You're staring at a blue screen with ERROR_RING2_STACK_IN_USE. Yeah, that's annoying. Let's fix it.
The Fastest Fix: Update or Remove the Culprit Driver
What's actually happening here is that a driver (usually a network adapter, GPU driver, or storage controller) is holding onto the ring 2 stack longer than it should. Windows detects this and throws the 0x000000CF BSOD to prevent a system crash from stack corruption.
Step 1: Boot into Safe Mode with Networking
Press Shift + Restart from the login screen, then go to Troubleshoot > Advanced options > Startup Settings > Restart. Press 4 for Safe Mode with Networking.
Step 2: Run Driver Verifier to find the offender
Open Command Prompt as admin and run:
verifier /standard /allReboot normally. If the system BSODs again, the next reboot will show which driver caused it. Write down the driver name. Then boot back into Safe Mode and run verifier /reset to stop it.
Step 3: Update or remove that driver
Right-click Start > Device Manager. Find the driver (likely under Network adapters, Display adapters, or Storage controllers). Right-click > Update driver > Search automatically. If no update is found, try the manufacturer's website directly (Intel, NVIDIA, Realtek—these are common culprits).
If updating doesn't work, right-click > Uninstall device (check "Delete the driver software for this device"). Reboot and let Windows install the generic version.
Why This Works
The ring 2 stack is part of the x86/x64 protection ring model. Ring 0 is the kernel, ring 3 is user space. Ring 2 is rarely used directly by drivers, but some legacy or poorly-written drivers try to use it for privilege escalation. The reason step 3 works is that updating the driver forces it to use the proper ring allocation, while uninstalling it removes the broken code entirely. The generic driver Windows provides skips the ring 2 stack entirely.
Less Common Variations
Corrupted system files
Sometimes a Windows update leaves ntoskrnl.exe or hal.dll in a bad state. Run these in an admin command prompt:
sfc /scannow
dism /online /cleanup-image /restorehealthThird-party antivirus
Antivirus software (I've seen this most with McAfee and Norton) injects drivers into ring 2 for low-level scanning. Try disabling or temporarily uninstalling it.
Virtualization software
Hyper-V, VirtualBox, and VMware all mess with ring levels. If the BSOD started after installing one, uninstall it and see if the error stops.
Faulty RAM
Unlikely but possible. Run Windows Memory Diagnostic (search for it in Start menu, restart, let it run).
How to Prevent This BSOD from Coming Back
Keep your drivers updated—especially the chipset, network, and GPU. I use SDI for this, but you can go manually if you prefer. Stick with WHQL-signed drivers; they've passed Microsoft's ring-level validation. If you're a developer writing kernel-mode code, never use ring 2 unless you really know what you're doing—and even then, test with Driver Verifier turned on.
One more thing: disable Windows' automatic driver updates if they've caused this before. Go to System Properties > Hardware > Device Installation Settings and choose "No."