Developers are advised to check their applications after Microsoft revealed that last week’s ASP.NET Core update inadvertently introduced a serious security flaw into the web framework’s Data Protection Library.
Microsoft describes the issue as a “regression,” coding jargon for an update that breaks something that was previously working correctly.
In this case, what was introduced was a CVSS 9.1-rated critical vulnerability, identified as CVE-2026-40372, that affects ASP.NET’s Core Data Protection application library distributed via the NuGet package manager. It impacts Linux, macOS and other non-Windows OSes, as well as Windows systems where the developer explicitly opted into managed algorithms via the UseCustomCryptographicAlgorithms API.
A bug in the .NET 10.0.6 package, released as part of the Patch Tuesday updates on April 14, causes the ManagedAuthenticatedEncryptor library to compute the validation tag for the Hash-based Message Authentication Code (HMAC) using an incorrect offset.
Incorrect calculation of security hashes results in the .AspNetCore application cookies and tokens being validated and trusted when they shouldn’t be.
“In these cases, the broken validation could allow an attacker to forge payloads that pass DataProtection’s authenticity checks, and to decrypt previously-protected payloads in auth cookies, anti-forgery tokens, TempData, OIDC state, etc,” said Microsoft’s GitHub advisory.
When embedded in applications, these long-lived tokens confer the sort of power attackers quickly jump on. “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,” the advisory noted.
This vulnerability arrives only six months after ASP.NET suffered one of its worst ever flaws, October’s CVSS 9.9-rated CVE-2025-55315 in the Kestrel web server component. But somewhat alarmingly, the current advisory goes on to compare the issue to MS10-070, an emergency patch for CVE-2010-3332, an infamous zero-day vulnerability in the way Windows ASP.NET handled cryptographic errors that caused a degree of panic in 2010.
Not a simple update
Normally, when flaws are uncovered, the drill involves merely applying an update, workaround, or mitigation. In this case, the update itself should have already happened automatically for server builds, taking runtimes to the patched version 10.0.7.
However, for developers using the popular Docker container platform, things are more complicated. For those projects, the Data Protection Library is also embedded in built applications. Addressing this requires updating and rebuilding any ASP.NET Core applications created after the April 14 update.
In addition, those using 10.0.x on the netstandard2.0 or net462 target framework asset from the flawed NuGet package, for compatibility with older operating systems including Windows, are also affected.
Detecting affected binaries
How will developers know if a vulnerable binary has been loaded? Microsoft’s security advisory offers the following advice:
“Check application logs. The clearest symptom is users being logged out and repeated The payload was invalid errors in your logs after upgrading to 10.0.6. Check your project file. Look for a PackageReference to Microsoft.AspNetCore.DataProtection version 10.0.6 in your .csproj file (or in a package that depends on it). You can also run dotnet list package to see resolved package versions.”
In summary, developers should rebuild affected applications to apply the fixed version, expire all affected authentication cookies and tokens to remove forgeries, and rotate to apply new ASP.NET Core Data Protection tokens.
While there is no evidence that the issue has been exploited by attackers, good security hygiene mandates also checking for unexpected or unusual logins failures, errors, or authentication failures, Microsoft advised.
This article originally appeared on CSOonline.
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