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Introduction:
Bring Your Own Vulnerable Driver (BYOVD) attacks have become a favorite among ransomware groups and advanced threat actors. They abuse legitimate, signed third‑party kernel drivers with known vulnerabilities to disable Endpoint Detection and Response (EDR) products from ring‑0. While EDRs try to detect post‑load malicious behavior, Windows Defender Application Control (WDAC) provides a proactive, policy‑based approach that stops unauthorized drivers from loading in the first place.
Learning Objectives:
- Understand how BYOVD attacks exploit vulnerable kernel drivers to disable security controls.
- Learn to configure WDAC on Windows 10 1607+ to whitelist only approved drivers.
- Implement a FilePublisher or WHQLPublisher policy to block unknown vulnerable drivers.
You Should Know:
1. BYOVD – The Silent Kernel Killer
Attackers no longer need to write rootkits from scratch. They search for a legitimate, digitally signed driver from vendors like ASUS, Gigabyte, or even game components (e.g., Capcom.sys) that contains a vulnerability allowing arbitrary kernel memory read/write. Once loaded, the driver’s vulnerability is exploited to terminate EDR processes, bypass Sysmon, or install persistent malware.
Example of a known vulnerable driver hash (Capcom.sys):
`7B6A8F5F9B8A3C4D2E1F0A9B8C7D6E5F4A3B2C1D`
To check for loaded vulnerable drivers on a live system (PowerShell as Admin):
Get-WindowsDriver -Online | Where-Object {$<em>.Driver -like "capcom" -or $</em>.ProviderName -eq "Capcom"}
Linux equivalent (detecting suspicious kernel modules):
lsmod | grep -i vulnerable sudo modprobe -r vulnerable_driver_name
The core issue: EDRs operate in user‑mode or as minifilters; once a malicious driver reaches the kernel, game over. WDAC shifts the paradigm from “detect” to “deny by default.”
- WDAC vs. Traditional EDR – A Fundamental Difference
Traditional EDR relies on signatures, heuristics, and behavioural analysis after a driver is loaded. BYOVD bypasses this because the driver is signed and legitimate. WDAC, on the other hand, enforces a whitelist at the kernel loading stage. If a driver isn’t explicitly allowed (by file hash, publisher, or WHQL certification), the kernel refuses to load it – no exceptions.
Why EDR alone fails:
- EDR hooks can be unloaded by kernel‑mode code.
- Signed vulnerable drivers are not flagged as malware.
- Exploit payloads run before EDR telemetry is generated.
Why WDAC wins:
- Enforced by the Secure Kernel / Hypervisor‑protected Code Integrity (HVCI).
- Blocklist of known vulnerable drivers shipped with Windows (updated via Windows Update).
- Can be set to “Allow only drivers from trusted publishers that you have already installed.”
3. Step‑by‑Step: Enabling WDAC Base Policy (Audit First)
Before locking down production, run WDAC in Audit Mode to identify which drivers your systems actually need.
Step 1 – Create a reference policy from a clean system
Install your critical drivers (storage, GPU, networking) on a reference machine. Then generate a policy that allows all currently loaded drivers:
Run as Administrator $PolicyPath = "C:\WDAC\BasePolicy.xml" New-CIPolicy -FilePath $PolicyPath -Level FilePublisher -UserPEs
Step 2 – Convert and deploy the policy to audit mode
Convert XML to binary policy $BinaryPath = "C:\WDAC\BasePolicy.bin" ConvertFrom-CIPolicy -XmlFilePath $PolicyPath -BinaryFilePath $BinaryPath Copy to EFI partition (requires admin) Copy-Item $BinaryPath -Destination "C:\EFI\Microsoft\Boot\BasePolicy.bin" -Force
Step 3 – Activate audit mode via registry
Set-ItemProperty -Path "HKLM\SYSTEM\CurrentControlSet\Control\DeviceGuard\Scenarios\SystemGuard" -Name "Enabled" -Value 1 -Type DWord Set-ItemProperty -Path "HKLM\SYSTEM\CurrentControlSet\Control\CI\Policy" -Name "PolicyStatus" -Value 2 -Type DWord Restart-Computer
After reboot, WDAC will log blocked drivers in Event Viewer `Applications and Services Logs\Microsoft\Windows\CodeIntegrity\Operational` (Event ID 3076). Review for 72 hours before enforcing.
- Creating a FilePublisher Rule – Whitelist Only Your Drivers
The default audit policy may be too permissive. For maximum security, create a FilePublisher rule that allows only drivers from publishers you already trust – not any new, unknown third‑party driver.
Generate a policy based on existing file publisher rules:
Scan system32\drivers for current publishers $ExistingDrivers = Get-ChildItem "C:\Windows\System32\drivers.sys" New-CIPolicy -FilePath "C:\WDAC\StrictPolicy.xml" -Level FilePublisher -Fallback Hash -UserPEs -DriverFiles $ExistingDrivers
Merge this with the default Microsoft signed driver allowlist (so Windows boots):
Merge-CIPolicy -OutputFilePath "C:\WDAC\MergedPolicy.xml" -PolicyPaths "C:\WDAC\StrictPolicy.xml", "C:\Windows\schemas\CodeIntegrity\ExamplePolicies\DefaultWindows_Enforced.xml"
Why FilePublisher over Publisher or WHQL?
