# CVE-2026-89787: ext4: check dir entry fits before reading the hash trailer in ext4_search_dir()

> A local attacker can trigger a use-after-free in the Linux kernel ext4 filesystem by crafting a directory entry, potentially causing a crash or leaking memory.

- **Severity:** Not scored
- **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:** Yes
- **Published:** 16 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-101439, BELL-CVE-2026-89787, DEBIAN-CVE-2026-89787, GHSA-cxfp-66mh-jhvv, UBUNTU-CVE-2026-89787

## What is CVE-2026-89787?

CVE-2026-89787 is an use after free vulnerability in Linux, published on 16 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: check dir entry fits before reading the hash trailer in ext4_search_dir()

For casefolded encrypted directories ext4 stores an 8-byte hash trailer
after the name (EXT4_DIRENT_HASHES()), at an offset derived from
de->name_len.  On the sb_no_casefold_compat_fallback() path ext4_match()
reads that trailer, but ext4_search_dir()'s by-hand pre-check only tests
de->name + de->name_len <= dlimit, which proves the name fits, not the
rounded trailer.  A crafted entry whose name ends at the block boundary
passes the check while EXT4_DIRENT_HASHES(de) lands past the block end,
so ext4_match() reads out of bounds on an ordinary lookup.  KASAN reports
it as a use-after-free when the page after the directory block holds a
freed object:

  BUG: KASAN: use-after-free in ext4_match (fs/ext4/namei.c:1435)
  Read of size 4 at addr ffff888010458000 by task exploit
  Call Trace:
   ext4_match (fs/ext4/namei.c:1435)
   ext4_search_dir (fs/ext4/namei.c:1470)
   __ext4_find_entry (fs/ext4/namei.c:1268 fs/ext4/namei.c:1632)
   ext4_lookup (fs/ext4/namei.c:1703 fs/ext4/namei.c:1769)
   ...
   filename_lookup (fs/namei.c:2842)
   vfs_statx (fs/stat.c:353)
   __do_sys_newfstatat (fs/stat.c:538)
   do_syscall_64 (arch/x86/entry/syscall_64.c:94)
   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)

Require, for hash-in-dirent directories, that the whole entry including
the rounded trailer fits before calling ext4_match().  This is the same
bound ext4_check_dir_entry() already enforces via ext4_dir_rec_len(), so
no well-formed entry is rejected.  The other caller, ext4_find_dest_de(),
runs ext4_check_dir_entry() first and is unaffected.

## Which versions are affected by CVE-2026-89787?


## How do I fix CVE-2026-89787?

Apply the vendor's patch.

## Is CVE-2026-89787 being exploited?

There is no known exploitation of CVE-2026-89787 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89787 and fix it if it is affected.

What it is: CVE-2026-89787 is an use after free vulnerability in Linux, published on 16 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-89787 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-89787/ (Markdown: https://insights.sbomify.com/CVE-2026-89787/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89787
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89787
- OSV: https://osv.dev/vulnerability/CVE-2026-89787

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89787/
Summaries are generated and may contain mistakes; the official description above is the record.