Impact

A mutation cross-site scripting (mXSS) vulnerability was discovered in TinyMCE’s core undo and redo functionality. When a carefully-crafted HTML snippet passes the XSS sanitisation layer, it is manipulated as a string by internal trimming functions before being stored in the undo stack. If the HTML snippet is restored from the undo stack, the combination of the string manipulation and reparative parsing by either the browser's native DOMParser API (TinyMCE 6) or the SaxParser API (TinyMCE 5) mutates the HTML maliciously, allowing an XSS payload to be executed.
​This vulnerability also impacts these related TinyMCE APIs and plugins:​

• tinymce.Editor.getContent({ format: 'raw' })
• tinymce.Editor.resetContent()
• Autosave Plugin

Patches

This vulnerability has been patched in TinyMCE 5.10.8 and TinyMCE 6.7.1 by ensuring HTML is trimmed using node-level manipulation instead of string manipulation.

Fix

To avoid this vulnerability:

• Upgrade to TinyMCE 5.10.8 or higher for TinyMCE 5.x.
• Upgrade to TinyMCE 6.7.1 or higher for TinyMCE 6.x.

Acknowledgements

Tiny Technologies would like to thank Masato Kinugawa of Cure53 for discovering this vulnerability.

References

• TinyMCE 5.10.8 Release Notes
• TinyMCE 6.7.1 Release Notes

For more information

If you have any questions or comments about this advisory:

• Email us at infosec@tiny.cloud
• Open an issue in the TinyMCE repo