- FilePublisher = Publisher + product name + version (most restrictive).
- WHQLPublisher = WHQL signature + publisher (less strict).
- Hash = exact file hash (breaks on updates).
For most enterprises, FilePublisher balances security with manageable updates.
5. Deploying and Enforcing WDAC via Group Policy
Once your merged policy is tested, enforce it.
Convert to binary and copy to all endpoints:
ConvertFrom-CIPolicy -XmlFilePath "C:\WDAC\MergedPolicy.xml" -BinaryFilePath "C:\WDAC\EnforcedPolicy.bin"
Group Policy deployment steps:
- Copy `EnforcedPolicy.bin` to a network share (authenticated users read).
- Open `gpmc.msc` → Edit GPO → Computer Config → Policies → Admin Templates → System → Device Guard.
- Enable “Turn On Virtualization Based Security” → Set “Platform Security Level” to “Secure Boot and DMA Protection”.
- Enable “Deploy Windows Defender Application Control” → Enter the UNC path to
EnforcedPolicy.bin. - Link GPO to OUs and run `gpupdate /force` on clients.
Verify enforcement on a client:
Get-CIPolicy | Format-List Should show "IsEnforced: True"
Windows Update caveat: Driver updates from allowed publishers will still load. If an allowed publisher later releases a vulnerable driver (e.g., a GPU vendor), you must revoke their certificate via an updated WDAC policy. Monitor WDAC logs continuously.
6. Managing Exceptions and Supplemental Policies
You will encounter legitimate drivers that your base policy blocks – think of a new printer driver or a security tool update. Instead of rebuilding the giant base policy, use a Supplemental Policy.
Create a supplemental policy that allows a specific blocked driver:
From the reference machine with the new driver installed New-CIPolicy -FilePath "C:\WDAC\Supplemental_Printer.xml" -Level FilePublisher -DriverFiles "C:\Windows\System32\drivers\usbprint.sys" Set-RuleOption -FilePath "C:\WDAC\Supplemental_Printer.xml" -Option 3 Enable Supplemental Policy flag ConvertFrom-CIPolicy -XmlFilePath "C:\WDAC\Supplemental_Printer.xml" -BinaryFilePath "C:\WDAC\Supplemental_Printer.bin"
Deploy the supplemental `.bin` alongside the base policy – same folder, no registry changes needed. WDAC merges them at next reboot.
Pro tip: Always timestamp and version your policies. Use `Set-CIPolicyVersion -Version “1.2.0.0”` to avoid stale policies lingering.
7. Monitoring BYOVD Attempts with WDAC Events
Even in enforcement mode, WDAC logs every blocked driver attempt. Set up monitoring to detect adversaries testing vulnerable drivers.
Event IDs to watch (CodeIntegrity Operational log):
- 3076 – Driver blocked (Policy failure) – immediate investigation required.
- 3077 – Driver allowed (Policy success) – baseline for normal operations.
- 3089 – Policy applied at boot.
PowerShell one‑liner to extract block events:
Get-WinEvent -FilterHashtable @{LogName="Microsoft-Windows-CodeIntegrity/Operational"; ID=3076} | Select-Object TimeCreated, Message | Export-Csv "C:\Logs\WDAC_Blocks.csv"
Integration with SIEM (e.g., Splunk, Sentinel):
Forward Event Logs via Windows Event Collector (WEC). Create alerts for more than 3 unique blocked drivers per host in 1 hour – often indicates BYOVD enumeration.
Linux parallel: For systems using `modprobe` with signature enforcement, monitor `auditd` for `DENIED` operations on kernel module loading:
auditctl -a always,exit -F arch=b64 -S init_module -S finit_module -k module_load ausearch -k module_load --format raw | grep denied
What Undercode Say:
- BYOVD is not a theoretical threat – ransomware groups like LockBit and BlackCat have publicly used vulnerable drivers (e.g., Genshin Impact driver, MSI Afterburner) to kill EDRs.
- WDAC is underutilized – most security engineers rely on EDR alone. Enabling WDAC with a FilePublisher policy raises the cost of kernel compromise astronomically.
- Audit mode is your friend – never enforce blindly. Use 72‑96 hours of audit logging to build a complete driver inventory, then enforce gradually.
- Combining WDAC with HVCI and Secure Boot creates a root‑of‑trust that even kernel‑mode exploits cannot trivially bypass. Attackers would need a UEFI firmware vulnerability – a much higher bar.
- Linux admins aren’t safe either – the same concept applies with Lockdown LSM + signed kernel modules (
CONFIG_MODULE_SIG_FORCE=y). Third‑party NVIDIA or VMware modules can be abused similarly.
Prediction:
Within 18 months, WDAC (or its successor under Microsoft’s Pluton security processor) will become a baseline requirement for cyber insurance policies covering ransomware. Attackers will shift from BYOVD to UEFI bootkits and DMA attacks via Thunderbolt. The industry will see a new class of “driver reputation services” integrated into WDAC, allowing cloud‑based allow/deny decisions for unknown drivers. Enterprises that fail to implement application whitelisting for kernel drivers will become low‑hanging fruit, as EDRs alone will no longer be considered sufficient against modern BYOVD toolkits.
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