# CVE-2026-98591: bpf: Fix bounds check for skb-backed dynptrs

> An integer underflow in Linux kernel BPF bounds checking allows attackers to access invalid memory, potentially leading to code execution or privilege escalation.

- **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:** 18 Oct 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-109223, BELL-CVE-2026-98591, DEBIAN-CVE-2026-98591, GHSA-4j29-hvp6-5v88, UBUNTU-CVE-2026-98591

## What is CVE-2026-98591?

CVE-2026-98591 is a vulnerability in Linux, published on 18 Oct 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:

bpf: Fix bounds check for skb-backed dynptrs

The skb_pointer_if_linear() function checks whether a
memory region of length len starting at offset off into
the skb is in the linear area, and returns a pointer to
the region if so. The check currently subtracts between
skb_headlen and offset of the check, and since skb_headlen
is unsigned the subtraction can underflow. This causes the
bounds check to spuriously pass and generate an arbitrary
pointer of the form *(skb->data + off).

The only user of this helper is currently skb-backed BPF
dynptr code. Returning the wrong pointer leads to the
dynptr erroneously being backed with invalid memory.

Ensure the subtraction cannot underflow, and fail the check if
it would. Use u64 arithmetic to also prevent overflow when
calculating (skb_headlen(skb) - off) since off is unsigned.

## Which versions are affected by CVE-2026-98591?


## How do I fix CVE-2026-98591?

Apply the vendor's patch.

## Is CVE-2026-98591 being exploited?

There is no known exploitation of CVE-2026-98591 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98591 and fix it if it is affected.

What it is: CVE-2026-98591 is a vulnerability in Linux, published on 18 Oct 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-2026-98591 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-2026-98591/ (Markdown: https://insights.sbomify.com/CVE-2026-98591/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98591
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98591
- OSV: https://osv.dev/vulnerability/CVE-2026-98591

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98591/
Summaries are generated and may contain mistakes; the official description above is the record.