# CVE-2021-28543: Null dereference in n/a

> Remote attackers can cause a denial of service by triggering a null pointer dereference in Varnish Cache modules, leading to daemon restarts.

- **Severity:** Medium 4.0 (`CVSS:3.1/AC:H/AV:N/A:L/C:N/I:N/PR:N/S:C/UI:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 16 Mar 2021
- **Updated:** 3 Aug 2024
- **Also known as:** GHSA-jpm5-7xh4-qgjv

## What is CVE-2021-28543?

CVE-2021-28543 is a medium-severity vulnerability in Fedoraproject Fedora, published on 16 Mar 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Varnish varnish-modules before 0.17.1 allows remote attackers to cause a denial of service (daemon restart) in some configurations. This does not affect organizations that only install the Varnish Cache product; however, it is common to install both Varnish Cache and varnish-modules. Specifically, an assertion failure or NULL pointer dereference can be triggered in Varnish Cache through the varnish-modules header.append() and header.copy() functions. For some Varnish Configuration Language (VCL) files, this gives remote clients an opportunity to cause a Varnish Cache restart. A restart reduces overall availability and performance due to an increased number of cache misses, and may cause higher load on backend servers.

Weakness: CWE-617 (Reachable Assertion), CWE-476 (NULL Pointer Dereference)

## Which versions are affected by CVE-2021-28543?

It affects 3 products.


Products: Fedoraproject Fedora, Varnish-Cache Varnish-Modules, Varnish-Cache Varnish-Modules Klarlack.

## How do I fix CVE-2021-28543?

No fix has been published for CVE-2021-28543 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-28543 being exploited?

There is no known exploitation of CVE-2021-28543 in the wild and no public exploit code.

## Other vulnerabilities in Fedoraproject Fedora

- [CVE-2024-4577](https://insights.sbomify.com/CVE-2024-4577/): PHP-CGI OS Command Injection (critical 9.8, exploited in the wild)
- [CVE-2021-39226](https://insights.sbomify.com/CVE-2021-39226/): Grafana Authentication Bypass (critical 9.8, exploited in the wild)
- [CVE-2019-5544](https://insights.sbomify.com/CVE-2019-5544/): VMware ESXi and Horizon DaaS OpenSLP Heap-Based Buffer Overflow (critical 9.8, exploited in the wild)
- [CVE-2024-4947](https://insights.sbomify.com/CVE-2024-4947/): Google Chromium V8 Type Confusion (critical 9.6, exploited in the wild)
- [CVE-2024-4671](https://insights.sbomify.com/CVE-2024-4671/): Google Chromium Visuals Use-After-Free (critical 9.6, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-28543 and fix it if it is affected.

What it is: CVE-2021-28543 is a medium-severity vulnerability in Fedoraproject Fedora, published on 16 Mar 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-28543 in the wild and no public exploit code.

Affected product: Fedoraproject Fedora. 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-2021-28543/ (Markdown: https://insights.sbomify.com/CVE-2021-28543/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-28543
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-28543
- OSV: https://osv.dev/vulnerability/CVE-2021-28543

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-28543/
Summaries are generated and may contain mistakes; the official description above is the record.