Microsoft has issued emergency out-of-band security updates to address a critical vulnerability in ASP.NET Core that could allow attackers to gain elevated system privileges. The flaw, tracked as CVE-2026-40372, affects the Data Protection component used widely in web applications to secure sensitive data such as authentication cookies and tokens.
The issue stems from a flaw in how cryptographic validation was handled in recent .NET updates. Due to this error, the system could incorrectly validate tampered data, potentially allowing attackers to forge authentication cookies. In practical terms, this means an unauthenticated attacker could impersonate a privileged user and gain SYSTEM-level access on affected systems.
Microsoft identified the vulnerability after users reported issues with data decryption following a recent update. Upon investigation, it was discovered that the cryptographic mechanism responsible for verifying data integrity was not functioning correctly in certain cases. This created a window where malicious payloads could bypass security checks and be treated as legitimate.
The implications are serious. If exploited, attackers could not only escalate privileges but also access sensitive data, manipulate application behavior, or generate valid authentication tokens for continued access—even after systems are patched. This persistence risk makes immediate remediation critical.
To address the issue, Microsoft has released updated packages and strongly urges organizations to upgrade to the latest version without delay. In addition to patching, security teams are advised to rotate cryptographic keys to invalidate any potentially compromised tokens and ensure that forged credentials can no longer be used.
This incident highlights how even small flaws in cryptographic implementations can lead to significant security risks. It also underscores the importance of rapid patching and proactive monitoring, especially for widely used frameworks like ASP.NET Core that form the backbone of many enterprise applications.
With active risks tied to authentication and access control, organizations should treat this update as a top priority to prevent potential compromise and protect sensitive systems.
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