Improper Neutralization of Directives in Statically Saved Code ('Static Code Injection')

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

This example attempts to write user messages to a message file and allow users to view them.

$MessageFile = "messages.out";
if ($_GET["action"] == "NewMessage") {
  $name = $_GET["name"];
  $message = $_GET["message"];
  $handle = fopen($MessageFile, "a+");
  fwrite($handle, "<b>$name</b> says '$message'<hr>\n");
  fclose($handle);
  echo "Message Saved!<p>\n";
}
else if ($_GET["action"] == "ViewMessages") {
  include($MessageFile);
}

While the programmer intends for the MessageFile to only include data, an attacker can provide a message such as:

name=h4x0r
message=%3C?php%20system(%22/bin/ls%20-l%22);?%3E

which will decode to the following:

<?php system("/bin/ls -l");?>

The programmer thought they were just including the contents of a regular data file, but PHP parsed it and executed the code. Now, this code is executed any time people view messages.

Notice that XSS (CWE-79) is also possible in this situation.

See Also