CVE-2026-21866: Dify - Stored XSS in chat

Severity: medium (CVSS 5.4 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-03-03 · updated 2026-06-17
Weakness: CWE-79

Description

Dify is an open-source LLM app development platform. Prior to 1.11.2, Dify is vulnerable to a stored XSS issue when rendering Mermaid diagrams within chats. This occurs because Dify’s default Mermaid configuration uses securityLevel: loose, which allows potentially unsafe content to execute. This vulnerability is fixed in 1.11.2.

Affected products

Dify: before 1.11.2 — fixed in 1.11.2

How to fix

Upgrade to a fixed version: Dify 1.11.2. Check the vendor advisory for mitigations.

References

https://github.com/langgenius/dify/commit/ae17537470bba417a8971fff705dd82ecb043564
https://github.com/langgenius/dify/pull/29811
https://github.com/langgenius/dify/security/advisories/GHSA-qpv6-75c2-75h4

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