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

Detect and mitigate CVE-2026-50659 with GitLab Dependency Scanning

Secure your software supply chain by verifying that all open source dependencies used in your projects contain no disclosed vulnerabilities. Dependency Scanning using SBOM also detects vulnerabilities from proprietary advisories that this site does not include. Learn more about Dependency Scanning using SBOM →