CISA says ‘Copy Fail’ flaw now exploited to root Linux systems

CISA has warned that threat actors have started exploiting the "Copy Fail" Linux security vulnerability in the wild, one day after Theori researchers disclosed it and shared a proof-of-concept (PoC) exploit.

Tracked as CVE-2026-31431, this security flaw was found in the Linux kernel's algif_aead cryptographic algorithm interface and enables unprivileged local users to gain root privileges on unpatched Linux systems by writing four controlled bytes to the page cache of any readable file.

Theori researchers disclosed it on Thursday and shared what they described as a "100% reliable" Python-based exploit that can be used to root Ubuntu 24.04 LTS, Amazon Linux 2023, RHEL 10.1, and SUSE 16 devices.

However, they also added that the same script can be used reliably against any Linux distribution shipped since 2017 with a vulnerable kernel version.

"Same script, four distributions, four root shells — in one take. The same exploit binary works unmodified on every Linux distribution," Theori said. "If your kernel was built between 2017 and the patch — which covers essentially every mainstream Linux distribution — you're in scope."

While major Linux distros began pushing the fix via kernel updates, Tharros' principal vulnerability analyst, Will Dormann, noted on Thursday that there were no "official updates" when Theori published its advisory.

Getting root shell on four Linux distros (Theori)

On Friday, CISA added the Copy Fail security flaw to its Known Exploited Vulnerabilities (KEV) Catalog, ordering Federal Civilian Executive Branch (FCEB) agencies to patch their Linux endpoints and servers within two weeks, by May 15, as mandated by Binding Operational Directive (BOD) 22-01.

"This type of vulnerability is a frequent attack vector for malicious cyber actors and poses significant risks to the federal enterprise," the U.S. cybersecurity agency warned.

"Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable."

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