# CVE-2020-26287: Stored XSS in mermaid diagrams

> Attackers can inject arbitrary JavaScript via mermaid diagrams in HedgeDoc notes, potentially executing code or taking over accounts.

- **Severity:** High 8.7 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 28 Dec 2020
- **Updated:** 4 Aug 2024
- **Also known as:** GHSA-g6w6-7xf9-m95p

## What is CVE-2020-26287?

CVE-2020-26287 is a high-severity cross-site scripting vulnerability in hedgedoc, published on 28 Dec 2020. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.

### Official description

HedgeDoc is a collaborative platform for writing and sharing markdown. In HedgeDoc before version 1.7.1 an attacker can inject arbitrary `script` tags in HedgeDoc notes using mermaid diagrams. Our content security policy prevents loading scripts from most locations, but `www.google-analytics.com` is allowed. Using Google Tag Manger it is possible to inject arbitrary JavaScript and execute it on page load. Depending on the configuration of the instance, the attacker may not need authentication to create or edit notes. The problem is patched in HedgeDoc 1.7.1. As a workaround one can disallow `www.google-analytics.com` in the `Content-Security-Policy` header. Note that other ways to leverage the `script` tag injection might exist.

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

## Which versions are affected by CVE-2020-26287?

It affects 1 product.


Products: Hedgedoc Hedgedoc.

## How do I fix CVE-2020-26287?

No fix has been published for CVE-2020-26287 yet. Limit exposure of the affected software until one is available.

## Is CVE-2020-26287 being exploited?

There is no known exploitation of CVE-2020-26287 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-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)
- [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-2020-26287 and fix it if it is affected.

What it is: CVE-2020-26287 is a high-severity cross-site scripting vulnerability in hedgedoc, published on 28 Dec 2020. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2020-26287 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-2020-26287/ (Markdown: https://insights.sbomify.com/CVE-2020-26287/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2020-26287
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2020-26287
- OSV: https://osv.dev/vulnerability/CVE-2020-26287

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From sbomify Insights: https://insights.sbomify.com/CVE-2020-26287/
Summaries are generated and may contain mistakes; the official description above is the record.