Linux Server Security: Hardening Web Interfaces to Block XXE Attacks.

Linux Server Security: Hardening Web Interfaces to Block XXE Attacks

Introduction:
With the widespread use of Web applications, server security has become the Internet An issue that users are increasingly concerned about. Over the past few years, external entities have assumed the role of accessing web servers and performing malicious actions that could lead to server compromise. Among them, XXE attacks are one of the most common and dangerous types of attacks. This article will introduce the principles of XXE attacks and provide steps on how to harden web interfaces to prevent XXE attacks and improve the security of Linux servers.

1. What is XXE attack?
XXE (XML External Entity) attack is an attack method that exploits vulnerabilities on the server by sending maliciously constructed XML files to the server. Attackers can use entity extensions and parameter entities to read files, execute remote code, and other malicious operations, thereby obtaining sensitive information and gaining unauthorized access to the server.

The following is a simple XML file used to demonstrate XXE attacks:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE root [
    <!ENTITY xxe SYSTEM "file:///etc/passwd">
]>
<root>
    <data>&xxe;</data>
</root>

In the above XML file, the /etc/ on the server is read by using an external entity. passwd file, causing sensitive information to be leaked.

2. Reinforce the Web interface to prevent XXE attacks
In order to prevent XXE attacks, we can take the following steps:

1. Disable External Entities:
   To prevent XXE attacks using entity extensions, we can solve it by disabling external entities. In the PHP configuration file php.ini, set libxml_disable_entity_loader to true to disable external entities.

libxml_disable_entity_loader(true);

2. Validate User Input:
   For the XML data input by the user, we must perform strict input verification to ensure that the input data conforms to the expected format. You can use XML Schema to define data types and structures and validate user input.

The following is a simple example showing how to use XML Schema to validate data:

<?xml version="1.0" encoding="UTF-8"?>
<root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="schema.xsd">
    <data>Valid data</data>
</root>

3. Use the whitelist mechanism (Whitelist) to filter entities:
   Use whitelist The list mechanism can limit the entities to be parsed, allowing only predefined entities to be parsed. Unnecessary entity definitions can be removed by preprocessing the parsed XML. The following is a sample code:

$xml = file_get_contents('php://input');
$xml = preg_replace('/<!ENTITY.*?>/', '', $xml);

The above code uses regular expressions to remove entity definitions in an XML document.

4. Use a secure XML parsing library:
   In order to prevent XXE attacks, we should use a secure XML parsing library as much as possible, such as using the SimpleXML library in PHP. SimpleXML provides some security mechanisms to prevent XXE attacks.

$dom = new DOMDocument();
$dom->loadXML($xml, LIBXML_NOENT | LIBXML_NOERROR | LIBXML_NOWARNING);

In the above example, by setting the LIBXML_NOENT | LIBXML_NOERROR | LIBXML_NOWARNING parameter, the DOMDocument class will disable external entities and not display parsing errors and warning messages.

Conclusion:
In order to ensure the security of Linux servers, it is very important to prevent XXE attacks. By disabling external entities, validating user input, using whitelisting mechanisms to filter entities, and using secure XML parsing libraries, we can effectively prevent XXE attacks. For server administrators, measures such as regularly updating server operating systems and applications, monitoring and analyzing log files, and setting strong passwords are also very important server security practices. Only by continuously strengthening the security of the server can we effectively protect the data security of the website and users.

Reference:

1. OWASP XXE Attack Prevention Guide - https://owasp.org/www-community/vulnerabilities/XML_External_Entity_(XXE)_Prevention_Cheat_Sheet
2. PHP : SimpleXML class - https://www.php.net/manual/zh/class.simplexml_element.php
3. DOMDocument class - https://www.php.net/manual/zh/class.domdocument. php

The above is the detailed content of Linux Server Security: Hardening Web Interfaces to Block XXE Attacks.. For more information, please follow other related articles on the PHP Chinese website!