CVE-2026-50659: Microsoft Security Advisory CVE-2026-50659 – .NET Spoofing Vulnerability

July 20, 2026

Microsoft is releasing this security advisory to provide information about a vulnerability in .NET SMTP client (System.Net.Mail). This advisory also provides guidance on what developers can do to update their applications to remove this vulnerability.

A spoofing vulnerability exists in the SMTP client implementation (System.Net.Mail) in .NET 8, .NET 9, and .NET 10, where an attacker can spoof messages during message routing.

References

github.com/advisories/GHSA-74jp-vm22-8q8x
github.com/dotnet/announcements/issues/423
github.com/dotnet/runtime/issues/130716
github.com/dotnet/runtime/security/advisories/GHSA-74jp-vm22-8q8x
msrc.microsoft.com/update-guide/vulnerability/CVE-2026-50659
nvd.nist.gov/vuln/detail/CVE-2026-50659

Code Behaviors & Features

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