David Zhang - $_SERVER['PHP_SELF'] and cross-site scripting

Monday, May 20, 2013

It's tempting to assume that PHP's $_SERVER array mostly contains fields out of the reach of an attacker, since these are "server" variables. However, that's not always the case; in particular, the seemingly innocuous PHP_SELF field can be a vector for cross-site scripting.

For example, consider the following foo.php:

<form method="POST" action="<?php echo $_SERVER['PHP_SELF'] ?>">
    <!-- ...form elements... -->
</form>

If I visit http://www.example.com/foo.php, $_SERVER['PHP_SELF'] will be /foo.php and everything will work correctly.

But what if I visit http://www.example.com/foo.php/"><script>alert('hello');</script> instead? Then the rendered HTML will be:

<form method="POST" action="/foo.php/"><script>alert('hello');</script>">
    <!-- ...form elements... -->
</form>

This allows injection of arbitrary script running under the host site's context, also known as XSS. Two ways to fix this are:

Use $_SERVER['SCRIPT_NAME'] instead of $_SERVER['PHP_SELF']. The former is the name of the actual script file and can't normally be manipulated by an attacker.
Use htmlspecialchars(), which by default will escape double-quotes and prevent a user-supplied string from breaking out of an HTML attribute context.

By the way, this was pretty surprising behavior to me for two reasons:

The documentation of PHP_SELF is misleading: The first sentence says:
The filename of the currently executing script, relative to the document root.
It seems odd that PHP would refer to something like /foo.php/"><script>alert('hello');</script> as a "filename."
It's pretty bizarre default behavior that PHP will execute /foo.php for a request of /foo.php/bar/baz.

Tags: php, xss, security | Posted at 11:13 | Comments (4)

Comments

David Annis on Wednesday, June 25, 2014 at 06:32

I have used the fact that php will execute /foo.php from a request that contains /foo.php/bar to make search engine friendly URLs because many search engines will not index both wheretodrink.php?answer=bar and wheretodrink.php?answer=home because they fear an infinite set of URLs.

JamesJoiva on Thursday, June 12, 2025 at 06:05

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Michaelwer on Sunday, June 15, 2025 at 16:05

<a href=https://copicsketch.ru/>https://copicsketch.ru/</a>

JamesJoiva on Sunday, June 15, 2025 at 18:07

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$_SERVER['PHP_SELF'] and cross-site scripting

Monday, May 20, 2013

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