In the forum plugin's functions.inc file, find the function plugin_dopluginsearch_forum(). Change this:
Text Formatted Code
if ( $keyType == 'phrase' ) {
$query = addslashes($query); // <-- new
$sqltmp .= "AND (comment LIKE '%$query%' OR subject LIKE '%$query%')";
} else if ( $keyType == 'any' ) {
Text Formatted Code
} else {
$query = addslashes($query); // <-- new
$sqltmp = "WHERE (comment LIKE '%$query%' OR subject LIKE '%$query%')";
}