Package 'MailKit' 4.7.1.1 has a known moderate severity vulnerability,

 Warning As Error: Package 'MailKit' 4.7.1.1 has a known moderate severity vulnerability, https://github.com/advisories/GHSA-9j88-vvj5-vhgr

The error message you're seeing is a security safeguard. Because your project is likely configured with TreatWarningsAsErrors enabled, NuGet is blocking your build due to a known security vulnerability in MailKit 4.7.1.1.

The Vulnerability: GHSA-9j88-vvj5-vhgr

This is a STARTTLS Response Injection vulnerability.

The Risk: A Man-in-the-Middle (MitM) attacker can inject commands during the "handshake" phase when your app switches from a plain-text connection to an encrypted one (TLS).

The Impact: This can be used to trick your application into "downgrading" its security—for example, forcing it to use a less secure authentication method (like sending passwords in plain text) even though you think you're using an encrypted connection.

How to Fix It

The fix is straightforward: Upgrade MailKit to a patched version.

1. Update via .NET CLI

Run the following command in your terminal at the project root:

Bash

dotnet add package MailKit --version 4.8.0

(Or the latest available version, as 4.8.0+ contains the fix).

2. Update via Package Manager Console (Visual Studio)

PowerShell

Update-Package MailKit

3. Update via Project File (.csproj)

Locate the PackageReference in your .csproj file and update the version manually:

XML

<PackageReference Include="MailKit" Version="4.8.0" />

Why am I seeing this now?

The .NET SDK and NuGet now integrate with the GitHub Advisory Database. When you build your project, it checks your dependencies against known security threats.

If you absolutely cannot upgrade right now (which is not recommended) and need to bypass the error to unblock your build, you can temporarily "suppress" this specific warning by adding the following to your .csproj file, though you should only do this if you have a specific mitigation in place:

XML

<PropertyGroup>

  <NoWarn>$(NoWarn);NU1901;NU1902;NU1903;NU1904</NoWarn>

</PropertyGroup>

Recommendation: Stick with the upgrade. MailKit updates are generally very stable and backward-compatible within the same major version