6.0.6 GA, 6.0.12 EE, 6.1.1 CE GA2, 6.1.20 EE GA2
REPRODUCTION STEPS: (6.0.x Community equals 6.1.x and trunk sites)
1. Create a Community and set its type to restricted.
2. Add portlet "My Communities" on a page.
3. Log in with a different user, who is not a member of the created community.
4. On the "My Communities" portlet change to tab "Available Communities".
5. Search for the created community and click on "Request Membership".
6. Type the following into comment field: <script>alert('XSS');</script>
7. Log out and log in as the owner or admin of the community.
8. Go to "My Communities" Portlet and select "View Membership Request" from the Actions menu.
And also reproduced the same behaviour with "Reply Comments" textbox.
LPE-7486 XSS in group membership requests