Mermaid improperly sanitizes sequence diagram labels leading to XSS
Summary
In the default configuration of mermaid 11.9.0, user supplied input for sequence diagram labels is passed to innerHTML during calculation of element size, causing XSS.
Details
Sequence diagram node labels with KaTeX delimiters are passed through calculateMathMLDimensions. This method passes the full label to innerHTML which allows allows malicious users to inject arbitrary HTML and cause XSS when mermaid-js is used in it's default configuration (with KaTeX support enabled).
The vulnerability lies here:
export const calculateMathMLDimensions = async (text: string, config: MermaidConfig) => {
text = await renderKatex(text, config);
const divElem = document.createElement('div');
divElem.innerHTML = text; // XSS sink, text has not been sanitized.
divElem.id = 'katex-temp';
divElem.style.visibility = 'hidden';
divElem.style.position = 'absolute';
divElem.style.top = '0';
const body = document.querySelector('body');
body?.insertAdjacentElement('beforeend', divElem);
const dim = { width: divElem.clientWidth, height: divElem.clientHeight };
divElem.remove();
return dim;
};The calculateMathMLDimensions method was introduced in 5c69e5fdb004a6d0a2abe97e23d26e223a059832 two years ago, which was released in Mermaid 10.9.0.
PoC
Render the following diagram and observe the modified DOM.
sequenceDiagram
participant A as Alice<img src="x" onerror="document.write(`xss on ${document.domain}`)">$$\\text{Alice}$$
A->>John: Hello John, how are you?
Alice-)John: See you later!Impact
XSS on all sites that use mermaid and render user supplied diagrams without further sanitization.
Remediation
The value of the text argument for the calculateMathMLDimensions method needs to be sanitized before getting passed on to innerHTML.
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| npm package | mermaid | >= 11.0.0-alpha.1, < 11.10.0, >= 10.9.0-rc.1, < 10.9.4 | 11.10.0, 10.9.4 |
References
- https://github.com/mermaid-js/mermaid/security/advisories/GHSA-7rqq-prvp-x9jh
- https://nvd.nist.gov/vuln/detail/CVE-2025-54881
- https://github.com/mermaid-js/mermaid/commit/5c69e5fdb004a6d0a2abe97e23d26e223a059832
- https://github.com/mermaid-js/mermaid/commit/685516a85ec1df64cefd4fd15f26533be87d458e
- https://github.com/mermaid-js/mermaid
- CVSS vector
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N- Weakness (CWE)
CWE-79- Platform
- npm
- Package
mermaid- Disclosed
- 19.08.2025
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-7rqq-prvp-x9jh - CC BY 4.0
Vulnerability history: mermaid
All published vulnerabilities of this package (npm package), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 16
- First → latest
- 09.2020 08.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 3
- Medium 12
- Low 1
Disclosures per year
- 2020: 1
- 2021: 1
- 2022: 2
- 2023: 0
- 2024: 1
- 2025: 2
- 2026: 9
-
Mermaid improperly sanitizes sequence diagram labels leading to XSS this entry
Know which vulnerabilities affect your application
With the DockRay integration and software inventory enabled, DockRay checks reported component versions against known vulnerabilities and notifies you about confirmed matches. Coverage depends on the supported technology and available advisory data.