CVE-2026-31431 (Copy Fail) – Ubuntu Security Mitigation Guide

🔐 CVE-2026-31431 (Copy Fail) – Ubuntu Security Mitigation Guide

The CVE-2026-31431 (Copy Fail) vulnerability in the Linux kernel may allow local privilege escalation attacks. This issue can be exploited via the algif_aead (AF_ALG AEAD) interface.

This guide explains how to securely mitigate the vulnerability on Ubuntu 22.04 and 24.04 systems step by step.

🎯 Objective

• Disable the algif_aead module
• Break the privilege escalation chain
• Close Qualys QID 387198 finding
• Apply a safe mitigation until a kernel patch is released

📌 1. Check System Status

First, verify whether your system is affected:

uname -r
echo "----"
grep -E '^CONFIG_CRYPTO_USER_API_AEAD=' /boot/config-$(uname -r) || echo "CONFIG line not found"
echo "----"
lsmod | grep algif_aead || echo "algif_aead not loaded"
            

Expected result: CONFIG_CRYPTO_USER_API_AEAD=m

📌 2. Create Mitigation File

Prevent the algif_aead module from loading by creating the following file:

nano /etc/modprobe.d/disable-algif-aead.conf
            

Add these lines inside the file:

install algif_aead /bin/false
blacklist algif_aead
            

Save and exit: CTRL + O → Enter → CTRL + X

📌 3. Verify Module Blocking

Confirm that the module can no longer be loaded:

modprobe -v algif_aead; echo "exit_code=$?"
            

Expected output:

install /bin/false
exit_code=1
            

📌 4. AF_ALG Exploit Test

Run this test to validate that the system is no longer vulnerable:

python3 -c "import socket,sys;
try:
 s=socket.socket(socket.AF_ALG, socket.SOCK_SEQPACKET, 0);
 s.bind(('aead','authencesn(hmac(sha256),cbc(aes))'));
 print('VULNERABLE');
 sys.exit(1)
except OSError as e:
 print('MITIGATED:', e);
 sys.exit(0)"
            

Expected output: MITIGATED

📌 5. Update initramfs (Recommended)

update-initramfs -u
            

This ensures the mitigation is applied during system boot.

✅ Summary

📝 Conclusion

After applying these steps, the CVE-2026-31431 (Copy Fail) vulnerability is effectively mitigated. The algif_aead module is blocked and the exploit chain is broken.

Once an official kernel patch is released, update your system and optionally remove this mitigation.