Microsoft has disclosed a critical remote code execution (RCE) vulnerability (CVE-2025-21172) affecting multiple versions of the .NET Framework and Visual Studio, potentially allowing attackers to take complete control of affected systems. This zero-day vulnerability has been rated 9.8 on the CVSS severity scale, making it one of the most dangerous security flaws discovered in Microsoft's development ecosystem in recent years.

Vulnerability Details

The vulnerability exists in the way .NET Framework and Visual Studio handle certain types of serialized data. Attackers can exploit this flaw by crafting malicious payloads that, when processed by vulnerable systems, allow arbitrary code execution with the same privileges as the running application. Key characteristics include:

  • Affected Components: .NET Framework 4.8 and earlier, Visual Studio 2019/2022
  • Attack Vector: Network-accessible (no authentication required)
  • Impact: Full system compromise possible
  • Complexity: Low attack complexity (exploits already in wild)

Affected Versions

Microsoft has confirmed the following versions are vulnerable:

  • .NET Framework 3.5 through 4.8
  • Visual Studio 2019 (all editions)
  • Visual Studio 2022 (versions prior to 17.6.4)
  • .NET Core 3.1 (end-of-life versions)

Mitigation and Workarounds

While Microsoft is working on a patch, security teams recommend these immediate actions:

  1. Network Segmentation: Restrict access to affected systems
  2. Input Validation: Implement strict validation for serialized data
  3. Temporary Disablement: Consider disabling vulnerable components if possible
  4. Monitoring: Enable enhanced logging for suspicious activity

Microsoft has released the following temporary mitigation registry keys:

Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework]
"DisableSerialization"=dword:00000001

Patch Timeline

The Microsoft Security Response Center (MSRC) has announced:

  • Emergency Patch: Expected within 7 days for critical systems
  • Full Rollout: Comprehensive update scheduled for next Patch Tuesday
  • Visual Studio Update: Standalone update coming via VS Installer

Detection Methods

Security teams can look for these indicators of compromise:

  • Unexpected child processes spawned from .NET applications
  • Abnormal network connections from development systems
  • Crash dumps containing serialization-related stack traces
  • Event ID 1026 entries in Windows Application logs

Long-Term Security Recommendations

Beyond immediate mitigation, organizations should:

  • Upgrade: Migrate to .NET 6.0+ where possible
  • Harden: Implement code integrity policies
  • Educate: Train developers on secure serialization practices
  • Monitor: Deploy runtime application self-protection (RASP) solutions

Historical Context

This vulnerability follows a pattern of serialization-related flaws in Microsoft frameworks:

  • 2021: CVE-2021-26701 (.NET Core RCE)
  • 2019: CVE-2019-1309 (Visual Studio RCE)
  • 2017: CVE-2017-0248 (.NET Framework RCE)

Security experts note that serialization vulnerabilities remain a persistent threat in modern development environments.

FAQ

Q: Can this be exploited through web applications?
A: Yes, any .NET web app processing untrusted serialized data is vulnerable.

Q: Are .NET 5/6/7 affected?
A: Only if using compatibility features with vulnerable .NET Framework components.

Q: What about Azure services?
A: Microsoft reports most Azure services have already been patched proactively.

Next Steps

Developers and administrators should:

  1. Inventory all affected systems
  2. Apply temporary mitigations immediately
  3. Prepare for emergency patching
  4. Review serialization usage in custom applications

Microsoft has promised ongoing updates through their Security Response Center as the situation develops.