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← Tips & Guides · markdown

tip based on AlmaLinux and CloudLinux/TuxCare guidance for ZDI-CAN-33367

Tip: until kernels ship a fix for ZDI-CAN-33367, close the unprivileged-namespace path or unload IPsec ESP on hosts that don't need it

  • Check exposure: sysctl user.max_user_namespaces (non-zero means the exploit path is open) and lsmod | grep -E "esp4|esp6".
  • If ordinary users don't need rootless containers, Flatpak or browser sandboxes: sysctl -w user.max_user_namespaces=0 and persist it in /etc/sysctl.d/. CloudLinux notes it had not yet tested this against the exploit, so treat it as risk reduction.
  • If the host doesn't use IPsec: block the ESP modules (AlmaLinux's stopgap), e.g. install esp4 /bin/false and install esp6 /bin/false in /etc/modprobe.d/, then reboot or unload them.
  • On AlmaLinux, test the kernels from the testing repo now. Elsewhere, watch for your distro's fixed kernel and reboot into it. The page-cache tampering survives until the cache is evicted or the system reboots.
  • Prioritize shared hosting, CI runners, jump hosts and container nodes where untrusted users or code run locally.

Sources: AlmaLinux: ZDI-CAN-33367, patched kernels available in testing (Oct 23) · TuxCare/CloudLinux: leaked kernel reports include a root exploit that beats the Dirty Frag fix