CVE-2025-40320 smb: client: fix potential cfid UAF in smb2_query_info_compound
A use-after-free vulnerability in the Linux kernel SMB client allows attackers to potentially execute code or escalate privileges via crafted SMB responses.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-40320?
CVE-2025-40320 is a critical use after free vulnerability in Linux, published on 8 Dec 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
smb: client: fix potential cfid UAF in smb2_query_info_compound
When smb2_query_info_compound() retries, a previously allocated cfid may have been freed in the first attempt. Because cfid wasn't reset on replay, later cleanup could act on a stale pointer, leading to a potential use-after-free.
Reinitialize cfid to NULL under the replay label.
Example trace (trimmed):
refcount_t: underflow; use-after-free. WARNING: CPU: 1 PID: 11224 at ../lib/refcount.c:28 refcount_warn_saturate+0x9c/0x110 [...] RIP: 0010:refcount_warn_saturate+0x9c/0x110 [...] Call Trace: <TASK> smb2_query_info_compound+0x29c/0x5c0 [cifs f90b72658819bd21c94769b6a652029a07a7172f] ? step_into+0x10d/0x690 ? __legitimize_path+0x28/0x60 smb2_queryfs+0x6a/0xf0 [cifs f90b72658819bd21c94769b6a652029a07a7172f] smb311_queryfs+0x12d/0x140 [cifs f90b72658819bd21c94769b6a652029a07a7172f] ? kmem_cache_alloc+0x18a/0x340 ? getname_flags+0x46/0x1e0 cifs_statfs+0x9f/0x2b0 [cifs f90b72658819bd21c94769b6a652029a07a7172f] statfs_by_dentry+0x67/0x90 vfs_statfs+0x16/0xd0 user_statfs+0x54/0xa0 __do_sys_statfs+0x20/0x50 do_syscall_64+0x58/0x80
Which versions are affected by CVE-2025-40320?
How do I fix CVE-2025-40320?
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-2025-40320 being exploited?
There is no known exploitation of CVE-2025-40320 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2025-40320, DEBIAN-CVE-2025-40320, GHSA-2vg8-pj5q-762f, ROOT-OS-DEBIAN-13-CVE-2025-40320, ROOT-OS-UBUNTU-2404-CVE-2025-40320, UBUNTU-CVE-2025-40320.