In PHP language development, path traversal vulnerability is a common security problem. Attackers can use this vulnerability to read or even tamper with any file in the system, greatly jeopardizing the security of the system. This article explains how to avoid path traversal vulnerabilities.

1. What is a path traversal vulnerability?

Path Traversal, also known as directory traversal vulnerability, means that an attacker successfully uses the "../" operator or other forms of relative paths to enter Directory outside the application's root directory, and read or modify any files related to the application.

For example, when the user enters "../../../../etc/passwd" as the file path, the application does not restrict the file path, but concatenates the paths provided by the user. into the file system, the attacker can obtain sensitive information in the system.

2. How to avoid path traversal vulnerabilities?

1. Validate user input

The most critical step to avoid path traversal vulnerabilities is to validate user input to ensure that the input only contains legal characters and path identifiers. Regular expressions, whitelists, etc. can be used to filter user input.

For example, a common directory traversal vulnerability in Apache uses a URL similar to the following:

http://localhost/index.php?page=../../../ ../etc/passwd

An attacker can read sensitive files in the system by sending malicious requests to this URL. To prevent this attack, you can limit the user's input to only contain specific characters, such as letters, numbers, and some special symbols.

2. Restrict file access permissions

Applications must restrict file access permissions to ensure that only authorized users can access files. Web applications, in particular, need to ensure that files and directories on the server are readable and writable only by web users.

For example, under Linux systems, you can use chroot to limit the file virtual root directory of the process. In PHP, you can use open_basedir to limit the access permissions of script files.

3. Use absolute paths

Using absolute paths can avoid path traversal vulnerabilities. In PHP, you can use $_SERVER['DOCUMENT_ROOT'] or __DIR__ to get the absolute path where the file is located.

For example, the following code can avoid the path traversal vulnerability:

$file = __DIR__ . '/data/' . $_GET['file'];
if(file_exists($file) {

}
?>

6. Update PHP version

If your PHP version is lower than 5.3.4, it is recommended You update to the latest version immediately. In PHP 5.3.4, a new checking function realpath_cache_get is introduced to avoid path traversal vulnerabilities.

7. Avoid information leakage

Finally, information leakage in the system should be avoided. Information leakage can be reduced in the following ways:

• Disable error reporting: Error reporting can be disabled through the error_reporting function.
• Separate log: Should Log PHP errors to the audit log instead of the public error output.
• Clean directory: Remove unnecessary files and directories and restrict access to the directory.

Summary:

Path traversal vulnerability is a serious security problem. If not dealt with in time, it will cause irreparable losses to the system. In PHP development, the above principles should be followed and effective security measures should be taken. This avoids path traversal vulnerabilities.

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