rules to identify malformed authentication tokens in HTTP traffic
• Utilize SentinelOne Singularity to detect anomalous process behavior indicative of exploitation attempts

Monitoring Recommendations

• Enable verbose logging for ASP.NET Core authentication subsystems to capture signature verification events
• Configure SIEM alerts for unusual patterns of failed authentication followed by successful privileged access
• Monitor for lateral movement following any detected authentication anomalies
• Review access logs for resources that should require cryptographic authentication

How to Mitigate CVE-2026-40372

Immediate Actions Required

• Review the Microsoft CVE-2026-40372 Advisory for official patch information
• Apply the latest ASP.NET Core security updates as soon as they are available
• Audit current cryptographic signature verification implementations in affected applications
• Implement additional authentication layers such as multi-factor authentication as defense-in-depth

Patch Information

Microsoft has published an official security advisory for this vulnerability. Administrators should consult the Microsoft CVE-2026-40372 Advisory for specific patch details, affected version information, and update guidance.

Workarounds

• Restrict network access to affected ASP.NET Core applications using firewall rules and network segmentation
• Implement additional authentication validation at the application gateway or reverse proxy level
• Enable strict cryptographic signature validation policies where configurable
• Monitor and audit all privileged actions until patches can be applied