CVE-2026-98432: Temporary nftables Filter with Auto-Rollback for Ubuntu Servers
CVE-2026-98432 affects Linux kernels and requires immediate temporary mitigation on systems where official patches remain unavailable. A temporary nftables filter has already been deployed across 26 servers to block malicious traffic until kernel updates can be applied.
Kernel Status Verification
Begin by checking the running kernel version with uname -r. Administrators should consult the Canonical kernel status page for their specific Ubuntu release. If the fix is listed as work in progress, schedule the update and apply the temporary filter during the waiting period. Red Hat recommends dropping IPv4 TCP packets that contain IP options as an additional layer of protection.
Single-Server Preparation and Rollback
Start with one node. Save existing rules using (umask 077; sudo nft list ruleset > "$HOME/nft-before.txt"). Prepare the rollback command sudo nft delete table inet cve_2026_98432 in advance. Access to hoster console is advised in case SSH connectivity is lost.
Rule Creation and Validation
Save the following rule set to cve-2026-98432.nft:
table inet cve_2026_98432 { chain input { type filter hook input priority -310; policy accept; ip protocol tcp ip hdrlength > 5 counter drop } }
Validate syntax with sudo nft --check --file cve-2026-98432.nft before loading. The condition targets IPv4 header length and does not affect standard TCP options such as MSS or timestamps.
Auto-Rollback with systemd
Before applying the rule, create a five-minute systemd timer: nft_bin=$(command -v nft) followed by sudo systemd-run --collect --unit=cve-98432-rollback --on-active=5m "$nft_bin" delete table inet cve_2026_98432. Confirm the timer is active before proceeding. Load the rule and verify with sudo nft list table inet cve_2026_98432.
Connectivity Testing and Production Rollout
Open a new SSH session from another host and test application traffic including HTTPS and API endpoints. If successful, stop the rollback timer. For persistent deployment, integrate the table into the existing firewall startup sequence without using flush ruleset. Repeat the full procedure on remaining nodes, always using per-server auto-rollback timers. After the official kernel patch is released, update, reboot, and remove the temporary table.
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