# CVE-2021-39521: Null dereference in n/a

> A NULL pointer dereference in libredwg allows attackers to cause a denial of service via crafted DWG files.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 20 Sep 2021
- **Updated:** 4 Aug 2024
- **Also known as:** GHSA-3fvf-wxcf-7v9q

## What is CVE-2021-39521?

CVE-2021-39521 is a medium-severity vulnerability in Gnu Libredwg, published on 20 Sep 2021. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

An issue was discovered in libredwg through v0.10.1.3751. A NULL pointer dereference exists in the function bit_read_BB() located in bits.c. It allows an attacker to cause Denial of Service.

Weakness: CWE-476 (NULL Pointer Dereference)

## Which versions are affected by CVE-2021-39521?

It affects 1 product.


Products: Gnu Libredwg.

## How do I fix CVE-2021-39521?

No fix has been published for CVE-2021-39521 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-39521 being exploited?

There is no known exploitation of CVE-2021-39521 in the wild and no public exploit code.

## Other vulnerabilities in Gnu Libredwg

- [CVE-2022-35164](https://insights.sbomify.com/CVE-2022-35164/): Use after free in n/a (critical 9.8)
- [CVE-2021-28237](https://insights.sbomify.com/CVE-2021-28237/): Memory corruption in n/a (critical 9.8)
- [CVE-2019-20914](https://insights.sbomify.com/CVE-2019-20914/): Null dereference in n/a (critical 9.8)
- [CVE-2019-9774](https://insights.sbomify.com/CVE-2019-9774/): Memory corruption in n/a (critical 9.1)
- [CVE-2019-9775](https://insights.sbomify.com/CVE-2019-9775/): Information disclosure in n/a (critical 9.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-39521 and fix it if it is affected.

What it is: CVE-2021-39521 is a medium-severity vulnerability in Gnu Libredwg, published on 20 Sep 2021. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2021-39521 in the wild and no public exploit code.

Affected product: Gnu Libredwg. 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-2021-39521/ (Markdown: https://insights.sbomify.com/CVE-2021-39521/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-39521
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-39521
- OSV: https://osv.dev/vulnerability/CVE-2021-39521

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-39521/
Summaries are generated and may contain mistakes; the official description above is the record.