How Cloudflare responded to the “Copy Fail” Linux vulnerability

This post details Cloudflare's rapid response to the "Copy Fail" Linux kernel vulnerability (CVE-2026-31431). It explains the vulnerability's mechanism involving the AF_ALG socket family, scatterlists, and out-of-bounds writes to the page cache, enabling privilege escalation. Cloudflare's response involved assessing fleet exposure, validating that existing behavioral detections flagged the exploit pattern within minutes, conducting threat hunting for pre-disclosure exploitation, and engineering a runtime mitigation (bpf-lsm) while simultaneously rolling out patched Linux kernels. The post emphasizes that no customer impact occurred and highlights the effectiveness of their proactive security posture and established patching procedures.