The Apache module for PHP 4.0.0 through PHP 4.0.4, when disabled with the 'engine = off' option for a virtual host, may disable PHP for other virtual hosts, which could cause Apache to serve the source code of PHP scripts.
PHP 3 and 4 do not properly cleanse user-injected format strings, which allows remote attackers to execute arbitrary commands by triggering error messages that are improperly written to the error logs.
The file upload capability in PHP versions 3 and 4 allows remote attackers to read arbitrary files by setting hidden form fields whose names match the names of internal PHP script variables.
PHP3 with safe_mode enabled does not properly filter shell metacharacters from commands that are executed by popen, which could allow remote attackers to execute commands.