# CVE-2026-43009: bpf: Fix incorrect pruning due to atomic fetch precision tracking

> Incorrect BPF verifier pruning due to atomic fetch precision tracking flaws may allow bypassing security checks or undefined behavior.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **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:** 1 May 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-85407, BELL-CVE-2026-43009, DEBIAN-CVE-2026-43009, GHSA-c84q-5rwr-h9h9, ROOT-OS-DEBIAN-12-CVE-2026-43009, ROOT-OS-DEBIAN-13-CVE-2026-43009, ROOT-OS-UBUNTU-2204-CVE-2026-43009, ROOT-OS-UBUNTU-2404-CVE-2026-43009, UBUNTU-CVE-2026-43009

## What is CVE-2026-43009?

CVE-2026-43009 is a high-severity vulnerability in Linux kernel, published on 1 May 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 incorrect pruning due to atomic fetch precision tracking

When backtrack_insn encounters a BPF_STX instruction with BPF_ATOMIC
and BPF_FETCH, the src register (or r0 for BPF_CMPXCHG) also acts as
a destination, thus receiving the old value from the memory location.

The current backtracking logic does not account for this. It treats
atomic fetch operations the same as regular stores where the src
register is only an input. This leads the backtrack_insn to fail to
propagate precision to the stack location, which is then not marked
as precise!

Later, the verifier's path pruning can incorrectly consider two states
equivalent when they differ in terms of stack state. Meaning, two
branches can be treated as equivalent and thus get pruned when they
should not be seen as such.

Fix it as follows: Extend the BPF_LDX handling in backtrack_insn to
also cover atomic fetch operations via is_atomic_fetch_insn() helper.
When the fetch dst register is being tracked for precision, clear it,
and propagate precision over to the stack slot. For non-stack memory,
the precision walk stops at the atomic instruction, same as regular
BPF_LDX. This covers all fetch variants.

Before:

  0: (b7) r1 = 8                        ; R1=8
  1: (7b) *(u64 *)(r10 -8) = r1         ; R1=8 R10=fp0 fp-8=8
  2: (b7) r2 = 0                        ; R2=0
  3: (db) r2 = atomic64_fetch_add((u64 *)(r10 -8), r2)          ; R2=8 R10=fp0 fp-8=mmmmmmmm
  4: (bf) r3 = r10                      ; R3=fp0 R10=fp0
  5: (0f) r3 += r2
  mark_precise: frame0: last_idx 5 first_idx 0 subseq_idx -1
  mark_precise: frame0: regs=r2 stack= before 4: (bf) r3 = r10
  mark_precise: frame0: regs=r2 stack= before 3: (db) r2 = atomic64_fetch_add((u64 *)(r10 -8), r2)
  mark_precise: frame0: regs=r2 stack= before 2: (b7) r2 = 0
  6: R2=8 R3=fp8
  6: (b7) r0 = 0                        ; R0=0
  7: (95) exit

After:

  0: (b7) r1 = 8                        ; R1=8
  1: (7b) *(u64 *)(r10 -8) = r1         ; R1=8 R10=fp0 fp-8=8
  2: (b7) r2 = 0                        ; R2=0
  3: (db) r2 = atomic64_fetch_add((u64 *)(r10 -8), r2)          ; R2=8 R10=fp0 fp-8=mmmmmmmm
  4: (bf) r3 = r10                      ; R3=fp0 R10=fp0
  5: (0f) r3 += r2
  mark_precise: frame0: last_idx 5 first_idx 0 subseq_idx -1
  mark_precise: frame0: regs=r2 stack= before 4: (bf) r3 = r10
  mark_precise: frame0: regs=r2 stack= before 3: (db) r2 = atomic64_fetch_add((u64 *)(r10 -8), r2)
  mark_precise: frame0: regs= stack=-8 before 2: (b7) r2 = 0
  mark_precise: frame0: regs= stack=-8 before 1: (7b) *(u64 *)(r10 -8) = r1
  mark_precise: frame0: regs=r1 stack= before 0: (b7) r1 = 8
  6: R2=8 R3=fp8
  6: (b7) r0 = 0                        ; R0=0
  7: (95) exit

Weakness: CWE-682 (Incorrect Calculation)

## Which versions are affected by CVE-2026-43009?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-43009?

Apply the vendor's patch.

## Is CVE-2026-43009 being exploited?

There is no known exploitation of CVE-2026-43009 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-43009 and fix it if it is affected.

What it is: CVE-2026-43009 is a high-severity vulnerability in Linux kernel, published on 1 May 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-2026-43009 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2026-43009/ (Markdown: https://insights.sbomify.com/CVE-2026-43009/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-43009
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-43009
- OSV: https://osv.dev/vulnerability/CVE-2026-43009

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-43009/
Summaries are generated and may contain mistakes; the official description above is the record.