Improper certificate validation in Active Directory Federation Services (AD FS) allows an unauthorized attacker to perform spoofing over a network.

Affected: NuGet: System.Security.Cryptography.Xml

## Executive summary

Microsoft is releasing this security advisory to provide information about a vulnerability in System.Security.Cryptography.Xml. This advisory also provides guidance on what developers can do to update their applications to remove this vulnerability.

`KeyInfoX509Data` can accept serialized certificate formats containing attacker-controlled chain metadata instead of requiring a plain X.509 certificate, contributing to certificate-forgery and token-spoofing paths.

## Announcement

Announcement for this issue can be found at https://github.com/dotnet/announcements/issues/474

## CVSS Details

- **Version:** 3.1 - **Severity:** High - **Score:** 7.4 - **Vector:** `CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N` - **Weakness:** CWE-295 (Improper Certificate Validation)

## Affected Platforms

- **Platforms:** Windows, Linux, macOS - **Architectures:** All

## <a name="affected-packages"></a>Affected Packages

The vulnerability affects Microsoft .NET projects that use an affected package version listed below.

Package name | Affected version | Patched version ------------ | ---------------- | --------------- [System.Security.Cryptography.Xml](https://www.nuget.org/packages/System.Security.Cryptography.Xml) | >= 8.0.0, <= 8.0.4 | 8.0.5

## Advisory FAQ

### <a name="how-affected"></a>How do I know if I am affected?

If using a package listed in [affected packages](#affected-packages), you're exposed to the vulnerability.

### <a name="how-fix"></a>How do I fix…