# 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.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **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:** 19 Jul 2026
- **Updated:** 17 Aug 2026
- **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

## 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.

## Is CVE-2026-63817 being exploited?

There is no known exploitation of CVE-2026-63817 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63817 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-63817 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-2026-63817/ (Markdown: https://insights.sbomify.com/CVE-2026-63817/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63817
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63817
- OSV: https://osv.dev/vulnerability/CVE-2026-63817

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63817/
Summaries are generated and may contain mistakes; the official description above is the record.