Debugging RDP Heap Corruption in mstsc.exe (ntdll.dll 0xc0000374 Crash After Password Entry)


2 views

When attempting remote desktop connections from Windows 10 workstations, many developers encounter a sudden crash after credential submission. The error manifests as:

Exception Code: 0xc0000374 (STATUS_HEAP_CORRUPTION)
Faulting Module: ntdll.dll (Windows NT Layer DLL)
Trigger Process: mstsc.exe (Remote Desktop Client)

Heap corruption in mstsc.exe typically occurs due to:

  • Memory mismanagement in credential handling routines
  • Third-party credential provider interference
  • Corrupted RDP session cache
  • Mismatched system DLL versions (particularly credssp.dll)

Capture a crash dump and analyze with:

.symfix
.reload
!analyze -v
!heap -s
!heap -p -a [faulting_address]

Registry Workaround

Disable CredSSP caching temporarily:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System\CredSSP\Parameters]
"AllowEncryptionOracle"=dword:00000002

DLL Reset Procedure

Replace potentially corrupted system files:

DISM /Online /Cleanup-Image /RestoreHealth
sfc /scannow

Network-Level Fixes

For development environments, try forcing TLS 1.2:

# PowerShell snippet
Set-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Client' -Name 'DisabledByDefault' -Value 0
Set-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Client' -Name 'Enabled' -Value 1

When building custom RDP clients using MSTSC libraries:

// C# example of proper credential disposal
public class SecureRdpConnection : IDisposable {
    private NetworkCredential _credential;
    
    public void Connect(string server) {
        using (var rdp = new AxMSTSCLib.AxMsRdpClient9NotSafeForScripting()) {
            rdp.Server = server;
            rdp.UserName = _credential.UserName;
            // Always encrypt in transit
            rdp.AdvancedSettings9.EncryptionEnabled = true;
        }
    }

    public void Dispose() {
        if (_credential != null) {
            _credential.SecurePassword.Dispose();
            _credential = null;
        }
    }
}

For the past week, I've been battling this exact RDP crash scenario across multiple Windows 10 machines. The error pattern is consistent:

Exception Code: 0xc0000374 (STATUS_HEAP_CORRUPTION)
Faulting Module: ntdll.dll (the Windows NT layer DLL)
Trigger Point: Immediately after password submission

Here's what I've observed consistently triggers the issue:

  • Windows 10 build 10586 (Threshold 2)
  • MSTSC version 10.0.10586.0
  • Both domain-joined and workgroup machines affected
  • Occurs with both FQDN and IP connections

After attaching WinDbg to mstsc.exe, here's the critical stack trace:

0:000> !analyze -v
*******************************************************************************
*                                                                             *
*                        Exception Analysis                                   *
*                                                                             *
*******************************************************************************

FAULTING_IP: 
ntdll!RtlpFreeHeap+dc
00007ffb7a6ee71c 488b8c2480000000 mov     rcx,qword ptr [rsp+80h]

EXCEPTION_RECORD:  (.exr -1)
ExceptionAddress: 00007ffb7a6ee71c (ntdll!RtlpFreeHeap+0x00000000000000dc)
   ExceptionCode: c0000374 (Heap corruption)

Through packet analysis (Wireshark), I noticed the crash consistently occurs during CredSSP negotiation. Here's a snippet of the relevant network flow:

Frame 42: 348 bytes on wire (2784 bits)
Transport Layer Security
    TLSv1.2 Record Layer: Handshake Protocol: Client Key Exchange
    CredSSP Protocol: TSPasswordCreds
        [Expert Info (Chat/Sequence): CredSSP auth attempt]

After testing dozens of variations, these are the solutions that resolved the issue:

Option 1: Registry Patch

Create this registry key to disable the problematic heap protection:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\mstsc.exe]
"HeapFlags"=dword:00000002

Option 2: CredSSP Version Forcing

Force an older CredSSP version via group policy:

gpedit.msc → Computer Configuration → Administrative Templates → System → Credentials Delegation
Set "Allow Delegating Default Credentials with NTLM-only Server Authentication" to Enabled

Option 3: .NET Framework Repair

Surprisingly, this often-overlooked fix worked for many cases:

DISM /Online /Cleanup-Image /RestoreHealth
sfc /scannow

The root cause appears to be a race condition in the credential serialization buffer. Microsoft has addressed this in later builds. The most reliable solution is to:

  1. Update to Windows 10 1607 (Anniversary Update) or later
  2. Apply KB4022715 if staying on Threshold 2

Here's a PowerShell snippet to check if your system is vulnerable:

function Test-RDPHeapVulnerability {
    $mstsc = Get-Item "$env:windir\system32\mstsc.exe"
    $ntdll = Get-Item "$env:windir\system32\ntdll.dll"
    
    return ($mstsc.VersionInfo.FileVersion -eq "10.0.10586.0") -and 
           ($ntdll.VersionInfo.FileVersion -like "10.0.10586.*")
}

if (Test-RDPHeapVulnerability) {
    Write-Warning "System appears vulnerable to RDP heap corruption crash"
}