PHP Security 2: Directory Traversal & Code Injection

In the first part of this guide, we focused on the most common and most dangerous (according to OWASP.org) security issues in PHP code: SQL Injection vulnerabilities. We explained, how important input validation is, how bad it is to include untrusted data (user input) directly in an SQL query, and how prepared statements help you avoid SQL Injection attacks. In the second part, we focus on two other common and dangerous PHP vulnerabilities and attack types: directory traversal and code injections attacks. In both cases, these vulnerabilities are also caused by unsanitized user data.

Directory Traversal

Directory traversal (path traversal) refers to an attack that affects the file system. In this type of attack, an authenticated or unauthenticated user can request and view or execute files that they should not be able to access. Such files usually reside outside of the root directory of a web application or outside of a directory to which the user is restricted (for example, /var/www). In many cases, an attacker may read any file accessible for the user that is running the web server (usually www-data). If the server has badly configured file permissions (very common), this attack can be escalated further.

Insecure Code Sample

In the following example, the script passes an unvalidated/unsanitized HTTP request value directly to the include() PHP function. This means that the script will try to include whatever path/filename is passed as a parameter:

$file = $_GET['file'];
include($file);

For example, if you pass /etc/passwd as the argument, this file is readable for all users. Therefore, the script returns the content of the file with information about all system users:

PHP Security

Secure Code Sample

This vulnerability may be mitigated in different ways, depending on the specific case. However, the most common and generic way to do it is by using the basename() and realpath() functions.

The basename() function returns only the filename part of a given path/filename: basename("../../../etc/passwd") = passwd. The realpath() function returns the canonicalized absolute pathname but only if the file exists and if the running script has executable permissions on all directories in the hierarchy: realpath("../../../etc/passwd") = /etc/passwd.

$file = basename(realpath($_GET['file']));
include($file);

Now, if we request the same file as above, we get an empty response:

PHP security

Avoid Blacklisting

Blacklisting is bad practice because there are more ways to make the same request. Intelligent attackers always find ways to bypass restrictions for user-supplied input. For example ../../../etc/ can also be written like this: ..%2F..%2F..%2Fetc%2F.

If you need to have access to specific files, use a whitelist instead.

Code Injection/Execution

In the case of PHP code injection attacks, an attacker takes advantage of a script that contains system functions/calls to read or execute malicious code on a remote server. This is synonymous to having a backdoor shell and under certain circumstances can also enable privilege escalation.

Insecure Code Sample

In this example, a script uses the exec() function to execute the ping command. However, the host is dynamic (passed via an HTTP GET request):

exec("ping -c 4 " . $_GET['host'], $output);
echo "&ltpre>";
print_r($output);
echo "&lt/pre>";

Passing www.google.com returns the output of the ping google.com command:

PHP Security

This snippet has a code injection vulnerability. It allows an attacker to pass multiple commands to the function using a semicolon. In Linux, this delimiter is used to execute multiple commands inline.

For example, if you pass www.google.com;whoami, the script returns the following output:

PHP Security

Secure Code Sample

There are two functions that you can use in PHP applications and that can help harden command line calls such as exec(), shell_exec(), passthru(), and system(): escapeshellcmd() and escapeshellarg(). The escapeshellcmd() function escapes any characters in a string that might be used to execute arbitrary commands. The following characters are escaped by including a backslash before them: &#;`|*?~<>^()[]{}$\, \x0A, and \xFF. Single and double quotes are escaped only if they are not paired. For example, escapeshellcmd("ping -c 4 www.google.com;ls -lah") = ping -c 4 www.google.com\;ls -lah.

The escapeshellarg() function adds single quotes around a string and escapes any existing single quotes. As a result, the entire string is being passed as a single argument to a shell command.

// #1 Restrict multiple commands
exec(escapeshellcmd("ping -c 4 " . $_GET['host']), $output);

// #2 Restrict multiple commands and multiple arguments
exec(escapeshellcmd("ping -c 4 " . escapeshellarg($_GET['host'])), $output);

If you pass www.google.com;whoami to the secure script, this is what you get in return:

PHP Security

To avoid security issues, we recommend that you disable exec(), shell_exec(), passthru(), and system() functions in PHP configuration unless it is absolutely necessary to use them. You can also create a whitelist of accepted commands/arguments.