CVE-2025-40205 btrfs: avoid potential out-of-bounds in btrfs_encode_fh()
A potential out-of-bounds write in the Linux kernel's btrfs filesystem allows attackers to corrupt memory, potentially leading to code execution or privilege escalation.
- 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-2025-40205?
CVE-2025-40205 is a high-severity memory corruption vulnerability in Linux, published on 12 Nov 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:
btrfs: avoid potential out-of-bounds in btrfs_encode_fh()
The function btrfs_encode_fh() does not properly account for the three cases it handles.
Before writing to the file handle (fh), the function only returns to the user BTRFS_FID_SIZE_NON_CONNECTABLE (5 dwords, 20 bytes) or BTRFS_FID_SIZE_CONNECTABLE (8 dwords, 32 bytes).
However, when a parent exists and the root ID of the parent and the inode are different, the function writes BTRFS_FID_SIZE_CONNECTABLE_ROOT (10 dwords, 40 bytes).
If *max_len is not large enough, this write goes out of bounds because BTRFS_FID_SIZE_CONNECTABLE_ROOT is greater than BTRFS_FID_SIZE_CONNECTABLE originally returned.
This results in an 8-byte out-of-bounds write at fid->parent_root_objectid = parent_root_id.
A previous attempt to fix this issue was made but was lost.
https://lore.kernel.org/all/[email protected]/
Although this issue does not seem to be easily triggerable, it is a potential memory corruption bug that should be fixed. This patch resolves the issue by ensuring the function returns the appropriate size for all three cases and validates that *max_len is large enough before writing any data.
Which versions are affected by CVE-2025-40205?
How do I fix CVE-2025-40205?
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-2025-40205 being exploited?
There is no known exploitation of CVE-2025-40205 in the wild and no public exploit code.
Sources
Also known as AZL-70097, BELL-CVE-2025-40205, DEBIAN-CVE-2025-40205, GHSA-7v4m-qg52-mxg3, ROOT-OS-UBUNTU-2404-CVE-2025-40205, UBUNTU-CVE-2025-40205.