# CVE-2026-98084: bpf: backtracking shouldn't clear outer frame R1-R5 for callbacks

> A flaw in the Linux kernel BPF verifier allows loading unsafe programs, potentially leading to privilege escalation or code execution.

- **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:** 25 Sep 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-104294, BELL-CVE-2026-98084, DEBIAN-CVE-2026-98084, GHSA-hcwg-2fch-4v4c, UBUNTU-CVE-2026-98084

## What is CVE-2026-98084?

CVE-2026-98084 is a vulnerability in Linux, published on 25 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:

bpf: backtracking shouldn't clear outer frame R1-R5 for callbacks

When processing calls to bpf_loop() verifier marks R1 (and R4) as
precise. R1 tracks loop iterations number and because of the
'callback_depth < R1' mechanics in check_helper_call() must be marked
precise. However, precision propagation for R1 was broken,
when bpf_loop() call was verified on a second iteration.

Consider the following verification trace:
- main: bpf_loop(nr_loops, callback ...)
- callback: BPF_EXIT
- main: bpf_loop(nr_loops, callback ...)
- ...

While the first visit of the call to bpf_loop() propagated R1
precision as expected, the second call to mark_chain_precision() in
the check_helper_call() set R1, but it was immediately reset when
backtrack_insn() processed preceding BPF_EXIT in the loop deleted in
this patch.

Because of that, the second visit of the call to bpf_loop() injected
checkpoint with R1 not marked as precise. Which could trick the
verifier into accepting unsafe programs. See the next patch for an
example of such program.

Commit is structured in a way to minimize conflicts when
'bpf' would be eventually merged with 'bpf-next'.

## Which versions are affected by CVE-2026-98084?


## How do I fix CVE-2026-98084?

Apply the vendor's patch.

## Is CVE-2026-98084 being exploited?

There is no known exploitation of CVE-2026-98084 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98084 and fix it if it is affected.

What it is: CVE-2026-98084 is a vulnerability in Linux, published on 25 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-2026-98084 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-2026-98084/ (Markdown: https://insights.sbomify.com/CVE-2026-98084/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98084
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98084
- OSV: https://osv.dev/vulnerability/CVE-2026-98084

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98084/
Summaries are generated and may contain mistakes; the official description above is the record.