# CVE-2021-21259: Stored XSS in slide mode

> Attackers can inject arbitrary JavaScript into HedgeDoc notes, executing it in slide mode, potentially leading to code execution or account takeover.

- **Severity:** High 7.4 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/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:** 22 Jan 2021
- **Updated:** 3 Aug 2024
- **Also known as:** GHSA-44w9-vm8p-3cxw

## What is CVE-2021-21259?

CVE-2021-21259 is a high-severity cross-site scripting vulnerability in hedgedoc, published on 22 Jan 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 open source software which lets you create real-time collaborative markdown notes. In HedgeDoc before version 1.7.2, an attacker can inject arbitrary JavaScript into a HedgeDoc note, which is executed when the note is viewed in slide mode. Depending on the configuration of the instance, the attacker may not need authentication to create or edit notes. The problem is patched in HedgeDoc 1.7.2. As a workaround, disallow loading JavaScript from 3rd party sites using the `Content-Security-Policy` header. Note that this will break some embedded content.

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

## Which versions are affected by CVE-2021-21259?

It affects 1 product.


Products: Hedgedoc Hedgedoc.

## How do I fix CVE-2021-21259?

No fix has been published for CVE-2021-21259 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-21259 being exploited?

There is no known exploitation of CVE-2021-21259 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-2021-29503](https://insights.sbomify.com/CVE-2021-29503/): Improper Neutralization of Script-Related HTML Tags in Notes (high 8.1)
- [CVE-2020-26286](https://insights.sbomify.com/CVE-2020-26286/): Arbitary file upload (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-21259 and fix it if it is affected.

What it is: CVE-2021-21259 is a high-severity cross-site scripting vulnerability in hedgedoc, published on 22 Jan 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-2021-21259 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-2021-21259/ (Markdown: https://insights.sbomify.com/CVE-2021-21259/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-21259
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-21259
- OSV: https://osv.dev/vulnerability/CVE-2021-21259

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-21259/
Summaries are generated and may contain mistakes; the official description above is the record.