Your site has been hacked. Take this as a warning. Improve your security.
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|Method|
+------+
SQL injection, and HTML redirect to deface.
+------------+
|Info and Fix|
+------------+
Use mod url_rewrite. If not, disable HTML coding in the category names.
+------+
|Greets|
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Team Intra, Hackforums, Uzi, Synyster. :D