CVE-2025-40214 af_unix: Initialise scc_index in unix_add_edge().
A missing initialization in the Linux kernel's AF_UNIX garbage collector can lead to use-after-free, allowing local attackers to execute code or escalate privileges.
- 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-2025-40214?
CVE-2025-40214 is a high-severity use after free 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:
af_unix: Initialise scc_index in unix_add_edge().
Quang Le reported that the AF_UNIX GC could garbage-collect a receive queue of an alive in-flight socket, with a nice repro.
The repro consists of three stages.
1) 1-a. Create a single cyclic reference with many sockets 1-b. close() all sockets 1-c. Trigger GC
2) 2-a. Pass sk-A to an embryo sk-B 2-b. Pass sk-X to sk-X 2-c. Trigger GC
3) 3-a. accept() the embryo sk-B 3-b. Pass sk-B to sk-C 3-c. close() the in-flight sk-A 3-d. Trigger GC
As of 2-c, sk-A and sk-X are linked to unix_unvisited_vertices, and unix_walk_scc() groups them into two different SCCs:
unix_sk(sk-A)->vertex->scc_index = 2 (UNIX_VERTEX_INDEX_START) unix_sk(sk-X)->vertex->scc_index = 3
Once GC completes, unix_graph_grouped is set to true. Also, unix_graph_maybe_cyclic is set to true due to sk-X's cyclic self-reference, which makes close() trigger GC.
At 3-b, unix_add_edge() allocates unix_sk(sk-B)->vertex and links it to unix_unvisited_vertices.
unix_update_graph() is called at 3-a. and 3-b., but neither unix_graph_grouped nor unix_graph_maybe_cyclic is changed because both sk-B's listener and sk-C are not in-flight.
3-c decrements sk-A's file refcnt to 1.
Since unix_graph_grouped is true at 3-d, unix_walk_scc_fast() is finally called and iterates 3 sockets sk-A, sk-B, and sk-X:
sk-A -> sk-B (-> sk-C) sk-X -> sk-X
This is totally fine. All of them are not yet close()d and should be grouped into different SCCs.
However, unix_vertex_dead() misjudges that sk-A and sk-B are in the same SCC and sk-A is dead.
unix_sk(sk-A)->scc_index == unix_sk(sk-B)->scc_index <-- Wrong! && sk-A's file refcnt == unix_sk(sk-A)->vertex->out_degree ^-- 1 in-flight count for sk-B -> sk-A is dead !?
The problem is that unix_add_edge() does not initialise scc_index.
Stage 1) is used for heap spraying, making a newly allocated vertex have vertex->scc_index == 2 (UNIX_VERTEX_INDEX_START) set by unix_walk_scc() at 1-c.
Let's track the max SCC index from the previous unix_walk_scc() call and assign the max + 1 to a new vertex's scc_index.
This way, we can continue to avoid Tarjan's algorithm while preventing misjudgments.
Which versions are affected by CVE-2025-40214?
How do I fix CVE-2025-40214?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2025-40214 being exploited?
There is no known exploitation of CVE-2025-40214 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2025-40214, DEBIAN-CVE-2025-40214, GHSA-g3xr-84mj-fvx6, ROOT-OS-DEBIAN-12-CVE-2025-40214, ROOT-OS-DEBIAN-13-CVE-2025-40214, UBUNTU-CVE-2025-40214.