View file upload/styles/default/xenforo/editor/icons-legal.txt

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icons.png includes icons from the CKEditor project which are triple licensed with GPL, LGPL, and MPL:

http://ckeditor.com/about/license

icons.png also includes icons from the Fugue set (http://p.yusukekamiyamane.com/) which are released under the Creative Commons Attribution 3.0 license:

http://creativecommons.org/licenses/by/3.0/