To reproduce:
1. Open a confluence instance in Firefox.
2. Create a space with key "TEST".
3. Create a page in that space called "<script>alert(0)</script>".
4. Create two pages with the page from step 3 as their parent.
5. Go to:

[base path]/panels/reorderpage.action?panelName=reorder&spaceKey=TEST&title=%3Cscript%3Ealert%280%29%3C/script%3E&movedPageId=0&pageTitle=

An alert should open. The strings in steps 2 and 3 are only important in that they need to match the URL (any space or XSS string can be used).

See reorder-page.vm, line 10.