CVE-2026-92499 ext4: validate readdir offset before accessing dirent
A local attacker can trigger a use-after-free in the Linux kernel ext4 filesystem by mounting a corrupted directory image, potentially causing a system crash or code execution.
- 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-2026-92499?
CVE-2026-92499 is an use after free vulnerability in Linux, published on 17 Sep 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:
ext4: validate readdir offset before accessing dirent
A corrupted directory can trigger the following KASAN report when ext4_readdir() resumes from an invalid position:
BUG: KASAN: use-after-free in __ext4_check_dir_entry+0x5ef/0x820 Read of size 2 at addr ffff88810a646000 by task repro_linear/509
Call Trace: <TASK> dump_stack_lvl+0x53/0x70 print_report+0xd0/0x630 kasan_report+0xce/0x100 __ext4_check_dir_entry+0x5ef/0x820 ext4_readdir+0xcde/0x2b70 iterate_dir+0x1a1/0x520 __x64_sys_getdents64+0x12b/0x220 do_syscall_64+0xf9/0x540 entry_SYSCALL_64_after_hwframe+0x77/0x7f </TASK>
KASAN reports use-after-free because the out-of-bounds access lands in an adjacent freed page. The directory buffer itself is still referenced.
ext4_dir_llseek() invalidates the directory cookie so that ext4_readdir() rescans directory entries from the start of the block. The rescan checks only the lower bound of rec_len before advancing. A corrupted rec_len can therefore place the offset where the block has insufficient space for a complete directory entry. The rescan itself may dereference that truncated entry, or the main loop may pass it to __ext4_check_dir_entry(). The latter reads de->rec_len before validating the range. For example:
block offset 0 4092 4096 |---- de1.rec_len = 4092 -----|----| de2.inode | de2.rec_len ^ OOB, reported as UAF
de2 starts at offset 4092 in this 4 KiB block. Its four-byte inode fits in the block, but its rec_len starts at offset 4096 and crosses the boundary.
The minimum safe length is inode-dependent. Encrypted and casefolded directory entries need eight additional hash bytes, while a valid metadata checksum tail is only 12 bytes.
Cache the metadata checksum feature state and derive the minimum directory entry length from the on-disk format. Use it to bound both the rescan and the offset passed to the main loop. Report an offset in a truncated block tail and skip the remainder of the block, while continuing to accept an offset exactly at the block boundary.
Which versions are affected by CVE-2026-92499?
How do I fix CVE-2026-92499?
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-2026-92499 being exploited?
There is no known exploitation of CVE-2026-92499 in the wild and no public exploit code.
Sources
Also known as AZL-101972, BELL-CVE-2026-92499, DEBIAN-CVE-2026-92499, GHSA-5fwr-5mg2-3p42, UBUNTU-CVE-2026-92499.