# CVE-2026-90203: Squashfs: check block offset is not negative

> A crafted Squashfs filesystem can cause an out-of-bounds access in the Linux kernel, potentially leading to denial of service or code execution.

- **Severity:** High 7.1 (`CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 17 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-102374, BELL-CVE-2026-90203, DEBIAN-CVE-2026-90203, GHSA-3vc4-628m-7f5q, UBUNTU-CVE-2026-90203

## What is CVE-2026-90203?

CVE-2026-90203 is a high-severity memory corruption vulnerability in Linux, published on 17 Sep 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

Squashfs: check block offset is not negative

If a negative offset is read off disk (for example the offset into the
decompressed fragment block), this will cause squashfs_copy_data() to
perform an out of bounds access.

Fix by checking if offset is negative, and returning 0.  This matches
existing behaviour where an offset beyond the block returns 0 bytes
copied.

To trigger this out of bounds access requires a crafted Squashfs
filesystem and CAP_SYS_ADMIN to mount it.  Unprivileged users will not be
able to mount such a filesystem, but once mounted, an unprivileged user
can trigger the out of bounds access by reading the crafted file with the
negative offset.

## Which versions are affected by CVE-2026-90203?


## How do I fix CVE-2026-90203?

Apply the vendor's patch.

## Is CVE-2026-90203 being exploited?

There is no known exploitation of CVE-2026-90203 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90203 and fix it if it is affected.

What it is: CVE-2026-90203 is a high-severity memory corruption vulnerability in Linux, published on 17 Sep 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-90203 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-2026-90203/ (Markdown: https://insights.sbomify.com/CVE-2026-90203/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90203
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90203
- OSV: https://osv.dev/vulnerability/CVE-2026-90203

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90203/
Summaries are generated and may contain mistakes; the official description above is the record.