GHSA-26gq-grmh-6xm6 Gogs vulnerable to Stored XSS via Mermaid diagrams

Not scored Published 6 Feb 2026

Attackers can inject malicious scripts via Mermaid diagrams in Markdown files, leading to account takeover or admin compromise when viewed.

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

What is GHSA-26gq-grmh-6xm6?

GHSA-26gq-grmh-6xm6 is a cross-site scripting vulnerability in gogs.io/gogs, published on 6 Feb 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.

Official description

Summary

Stored XSS via mermaid diagrams due to usage of vulnerable renderer library

Details

Gogs introduced support for rendering mermaid diagrams in version 0.13.0.

Currently used version of the library mermaid 11.9.0 is vulnerable to at least two XSS scenarios with publicly available payloads

Resources: https://github.com/mermaid-js/mermaid/security/advisories/GHSA-7rqq-prvp-x9jh https://github.com/mermaid-js/mermaid/security/advisories/GHSA-8gwm-58g9-j8pw

PoC

Create a markdown file eg. README.md containing following malicious mermaid diagram (payload based on CVE-2025-54880)

architecture-beta
    group api(cloud)[API]
    service db &#34;&lt;img src=x onerror=\&#34;alert(document.domain)\&#34;&gt;&#34; [Database] in api

The XSS should pop whenever either repository or file is viewed

Demo

https://github.com/user-attachments/assets/98320f62-6c1c-4254-aa61-95598c725235

Impact

The attacker can potentially achieve account takeover In a worst case scenario if the victim were an instance admin this could lead to a compromise of the entire deployment

Proposed remediation steps

Upgrade to a patched version of the third party library https://github.com/mermaid-js/mermaid/releases/tag/v10.9.5
Consider running mermaid using sandbox level which would mitigate impact of future potential cross-site scripting issues https://mermaid.js.org/config/usage.html#securitylevel

Which versions are affected by GHSA-26gq-grmh-6xm6?

3 open source packages ship an affected version, including gogs.io/gogs.

Package	Affected	Fixed in
`gogs.io/gogs` Go	All versions	None
`gogs.io/gogs` Go	All versions	`0.13.4`
`gogs.io/gogs` Go	All versions	`0.13.4`

How do I fix GHSA-26gq-grmh-6xm6?

Upgrade gogs.io/gogs to 0.13.4 or later.

Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.

Is GHSA-26gq-grmh-6xm6 being exploited?

There is no known exploitation of GHSA-26gq-grmh-6xm6 in the wild and no public exploit code.

Sources

Also known as GO-2026-4454.