# CVE-2021-29503: Improper Neutralization of Script-Related HTML Tags in Notes

> Attackers can inject JavaScript via YAML metadata in HedgeDoc notes, potentially leading to code execution or account takeover.

- **Severity:** High 8.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N`)
- **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:** 19 May 2021
- **Updated:** 3 Aug 2024
- **Also known as:** GHSA-gjg7-4j2h-94fq

## What is CVE-2021-29503?

CVE-2021-29503 is a high-severity cross-site scripting vulnerability in hedgedoc, published on 19 May 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

HedgeDoc is a platform to write and share markdown. HedgeDoc before version 1.8.2 is vulnerable to a cross-site scripting attack using the YAML-metadata of a note. An attacker with write access to a note can embed HTML tags in the Open Graph metadata section of the note, resulting in the frontend rendering the script tag as part of the `<head>` section. Unless your instance prevents guests from editing notes, this vulnerability allows unauthenticated attackers to inject JavaScript into notes that allow guest edits. If your instance prevents guests from editing notes, this vulnerability allows authenticated attackers to inject JavaScript into any note pages they have write-access to. This vulnerability is patched in version 1.8.2. As a workaround, one can disable guest edits until the next update.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation), CWE-80 (Improper Neutralization of Script-Related HTML Tags in a Web Page)

## Which versions are affected by CVE-2021-29503?

It affects 1 product.


Products: Hedgedoc Hedgedoc.

## How do I fix CVE-2021-29503?

No fix has been published for CVE-2021-29503 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-29503 being exploited?

There is no known exploitation of CVE-2021-29503 in the wild and no public exploit code.

## Other vulnerabilities in hedgedoc

- [CVE-2021-29475](https://insights.sbomify.com/CVE-2021-29475/): PDF export allows arbitrary file reads (critical 10.0)
- [CVE-2020-26287](https://insights.sbomify.com/CVE-2020-26287/): Stored XSS in mermaid diagrams (high 8.7)
- [CVE-2021-39175](https://insights.sbomify.com/CVE-2021-39175/): XSS vector in slide mode speaker-view (high 8.1)
- [CVE-2020-26286](https://insights.sbomify.com/CVE-2020-26286/): Arbitary file upload (high 7.5)
- [CVE-2021-21259](https://insights.sbomify.com/CVE-2021-21259/): Stored XSS in slide mode (high 7.4)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-29503 and fix it if it is affected.

What it is: CVE-2021-29503 is a high-severity cross-site scripting vulnerability in hedgedoc, published on 19 May 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-2021-29503 in the wild and no public exploit code.

Affected product: hedgedoc. 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-2021-29503/ (Markdown: https://insights.sbomify.com/CVE-2021-29503/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-29503
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-29503
- OSV: https://osv.dev/vulnerability/CVE-2021-29503

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-29503/
Summaries are generated and may contain mistakes; the official description above is the record.