# CVE-2026-98061: bpf: Reject tail calls directly from callback frames

> A local attacker with CAP_BPF privileges can trigger a kernel warning and denial of service by loading a BPF program that performs tail calls from callback frames.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** DEBIAN-CVE-2026-98061, GHSA-3q38-vxgf-8598, UBUNTU-CVE-2026-98061

## What is CVE-2026-98061?

CVE-2026-98061 is a denial of service 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: Reject tail calls directly from callback frames

A tail call from a non-zero frame is modeled as a return from that frame.
The verifier makes R0 unknown and calls prepare_func_exit() for the taken
branch.

When the current frame is a synchronous callback, prepare_func_exit()
enforces the callback return-value contract and marks R0 precise. Since the
tail-call path synthesized R0 rather than deriving it from an instruction,
precision backtracking reaches the callback-calling instruction with R0
still requested and triggers the "callback unexpected regs" verifier bug.
A CAP_BPF task can therefore cause a WARN and an -EFAULT BPF_PROG_LOAD.

Tail calls reachable from callbacks are already rejected later by
check_max_stack_depth(). Reject a tail call made directly by a callback
before constructing the inconsistent return state, using the existing
diagnostic. Tail calls from ordinary subprograms keep their current
behavior.

## Which versions are affected by CVE-2026-98061?


## How do I fix CVE-2026-98061?

Apply the vendor's patch.

## Is CVE-2026-98061 being exploited?

There is no known exploitation of CVE-2026-98061 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98061 and fix it if it is affected.

What it is: CVE-2026-98061 is a denial of service 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-2026-98061 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-2026-98061/ (Markdown: https://insights.sbomify.com/CVE-2026-98061/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98061
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98061
- OSV: https://osv.dev/vulnerability/CVE-2026-98061

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98061/
Summaries are generated and may contain mistakes; the official description above is the record.