# CVE-2026-98040: bpf: Mark the zero register precise for a register-form NULL check

> A flaw in the Linux kernel BPF verifier allows local attackers to bypass NULL checks and dereference null pointers, 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:** 19 Oct 2026
- **Also known as:** DEBIAN-CVE-2026-98040, GHSA-xg34-pxxf-5frp, UBUNTU-CVE-2026-98040

## What is CVE-2026-98040?

CVE-2026-98040 is a memory corruption 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: Mark the zero register precise for a register-form NULL check

check_cond_jmp_op() accepts "if rA <op> rB" as a NULL check for a
nullable pointer rA when rB is a scalar known to be zero,
lifts PTR_MAYBE_NULL from rA in the corresponding branch and does not
mark rB precise. Consider the following program:

  r0 = bpf_get_prandom_u32();
  r6 = 1;                  /* the r6 == 0 path is explored first */
  if (r0 == 0) goto 1f;
  r6 = 0;
1:
  r0 = bpf_map_lookup_elem(map, &0);  /* absent, NULL at runtime */
  if (r0 == r6) goto 2f;   /* taken as a NULL check for r0       */
  *(u8 *)(r0 + 0);         /* verifier: map value; runtime: zero */
2:
  return 0;

The r6 == 0 path is explored first and the dereference is accepted.
The r6 == 1 path is pruned at the checkpoint recorded for (1),
so the comparison is never verified with a non-zero r6. At runtime a
failed lookup returns NULL, NULL != 1 takes the non-NULL edge and the
program dereferences a pointer that is zero.

## Which versions are affected by CVE-2026-98040?


## How do I fix CVE-2026-98040?

Apply the vendor's patch.

## Is CVE-2026-98040 being exploited?

There is no known exploitation of CVE-2026-98040 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98040 and fix it if it is affected.

What it is: CVE-2026-98040 is a memory corruption 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-2026-98040 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-2026-98040/ (Markdown: https://insights.sbomify.com/CVE-2026-98040/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98040
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98040
- OSV: https://osv.dev/vulnerability/CVE-2026-98040

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98040/
Summaries are generated and may contain mistakes; the official description above is the record.