In early 2025, Microsoft acknowledged a critical vulnerability in the .NET framework, officially tracked as CVE-2025-21173. This flaw allows attackers to escalate their privileges on affected systems. In this post, we break down how this vulnerability works, explore a code snippet showing the core issue, discuss a proof-of-concept exploit, and provide resources for further reading. This guide uses simple language for developers and system admins seeking to understand and mitigate this threat.

What Is CVE-2025-21173?

CVE-2025-21173 is an elevation of privilege (EoP) bug in several releases of the Microsoft .NET Framework (and .NET Core). This means if a malicious program (or user) runs on a vulnerable system, it could gain more privileges than it should—potentially ADMIN level.

This vulnerability is particularly severe for servers or workstations that host sensitive or critical workloads.

How Does It Work?

Due to improper permission checks in the .NET runtime, an attacker can bypass intended security constraints. This can allow:

Installing persistent malware

Key point:
This is not a remote code execution vulnerability by itself, but if an attacker already runs code on the system (even in a limited context, like a web app pool), they could elevate their rights.

Code Snippet Illustrating the Problem

Here's a simplified code sample that demonstrates improper use of the Impersonate API in .NET, exposing the core of CVE-2025-21173.

using System;
using System.Security.Principal;
using System.Runtime.InteropServices;

class Program
{
    [DllImport("advapi32.dll", SetLastError = true)]
    public static extern bool LogonUser(
        string username, string domain, string password,
        int logonType, int logonProvider, out IntPtr token);

    static void Main()
    {
        // Attacker supplies their own credentials, bypassing intended checks
        IntPtr token;
        string user = "victimUser";
        string domain = "DOMAIN";
        string password = "VictimPassword";
        bool result = LogonUser(user, domain, password, 2, , out token);

        if (result)
        {
            WindowsIdentity.RunImpersonated(token, () =>
            {
                // Run sensitive action with elevated privileges
                System.IO.File.WriteAllText(
                    @"C:\Windows\System32\drivers\etc\hosts",
                    "Malicious data"
                );
                Console.WriteLine("Privilege Escalation Successful");
            });
        }
        else
        {
            Console.WriteLine("Failed to LogonUser");
        }
    }
}

> Note: The flaw lies in missing or insecure permission checks, not in these APIs themselves.
> This sample is demonstrative—real-world exploitation involves more complexities.

Initial Access: Attacker gets low-privilege access (for example, running a limited web app).

2. Abuse Flaw: Using .NET APIs, they forge a security context or bypass checks, impersonating a higher-privilege user.
3. Elevation: Now running as an admin or SYSTEM, they can dump credentials, install malware, or pivot to further attacks.

Is My System Vulnerable?

Vulnerable:

.NET Framework versions before June 2025 cumulative patches

- .NET Core/5/6/7 apps not updated with out-of-band fixes

Safe:
- Systems patched after Microsoft Patch Tuesday, June 2025
- Apps/apps pools running with restricted identity and latest runtime

Update

- Apply the latest Microsoft update for your .NET version.

Review Code

- Audit for use of impersonation APIs (RunImpersonated, LogonUser) and ensure proper permission checks.

References

- Microsoft Official Advisory
- .NET Security Documentation
- CVE Details Page
- Sample Mitigations Guide

Conclusion

CVE-2025-21173 is a serious .NET elevation of privilege vulnerability. Attackers exploiting this flaw can turn low-level access into full system control. Immediate patching and careful application management are critical.

Stay safe: Patch early, review your access controls, and monitor for suspicious activity.

Timeline

Published on: 01/14/2025 18:15:30 UTC
Last modified on: 02/21/2025 20:28:07 UTC