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Copy Fail (CVE-2026-31431): A 4‑Byte Kernel Bug That Lets Attackers Gain Root on Major Linux Distros

Microsoft Defender Security Research recently disclosed CVE-2026-31431—nicknamed “Copy Fail”—a high‑severity local privilege escalation in the Linux kernel’s crypto subsystem that enables an unprivileged user to escalate to root. The vulnerability affects kernels released since 2017 and has broad implications for cloud and container environments because the exploit can corrupt in-memory representations of readable files (including setuid binaries) without changing the on-disk file. Although public proof-of-concept code exists and detected exploitation has been limited so far, the combination of a reliable…