Mermaid: Improper sanitization of `classDefs` in diagrams leads to CSS injection

Details

The state diagram and any other diagram type that routes user-controlled style strings through createCssStyles parser for Mermaid v11.14.0 and earlier captures classDef values with an unrestricted regex:

// packages/mermaid/src/diagrams/state/parser/stateDiagram.jison:83
<CLASSDEFID>[^\n]*   { this.popState(); return 'CLASSDEF_STYLEOPTS' }

The value passes unsanitized through addStyleClass() -> createCssStyles() -> style.innerHTML (mermaidAPI.ts:418). A } in the value closes the generated CSS selector, and everything after becomes a new CSS rule on the page.

PoC

stateDiagram-v2 
      classDef x }*{ background-image: url("http://media.giphy.com/media/SggILpMXO7Xt6/giphy.gif")}

Live demo:
https://mermaid.live/edit#pako:eNpFjzFvgzAQhf-KdVNbEcBgMHhtlkqtOnSJKi8ONsYKBmRMlRTx3-skanvTfbp7996t0IxSAYPZC6_2Rmgn7O4rQ00v5nmvWnRG29OKjqI5aTcug9wZK7RiaHH9A4fO-4kliVXSiFibqbvEzWjvnHxo_fI6vR3e6cGXyX2qTcvhcYMItDMSmHeLisAqZ8UVYeUDQhx8p6ziwEIrhTtx4MNVM4nhcxztrywE0h2wVvRzoGWS_z_8rahBKvcckntgmN5OAFvhDIzUNCZZQXCR5nVaZkUEF2BVFpOcEkoxxhUuyRbB980yjStapKHqoKFlhvPtB7BFZEU

Patches

This has been patched in:

• v11.15.0 (see e9b0f34d8d82a6260077764ee45e1d7d90957a0f)
• v10.9.6 (see 8fead23c59166b7bab6a39eac81acebee2859102)

Workarounds

Setting "securityLevel": "sandbox" will prevent this, by rendering the mermaid diagram in a sandboxed <iframe>.

Impact

Enables page defacement, user tracking via url() callbacks, and DOM attribute exfiltration via CSS :has() selectors.