How to protect against path traversal vulnerabilities using PHP

Vulnerabilities are inevitable problems in code, and path traversal vulnerabilities are one of the common types of vulnerabilities. Path traversal vulnerabilities are usually caused by lax input legality checking or unclear logic. An attacker can exploit this vulnerability to access resources outside the application system.

This article will introduce the principles and hazards of path traversal vulnerabilities, and provide some effective PHP techniques to prevent path traversal vulnerabilities.

1. The harm of path traversal vulnerability

Path traversal vulnerability is a common type of vulnerability. An attacker can use this vulnerability to bypass the access control of the application, thereby accessing unknown files. Authorized files and directories.

In PHP, you can read and execute PHP script files or other files through some functions (such as include, require, file_get_contents, etc.). Attackers can use relative path symbols such as ../ in function parameters to achieve path traversal.

For example, an attacker can change the following URL:

http://example.com/index.php?file=../../../../../ ../etc/password

is submitted to the target web application, thereby reading and leaking the contents of the /etc/password file.

2. How to prevent path traversal vulnerabilities

In order to avoid path traversal vulnerabilities, we need to make the following three efforts:

1. Filter user input

For any input from the user, effective filtering is required to prevent attackers from injecting illegal characters and bypassing the path checking function. We can use PHP's built-in functions (such as realpath, basename, dirname, etc.) to filter user input.

For example, when using the include function, the file name input by the user can be filtered by path and file name first, for example:

$filename = $_GET['filename'];
$filename = str_replace(array('/', '\'), '', $filename);
$filename = realpath(__DIR__.'/'.$filename);

Or use This is achieved by combining basename and dirname:

$filename = $_GET['filename'];
$filename = basename($filename);
$filename = realpath(__DIR__.'/ '.dirname($filename).'/'.$filename);

These filtering methods will eliminate relative paths and illegal characters, and return absolute paths at the end, effectively avoiding path traversal attacks.

2. Restrict Web Server Access Permissions

In order to limit attackers' attempts to access system files, we should run the Web server with minimal privileges. The web server should be configured to access only the directories and files required by the application and nothing beyond that.

In addition, in order to prevent certain configuration files or key files in the application from being leaked, we should move these files out of the accessible directory of the web server.

3. Regular updates

Although the risk of path traversal vulnerabilities can be reduced through filtering and permission control, you must be aware of the ongoing risk of this vulnerability. Therefore, we should constantly update and improve the defense measures in the code, fix vulnerabilities in a timely manner and conduct vulnerability scanning to ensure the security of web applications.

3. Conclusion

Path traversal vulnerability is a common type of security vulnerability, but it can be effectively defended through rigorous coding practices and preventive measures. By filtering user input, limiting web server access, and regularly updating code and conducting vulnerability scans, we can effectively protect web applications from path traversal attacks.

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