CVE-2025-40256 xfrm: also call xfrm_state_delete_tunnel at destroy time for states that were never added
A memory leak in the Linux kernel's xfrm subsystem allows local attackers to cause denial of service by triggering specific IPcomp tunnel state failures.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-40256?
CVE-2025-40256 is a memory corruption vulnerability in Linux, published on 4 Dec 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
xfrm: also call xfrm_state_delete_tunnel at destroy time for states that were never added
In commit b441cf3f8c4b ("xfrm: delete x->tunnel as we delete x"), I missed the case where state creation fails between full initialization (->init_state has been called) and being inserted on the lists.
In this situation, ->init_state has been called, so for IPcomp tunnels, the fallback tunnel has been created and added onto the lists, but the user state never gets added, because we fail before that. The user state doesn't go through __xfrm_state_delete, so we don't call xfrm_state_delete_tunnel for those states, and we end up leaking the FB tunnel.
There are several codepaths affected by this: the add/update paths, in both net/key and xfrm, and the migrate code (xfrm_migrate, xfrm_state_migrate). A "proper" rollback of the init_state work would probably be doable in the add/update code, but for migrate it gets more complicated as multiple states may be involved.
At some point, the new (not-inserted) state will be destroyed, so call xfrm_state_delete_tunnel during xfrm_state_gc_destroy. Most states will have their fallback tunnel cleaned up during __xfrm_state_delete, which solves the issue that b441cf3f8c4b (and other patches before it) aimed at. All states (including FB tunnels) will be removed from the lists once xfrm_state_fini has called flush_work(&xfrm_state_gc_work).
Which versions are affected by CVE-2025-40256?
How do I fix CVE-2025-40256?
Apply the vendor's patch.
Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.
Is CVE-2025-40256 being exploited?
There is no known exploitation of CVE-2025-40256 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2025-40256, DEBIAN-CVE-2025-40256, GHSA-wxqp-f8fg-rxmv, UBUNTU-CVE-2025-40256.