What’s Vulnerable › Vulnerabilities
CVE-2020-26287: Stored XSS in mermaid diagrams
- Severity: high (CVSS 8.7 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 1.5% probability of exploitation in the next 30 days
- Public exploit or detection template: yes
- Published: 2020-12-29 · updated 2026-06-17
- Weakness: CWE-79
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.
Affected products
- hedgedoc: before 1.7.1 — fixed in 1.7.1
How to fix
Upgrade to a fixed version: hedgedoc 1.7.1. Check the vendor advisory for mitigations.
References
- https://github.com/Alemmi/ctf-writeups/blob/main/hxpctf-2020/hackme/solution.md
- https://github.com/hackmdio/codimd/issues/1630
- https://github.com/hedgedoc/hedgedoc/commit/58276ebbf4504a682454a3686dcaff88bc1069d4
- https://github.com/hedgedoc/hedgedoc/releases/tag/1.7.1
- https://github.com/hedgedoc/hedgedoc/security/advisories/GHSA-g6w6-7xf9-m95p
- https://github.com/Alemmi/ctf-writeups/blob/main/hxpctf-2020/hackme/solution.md
- https://github.com/hackmdio/codimd/issues/1630
- https://github.com/hedgedoc/hedgedoc/commit/58276ebbf4504a682454a3686dcaff88bc1069d4
- https://github.com/hedgedoc/hedgedoc/releases/tag/1.7.1
- https://github.com/hedgedoc/hedgedoc/security/advisories/GHSA-g6w6-7xf9-m95p
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