Microsoft fixed critical ASP.NET Core vulnerability, tracked as CVE-2026-40372 (CVSS score of 9.1), that lets attackers escalate privileges.

Microsoft released out-of-band updates to address a serious ASP.NET Core vulnerability tracked as CVE-2026-40372 (CVSS score of 9.1). Microsoft fixed the flaw in ASP.NET Core version 10.0.7.

An attacker could exploit the flaw to gain SYSTEM-level privileges, access sensitive files, and modify data, but they cannot disrupt system availability.

An anonymous researcher reported the flaw, prompting out-of-band patches to reduce risk and protect affected systems.

“Improper verification of cryptographic signature in ASP.NET Core allows an unauthorized attacker to elevate privileges over a network.” reads the advisory. “An attacker who successfully exploited this vulnerability could gain SYSTEM privileges. Exploiting this vulnerability could allow an attacker to disclose files and modify data, but the attacker cannot impact the availability of the system.”

According to Microsoft, the exploitation of the flaw in attacks in the wild is currently less likely.

According to Microsoft, a bug in Microsoft.AspNetCore.DataProtection 10.0.0–10.0.6 caused incorrect HMAC validation, sometimes ignoring the correct hash. This could let attackers forge or decrypt protected data like cookies and antiforgery tokens, possibly impersonating users and getting valid sessions or tokens. Even after upgrading to 10.0.7, old tokens may remain valid unless the key ring is rotated.

“If an attacker used forged payloads to authenticate as a privileged user during the vulnerable window, they may have induced the application to issue legitimately-signed tokens (session refresh, API key, password reset link, etc.) to themselves.” states Microsoft. “Those tokens remain valid after upgrading to 10.0.7 unless the DataProtection key ring is rotated.”

The tech giant states that the exploitation requires three conditions: the app uses Microsoft.AspNetCore.DataProtection 10.0.6 from NuGet (directly or via dependencies like StackExchangeRedis), the NuGet version is loaded at runtime, and the application runs on a non-Windows OS such as Linux or macOS.

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Pierluigi Paganini

(SecurityAffairs – hacking, ASP.NET Core)

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