CVE-2026-63817 f2fs: validate compress cache inode only when enabled
Attackers can bypass inode validation in F2FS when compression cache is disabled, potentially accessing internal kernel structures or escalating 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-2026-63817?
CVE-2026-63817 is a high-severity missing authorization vulnerability in Linux, published on 19 Jul 2026. 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:
f2fs: validate compress cache inode only when enabled
F2FS_COMPRESS_INO() uses NM_I(sbi)->max_nid as the synthetic inode number for the compressed page cache inode. That inode only exists when the compress_cache mount option is enabled.
When compress_cache is disabled, max_nid is outside the valid inode range. A corrupted directory entry that points to ino == max_nid should therefore be rejected by f2fs_check_nid_range(). However, is_meta_ino() currently treats F2FS_COMPRESS_INO() as a meta inode unconditionally, so f2fs_iget() bypasses do_read_inode() and its nid range check, and instantiates a fake internal inode instead.
Gate the compressed cache inode case on COMPRESS_CACHE, matching f2fs_init_compress_inode(). With compress_cache disabled, ino == max_nid now follows the normal inode path and is rejected as an out-of-range nid.
Which versions are affected by CVE-2026-63817?
How do I fix CVE-2026-63817?
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-2026-63817 being exploited?
There is no known exploitation of CVE-2026-63817 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-63817, DEBIAN-CVE-2026-63817, GHSA-cv79-fx5q-34h3, ROOT-OS-UBUNTU-2204-CVE-2026-63817, UBUNTU-CVE-2026-63817.