CVE-2026-98432

Description

This vulnerability affects the IPv4 TCP connection handling in the Linux kernel and can lead to remote code execution. In the Linux kernel, the following vulnerability has been resolved: ipv4: free inet_opt and ireq_opt after an RCU grace period tcp_v4_syn_recv_sock() transfers ownership of ireq->ireq_opt to the child socket (newinet->inet_opt) without copying it. Another cpu can concurrently process a retransmitted SYN for the same request socket, and send a SYNACK from tcp_check_req(). tcp_v4_send_synack() and inet_csk_route_req() read ireq->ireq_opt under rcu_read_lock() only, and ip_build_and_send_pkt() and ip_options_build() then read opt->optlen twice. Note that the SYNACK timer itself is not an issue: it holds a reference on its request socket, and inet_csk_reqsk_queue_drop() calls timer_delete_sync() before the child can be freed. Since commit 079096f103fa ("tcp/dccp: install syn_recv requests into ehash table"), request sockets are processed without holding the listener lock, so nothing prevents the child socket from being freed while the SYNACK is still being built. TCP child sockets do not have SOCK_RCU_FREE, and inet_sock_destruct() frees inet_opt with a plain kfree(), leading to a use-after-free in ip_options_build(). A similar issue exists with request socket migration (net.ipv4.tcp_migrate_req=1, or a BPF_SK_REUSEPORT_SELECT_OR_MIGRATE program). reqsk_timer_handler() clones the request socket with inet_reqsk_clone(), so that the old request socket and its clone share the same ireq_opt, then reqsk_migrate_reset() clears the pointer in the old one. Another cpu holding a reference on the old request socket can still be using these options (sending a SYNACK, or creating a child in tcp_v4_syn_recv_sock()) when the clone is freed, and tcp_v4_reqsk_destructor() also uses a plain kfree(). Readers already use RCU, and other paths replacing inet_opt (do_ip_setsockopt(), cipso_v4_sock_setattr()...) already use kfree_rcu(). Use kfree_rcu() in inet_sock_destruct() and tcp_v4_reqsk_destructor() as well. IPv6 is not affected by the first issue, because tcp_v6_syn_recv_sock() duplicates the options. tcp_v6_reqsk_destructor() has the same migration issue with ipv6_opt, which is only set by CALIPSO. This will be addressed in a separate patch.

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Mitigation

cengizcan> IPv4 TCP packet filtering: - Requires: nftables and root access through sudo. - Assumes: IPv4 options are not needed. For boot persistence, /etc/nftables.conf manages the firewall, without UFW or firewalld. - Blocks: incoming IPv4 TCP packets that contain IP options. - Keeps working: IPv4 TCP connections without IP options. TCP options such as timestamps and window scaling are unaffected, as is IPv6. Existing firewall rules still apply. - Breaks: TCP connections that depend on IPv4 options. - Covers: the host, including connections from the host to itself. Containers with separate networking need the rule applied separately. - Limit: reduces the risk but does not fully prevent exploitation. To apply the rule: 1. Create the rule file: printf '%s\n' 'table inet cve_2026_98432 { chain input { type filter hook input priority -310; policy accept; ip protocol tcp ip hdrlength > 5 counter drop } }' | sudo tee /etc/cve-2026-98432.nft >/dev/null 2. Check and apply the rule: sudo nft -c -f /etc/cve-2026-98432.nft && sudo nft -f /etc/cve-2026-98432.nft 3. Keep the rule after reboot (run once): echo 'include "/etc/cve-2026-98432.nft"' | \ sudo tee -a /etc/nftables.conf sudo nft -c -f /etc/nftables.conf && sudo systemctl enable nftables.service To remove the rule and its boot setting: sudo sed -i '\|^include "/etc/cve-2026-98432.nft"$|d' \ /etc/nftables.conf sudo nft delete table inet cve_2026_98432

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Patch details

Severity score breakdown

CVSS version: CVSS v4.0 CVSS v4.0 CVSS v3.0

References

• MITRE
• NVD
• Launchpad
• Debian

Related Ubuntu Security Notices (USN)

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• Linux kernel (Azure CVM) vulnerabilities
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• USN-9040-1
• Linux kernel (Low Latency NVIDIA) vulnerabilities
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Other references

• https://www.cve.org/CVERecord?id=CVE-2026-98432
• https://ubuntu.com/blog/fixes-available-in-proposed-pocket-for-cve-2026-98432
• https://git.kernel.org/linus/e3f33b0a1d89d738e2b913dab519226eaf76e15b (7.3-rc7)