CVE-2025-22021
CriticalCVSS 10.0Exploitation Probability (EPSS)
Low risk35th percentile - higher than 35% of all known CVEs
Summary
In the Linux kernel's netfilter subsystem, the nf_sk_lookup_slow_v6 function lacks a conntrack lookup for SNATed IPv6 packets, unlike its IPv4 counterpart. This causes xt_socket rules with --transparent to fail for IPv6 traffic after address translation.
Risk Assessment
In Kubernetes clusters using Cilium and Envoy, iptables rules based on -m socket --transparent do not correctly intercept SNATed IPv6 traffic, potentially bypassing L7 security policies.
Recommendation
Install the Linux kernel update that fixes CVE-2025-22021 by adding conntrack lookup in nf_sk_lookup_slow_v6.
Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: netfilter: socket: Lookup orig tuple for IPv6 SNAT nf_sk_lookup_slow_v4 does the conntrack lookup for IPv4 packets to restore the original 5-tuple in case of SNAT, to be able to find the right socket (if any). Then socket_match() can correctly check whether the socket was transparent. However, the IPv6 counterpart (nf_sk_lookup_slow_v6) lacks this conntrack lookup, making xt_socket fail to match on the socket when the packet was SNATed. Add the same logic to nf_sk_lookup_slow_v6. IPv6 SNAT is used in Kubernetes clusters for pod-to-world packets, as pods' addresses are in the fd00::/8 ULA subnet and need to be replaced with the node's external address. Cilium leverages Envoy to enforce L7 policies, and Envoy uses transparent sockets. Cilium inserts an iptables prerouting rule that matches on `-m socket --transparent` and redirects the packets to localhost, but it fails to match SNATed IPv6 packets due to that missing conntrack lookup.