# CVE-2021-47607: bpf: Fix kernel address leakage in atomic cmpxchg's r0 aux reg

> An unprivileged user can leak kernel pointers via BPF atomic cmpxchg operations, potentially aiding further attacks.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N`)
- **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:** 19 Jun 2024
- **Updated:** 11 May 2026
- **Also known as:** DEBIAN-CVE-2021-47607, GHSA-fp67-w4xm-4hj6, UBUNTU-CVE-2021-47607

## What is CVE-2021-47607?

CVE-2021-47607 is a medium-severity vulnerability in Linux kernel, published on 19 Jun 2024. 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 kernel address leakage in atomic cmpxchg's r0 aux reg

The implementation of BPF_CMPXCHG on a high level has the following parameters:

  .-[old-val]                                          .-[new-val]
  BPF_R0 = cmpxchg{32,64}(DST_REG + insn->off, BPF_R0, SRC_REG)
                          `-[mem-loc]          `-[old-val]

Given a BPF insn can only have two registers (dst, src), the R0 is fixed and
used as an auxilliary register for input (old value) as well as output (returning
old value from memory location). While the verifier performs a number of safety
checks, it misses to reject unprivileged programs where R0 contains a pointer as
old value.

Through brute-forcing it takes about ~16sec on my machine to leak a kernel pointer
with BPF_CMPXCHG. The PoC is basically probing for kernel addresses by storing the
guessed address into the map slot as a scalar, and using the map value pointer as
R0 while SRC_REG has a canary value to detect a matching address.

Fix it by checking R0 for pointers, and reject if that's the case for unprivileged
programs.

## Which versions are affected by CVE-2021-47607?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2021-47607?

Apply the vendor's patch.

## Is CVE-2021-47607 being exploited?

There is no known exploitation of CVE-2021-47607 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-2021-47607 and fix it if it is affected.

What it is: CVE-2021-47607 is a medium-severity vulnerability in Linux kernel, published on 19 Jun 2024. 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-2021-47607 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-2021-47607/ (Markdown: https://insights.sbomify.com/CVE-2021-47607/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-47607
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-47607
- OSV: https://osv.dev/vulnerability/CVE-2021-47607

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-47607/
Summaries are generated and may contain mistakes; the official description above is the record.