ConnectWise ScreenConnect Breach: Exploiting ASPNET ViewState Deserialization Vulnerability

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A critical vulnerability (CVE-2025-3935) in ConnectWise ScreenConnect allowed attackers to achieve remote code execution (RCE) via ViewState deserialization in ASP.NET. This exploit targeted versions 25.2.3 and earlier, enabling attackers to inject malicious code by forging ViewState data.

Key Details of the Exploit

  • Vulnerable Component: ASP.NET ViewState (used for maintaining web page state).
  • Attack Vector: Tampering with Base64-encoded `__VIEWSTATE` hidden field.
  • Root Cause: Hardcoded or exposed machine keys allowed attackers to bypass integrity checks.
  • Patch: ConnectWise disabled ViewState entirely in the latest update.

Relevant URLs

You Should Know: How to Detect & Mitigate ViewState Exploits

1. Check if Your System is Vulnerable

 Check ASP.NET version (Linux/Windows) 
grep -i "ViewState" /var/www/html/web.config

Windows (PowerShell):

Get-ChildItem "C:\inetpub\wwwroot\" -Recurse -Filter "web.config" | Select-String "ViewState"

2. Verify Machine Key Security

Ensure machine keys are not hardcoded and are dynamically generated:

<!-- Secure web.config Example --> 
<system.web> 
<machineKey validationKey="AutoGenerate" decryptionKey="AutoGenerate" validation="SHA1" /> 
</system.web>

3. Manual Exploit Simulation (For Testing)

Use ysoserial.net to generate a malicious ViewState payload:

 Generate payload 
ysoserial.exe -f ViewState -g TypeConfuseDelegate -o base64 -c "calc.exe"

Then send it via Burp Suite or curl:

curl -X POST "http://vulnerable-site.com/page.aspx" --data "__VIEWSTATE=<malicious_payload>"

4. Patch & Mitigation Steps

  • Disable ViewState if unused:
    <%@ Page EnableViewState="false" %>
  • Enable ViewState MAC (Message Authentication Code):
    <pages enableViewStateMac="true" />
  • Rotate machine keys post-breach:
    Generate new machine keys (Windows) 
Add-Type -AssemblyName System.Web; [System.Web.Security.MachineKey]::Generate()

5. Log Analysis for Exploitation Attempts

Check IIS logs for suspicious ViewState activity:

Get-Content C:\inetpub\logs\LogFiles\.log | Select-String "__VIEWSTATE"

Linux (Apache/Nginx):

grep "__VIEWSTATE" /var/log/nginx/access.log

What Undercode Say

The ConnectWise breach highlights the dangers of insecure deserialization in ASP.NET. Similar flaws (like CVE-2025-30406 in CentreStack/Triofox) prove that hardcoded keys and legacy frameworks remain high-risk.

Key Takeaways

✔ Always disable unused features (like ViewState).

✔ Enforce MAC validation for serialized data.

✔ Monitor logs for abnormal ViewState payloads.

✔ Assume breach: Rotate keys after any exploit attempt.

Expected Output

  • A patched ScreenConnect instance with ViewState disabled.
  • Audited machine keys in all ASP.NET apps.
  • Active monitoring for ViewState tampering in logs.

Prediction

Future attacks will increasingly target legacy enterprise software (like RMM tools) due to their widespread use in MSPs. Expect more deserialization-based RCE exploits in 2025-2026.

Expected Output: More CVEs related to hardcoded credentials and insecure serialization in enterprise apps.

IT/Security Reporter URL:

Reported By: Mthomasson There – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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