# CVE-2021-43827: Inline footnotes wrapped in <a> tags can cause errors in discourse-footnotes

> A null reference error in JavaScript occurs when rendering inline footnotes wrapped in anchor tags, causing errors on topic pages for Discourse users.

- **Severity:** Medium 4.3 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 14 Dec 2021
- **Updated:** 4 Aug 2024
- **Also known as:** GHSA-58vr-c56v-qr57

## What is CVE-2021-43827?

CVE-2021-43827 is a medium-severity vulnerability in discourse-footnote, published on 14 Dec 2021. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

discourse-footnote is a library providing footnotes for posts in Discourse. ### Impact When posting an inline footnote wrapped in `<a>` tags (e.g. `<a>^[footnote]</a>`, the resulting rendered HTML would include a nested `<a>`, which is stripped by Nokogiri because it is not valid. This then caused a javascript error on topic pages because we were looking for an `<a>` element inside the footnote reference span and getting its ID, and because it did not exist we got a null reference error in javascript. Users are advised to update to version 0.2. As a workaround editing offending posts from the rails console or the database console for self-hosters, or disabling the plugin in the admin panel can mitigate this issue.

Weakness: CWE-755 (Improper Handling of Exceptional Conditions)

## Which versions are affected by CVE-2021-43827?

It affects 1 product.


Products: Discourse Discourse Footnote.

## How do I fix CVE-2021-43827?

No fix has been published for CVE-2021-43827 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-43827 being exploited?

There is no known exploitation of CVE-2021-43827 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-43827 and fix it if it is affected.

What it is: CVE-2021-43827 is a medium-severity vulnerability in discourse-footnote, published on 14 Dec 2021. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-43827 in the wild and no public exploit code.

Affected product: discourse-footnote. 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-2021-43827/ (Markdown: https://insights.sbomify.com/CVE-2021-43827/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-43827
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-43827
- OSV: https://osv.dev/vulnerability/CVE-2021-43827

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-43827/
Summaries are generated and may contain mistakes; the official description above is the record.