CVE-2025-21864
MediumCVSS 5.5Exploitation Probability (EPSS)
Low risk10th percentile - higher than 10% of all known CVEs
Summary
In the Linux kernel, a vulnerability in the TCP stack causes the security path (secpath) not to be dropped together with the dst object. This retains a reference to the xfrm_state, preventing its proper cleanup when network namespaces are deleted. The issue manifests as a WARN warning in xfrm6_tunnel_net_exit.
Risk Assessment
Organizations may experience kernel memory leaks and unexpected WARN warnings when dynamically creating and deleting network namespaces with IPComp traffic. In extreme cases, this could lead to system instability or denial of service (DoS) in environments heavily using network virtualization.
Recommendation
Apply the security patch for the Linux kernel containing the commit that fixes the issue (dropping secpath together with dst in the TCP receive path). Update to a kernel version that includes this fix or manually backport the commit.
Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: tcp: drop secpath at the same time as we currently drop dst Xiumei reported hitting the WARN in xfrm6_tunnel_net_exit while running tests that boil down to: - create a pair of netns - run a basic TCP test over ipcomp6 - delete the pair of netns The xfrm_state found on spi_byaddr was not deleted at the time we delete the netns, because we still have a reference on it. This lingering reference comes from a secpath (which holds a ref on the xfrm_state), which is still attached to an skb. This skb is not leaked, it ends up on sk_receive_queue and then gets defer-free'd by skb_attempt_defer_free. The problem happens when we defer freeing an skb (push it on one CPU's defer_list), and don't flush that list before the netns is deleted. In that case, we still have a reference on the xfrm_state that we don't expect at this point. We already drop the skb's dst in the TCP receive path when it's no longer needed, so let's also drop the secpath. At this point, tcp_filter has already called into the LSM hooks that may require the secpath, so it should not be needed anymore. However, in some of those places, the MPTCP extension has just been attached to the skb, so we cannot simply drop all extensions.