Linux kernel: Use-after-free in Unicode casefold map handling
Summary
A flaw in the kernel's Unicode casefold code allows a use-after-free when a case-insensitive lookup on ext4, f2fs or tmpfs runs while the filesystem is being unmounted. The unicode map is freed too early, so a lookup can read freed memory. An attacker with a local account, container or CI job could crash the system or possibly escalate privileges; the race is hard to trigger because normal unmount waits for a grace period.
Affected
- Linux kernel from commit b886ee3e778ec2ad43e276fd378ab492cf6819b7 (version 5.2) up to the fixing commits
- Only systems using casefold (case-insensitive) file name lookup on ext4, f2fs or tmpfs are relevant
- Exact fixed stable release per branch: Unknown.
Fixed in
- Fix commits: 092e14e1479a39157ca01668d43e4649793f39ed, 2b54d3c971a1e5df3c9b0bcc10311ae6c29e79d8, 5d7687b0b71601cc32f1bfd62f56cf5eea3e5a07, 6472455f2b015cad43adb790cb63fe1d3cff2f57, 6560c3d9a6925e3747d9ad8463e16010c31ba715, 69a0d6b31d05e856c7335509222f25321e3c69ca, a96c5299bd7305eb984377681eb3222a57376405, f96c2731a105d6663c606388f5dc21781e00ade5
- Stable branches per CNA/OSV data: 5.10.273, 5.15.224, 6.1.191, 6.6.161, 6.12.115, 6.18.59, 7.2.13 and later.
- Distribution packages: Unknown.
Workaround
- Unknown. No vendor workaround found; the remedy is the kernel update.
- Avoid mounting casefold filesystems you do not need.
- On multi-user hosts, restrict unprivileged user namespaces if they are not required.
Exploitation
- Not known to be exploited (Offseq); CISA KEV: No.
- No public proof of concept found.
- EPSS: 0.18 %.
Our recommendation
- Check whether casefold is used on ext4, f2fs or tmpfs mounts.
- Apply the vendor kernel update in the normal patch cycle once your distribution ships it; prioritise shared, multi-user and build systems.
- Verify fixed versions against your distribution's changelog, as the exact fixed release per branch is not confirmed.
- Limit local access to untrusted users and monitor for kernel crashes (KASAN or oops messages).
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Linux stable commit 092e14e1 git.kernel.org
- Linux stable commit 2b54d3c9 git.kernel.org
- Linux stable commit a96c5299 git.kernel.org
- Ubuntu CVE-2026-99180 ubuntu.com
- Strix CVE-2026-99180 www.strix.ai
- Offseq threat record radar.offseq.com
- Vulners OSV Debian record vulners.com
- Vulners Debian CVE record vulners.com
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 1 Nov 2026, 21:57.
Original description (NVD)
In the Linux kernel, the following vulnerability has been resolved: unicode: free the map after a grace period generic_ci_d_hash() and generic_ci_d_compare() read sb->s_encoding and dereference the unicode map while a pathwalk in rcu mode holds nothing but rcu_read_lock(). The filesystems that use them, ext4, f2fs and tmpfs, drop the map with utf8_unload() from ->put_super(), which runs when the last reference to the mount goes away. A lookup that is still inside such a filesystem at that point reads a freed map. With a pathwalk held up in utf8_casefold_hash() while the last reference to a casefolded tmpfs goes away: BUG: KASAN: slab-use-after-free in utf8nlookup+0x74c/0x950 Read of size 8 at addr ff110001030fe658 by task walker/98 Call Trace: <TASK> utf8nlookup+0x74c/0x950 utf8byte+0x160/0x1560 utf8_casefold_hash+0x167/0x230 generic_ci_d_hash+0xdd/0x1c0 link_path_walk+0x11ad/0x1d30 path_lookupat+0x63/0xb00 filename_lookup+0x1f1/0x5b0 vfs_statx+0xd7/0x3b0 [...] Allocated by task 94: __kmalloc_cache_noprof+0x249/0x560 utf8_load+0x4e/0x570 shmem_parse_opt_casefold.isra.0+0x92/0x270 [...] Freed by task 97: kfree+0x1db/0x5b0 shmem_put_super+0x72/0x170 generic_shutdown_super+0x14c/0x530 kill_anon_super+0x3a/0x60 deactivate_locked_super+0x9c/0xf0 cleanup_mnt+0x9f/0x130 So far that has been hard to hit. umount(2) waits for a grace period in namespace_unlock() before it drops the mount, so only a mount whose last reference goes away elsewhere, like the close of a file or a chdir out of a lazily unmounted directory, can race a walker. So it is reachable. Delay the free until a grace period has passed. The map and the data table it holds a reference to then outlive any pathwalk that found the superblock before the mount went away. The reference on the data table module is dropped from the rcu callback as well. utf8-core is built in whenever CONFIG_UNICODE is set, so there is no module exit that would have to wait for the callbacks. Overlayfs points the s_encoding of an overlay at the map of its first layer without taking a reference of its own. That stays correct. The layer is held by the overlay, so its map is dropped only after the overlay is gone.
NVD references (8)
- https://git.kernel.org/stable/c/092e14e1479a39157ca01668d43e4649793f39ed
- https://git.kernel.org/stable/c/2b54d3c971a1e5df3c9b0bcc10311ae6c29e79d8
- https://git.kernel.org/stable/c/5d7687b0b71601cc32f1bfd62f56cf5eea3e5a07
- https://git.kernel.org/stable/c/6472455f2b015cad43adb790cb63fe1d3cff2f57
- https://git.kernel.org/stable/c/6560c3d9a6925e3747d9ad8463e16010c31ba715
- https://git.kernel.org/stable/c/69a0d6b31d05e856c7335509222f25321e3c69ca
- https://git.kernel.org/stable/c/a96c5299bd7305eb984377681eb3222a57376405
- https://git.kernel.org/stable/c/f96c2731a105d6663c606388f5dc21781e00ade5
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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