CVE-2025-40362 ceph: fix multifs mds auth caps issue
A flaw in the Linux kernel Ceph client allows users to bypass read-only restrictions on one filesystem by leveraging write permissions on another in multi-filesystem clusters.
- 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-2025-40362?
CVE-2025-40362 is a high-severity missing authorization vulnerability in Linux, published on 16 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:
ceph: fix multifs mds auth caps issue
The mds auth caps check should also validate the fsname along with the associated caps. Not doing so would result in applying the mds auth caps of one fs on to the other fs in a multifs ceph cluster. The bug causes multiple issues w.r.t user authentication, following is one such example.
Steps to Reproduce (on vstart cluster): 1. Create two file systems in a cluster, say 'fsname1' and 'fsname2' 2. Authorize read only permission to the user 'client.usr' on fs 'fsname1' $ceph fs authorize fsname1 client.usr / r 3. Authorize read and write permission to the same user 'client.usr' on fs 'fsname2' $ceph fs authorize fsname2 client.usr / rw 4. Update the keyring $ceph auth get client.usr >> ./keyring
With above permssions for the user 'client.usr', following is the expectation. a. The 'client.usr' should be able to only read the contents and not allowed to create or delete files on file system 'fsname1'. b. The 'client.usr' should be able to read/write on file system 'fsname2'.
But, with this bug, the 'client.usr' is allowed to read/write on file system 'fsname1'. See below.
5. Mount the file system 'fsname1' with the user 'client.usr' $sudo bin/mount.ceph [email protected]=/ /kmnt_fsname1_usr/ 6. Try creating a file on file system 'fsname1' with user 'client.usr'. This should fail but passes with this bug. $touch /kmnt_fsname1_usr/file1 7. Mount the file system 'fsname1' with the user 'client.admin' and create a file. $sudo bin/mount.ceph [email protected]=/ /kmnt_fsname1_admin $echo "data" > /kmnt_fsname1_admin/admin_file1 8. Try removing an existing file on file system 'fsname1' with the user 'client.usr'. This shoudn't succeed but succeeds with the bug. $rm -f /kmnt_fsname1_usr/admin_file1
For more information, please take a look at the corresponding mds/fuse patch and tests added by looking into the tracker mentioned below.
v2: Fix a possible null dereference in doutc v3: Don't store fsname from mdsmap, validate against ceph_mount_options's fsname and use it v4: Code refactor, better warning message and fix possible compiler warning
[ Slava.Dubeyko: "fsname check failed" -> "fsname mismatch" ]
Which versions are affected by CVE-2025-40362?
How do I fix CVE-2025-40362?
Apply the vendor's patch.
Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.
Is CVE-2025-40362 being exploited?
There is no known exploitation of CVE-2025-40362 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2025-40362, DEBIAN-CVE-2025-40362, GHSA-p5mp-qv23-7r77, ROOT-OS-DEBIAN-13-CVE-2025-40362, ROOT-OS-UBUNTU-2204-CVE-2025-40362, ROOT-OS-UBUNTU-2404-CVE-2025-40362 and 1 more.