EDRStartupHinder: EDR Startup Process Blocker

Summary

The article outlines a research tool called EDRStartupHinder that abuses the Windows Bindlink mechanism to redirect a critical System32 DLL load, causing a protected EDR process to crash during startup. By installing a higher-priority service and supplying an unsigned replacement DLL, the method prevents the EDR component from initializing, effectively stripping the host of endpoint protection at boot.

Investigation

The author determined that Microsoft Defender loads msvcp_win.dll during startup and that a service associated with the TDI group launches earlier than Defender. The tool copies the legitimate DLL, alters the PE header to create a tampered variant, registers a service named DusmSVC-01, and monitors the Defender process (MsMpEng.exe). It then uses Bindlink to redirect the DLL resolution so that Defender attempts to load the unsigned DLL. Because the process runs under PPL protection, the unsigned module load triggers termination of the Defender process.

Mitigation

Monitor for suspicious or unexpected service creation events and for telemetry indicating bindlink.dll usage. Enforce integrity validation for System32 DLLs and alert on unexpected duplicates, mutations, or load-path redirections. Reduce attack feasibility by restricting write permissions that could enable DLL redirection, tightening controls around KnownDLLs, and enforcing code-signing policies where operationally viable.

Response

If a new service named DusmSVC-01 or anomalous Bindlink behavior is detected, isolate the endpoint and validate the integrity of key System32 DLLs. Restore any altered files from trusted sources or backups, then remove the malicious service and any related artifacts. Complete a forensic review to identify additional persistence, privilege escalation, or follow-on activity associated with the startup-time protection bypass.

Simulation Execution

Prerequisite: The Telemetry & Baseline Pre‑flight Check must have passed.

Attack Narrative & Commands

The adversary aims to cripple the endpoint detection platform by delivering a tampered copy of msvcp_win.dll. The steps are:

1. Locate the trusted DLL in C:WindowsSystem32.
2. Copy it to a hidden attacker‑controlled directory whose name includes “FakeLib” (e.g., C:ProgramDataFakeLib).
3. Corrupt the signature by appending arbitrary bytes to the end of the file, ensuring the binary hash changes and the Authenticode signature becomes invalid.
4. Trigger the EDR’s load path (not simulated here) – the presence of the malformed DLL will cause the EDR to fail during its initialization.

Regression Test Script

# ---------------------------------------------------------------
# EDRStartupHinder Simulation – copies & corrupts msvcp_win.dll
# ---------------------------------------------------------------

# 1. Define source and malicious destination
$src  = "$env:SystemRootSystem32msvcp_win.dll"
$dstDir = "C:ProgramDataFakeLib"
$dst  = Join-Path $dstDir "msvcp_win.dll"

# 2. Create destination folder (hidden)
if (-not (Test-Path $dstDir)) {
    New-Item -Path $dstDir -ItemType Directory | Out-Null
    # Hide the folder to mimic stealth
    (Get-Item $dstDir).Attributes += 'Hidden'
}

# 3. Copy the legitimate DLL
Copy-Item -Path $src -Destination $dst -Force

# 4. Corrupt the DLL – append 4 random bytes
$rand = -join ((65..90) + (97..122) | Get-Random -Count 4 | % {[char]$_})
[IO.File]::OpenWrite($dst).Seek(0, [IO.SeekOrigin]::End) | Out-Null
[IO.File]::WriteAllBytes($dst, [byte[]]($rand.ToCharArray() | ForEach-Object {[byte][char]$_}))

Write-Host "EDRStartupHinder simulation completed. DLL copied to $dst and corrupted."

Cleanup Commands

# Remove the malicious copy and optionally the hidden folder
$dstDir = "C:ProgramDataFakeLib"
if (Test-Path $dstDir) {
    Remove-Item -Path $dstDir -Recurse -Force
    Write-Host "Cleaned up FakeLib directory."
}

End of Report