# CVE-2021-43523: Cross-site scripting in n/a

> Attackers can inject malicious content via DNS responses in uClibc, leading to remote code execution or XSS in applications using affected functions.

- **Severity:** Critical 9.6 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/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:** Yes
- **Published:** 10 Nov 2021
- **Updated:** 5 May 2025
- **Also known as:** AZL-6928, DEBIAN-CVE-2021-43523, GHSA-65xv-vmv6-8r3f

## What is CVE-2021-43523?

CVE-2021-43523 is a critical cross-site scripting vulnerability in n/a, published on 10 Nov 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

In uClibc and uClibc-ng before 1.0.39, incorrect handling of special characters in domain names returned by DNS servers via gethostbyname, getaddrinfo, gethostbyaddr, and getnameinfo can lead to output of wrong hostnames (leading to domain hijacking) or injection into applications (leading to remote code execution, XSS, applications crashes, etc.). In other words, a validation step, which is expected in any stub resolver, does not occur.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2021-43523?

It affects 2 products.


Products: Uclibc-Ng Project Uclibc-Ng, Uclibc Uclibc.

## How do I fix CVE-2021-43523?

Upgrade to a fixed version.

## Is CVE-2021-43523 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2021-43523 exists.

## Other vulnerabilities in n/a

- [CVE-2016-2224](https://insights.sbomify.com/CVE-2016-2224/): Denial of service in n/a (high 7.5)
- [CVE-2016-2225](https://insights.sbomify.com/CVE-2016-2225/): Denial of service in n/a (high 7.5)
- [CVE-2016-6264](https://insights.sbomify.com/CVE-2016-6264/): Integer overflow in n/a (high 7.5)
- [CVE-2021-27419](https://insights.sbomify.com/CVE-2021-27419/): uClibc-ng Integer Overflow or Wraparound (high 7.3)
- [CVE-2022-30295](https://insights.sbomify.com/CVE-2022-30295/): Other in n/a (medium 6.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-43523 and fix it if it is affected.

What it is: CVE-2021-43523 is a critical cross-site scripting vulnerability in n/a, published on 10 Nov 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: It is not confirmed as exploited in the wild, but public exploit code for CVE-2021-43523 exists.

Affected product: n/a. 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-2021-43523/ (Markdown: https://insights.sbomify.com/CVE-2021-43523/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-43523
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-43523
- OSV: https://osv.dev/vulnerability/CVE-2021-43523

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-43523/
Summaries are generated and may contain mistakes; the official description above is the record.