How does PHP defend against malicious XML parsing and entity attacks?

How to use PHP to defend against malicious XML parsing and XML external entity attacks

Introduction:
As network security threats continue to increase, the need to protect applications from malicious attacks is becoming more and more urgent. XML (Extensible Markup Language), a popular data exchange format, is a common input source for web applications. However, there are some security risks in XML parsing, such as malicious XML parsing and XML External Entity (XXE) attacks. This article will focus on how to use PHP to defend against these two types of attacks.

1. Malicious XML parsing attack defense
Malicious XML parsing attack refers to an attacker using maliciously constructed XML data to trigger vulnerabilities in the XML parser, thereby executing malicious code or obtaining sensitive information. Here are some defensive measures:

1. Use a secure XML parser: Choose to use a security-audited and updated XML parser, such as PHP's built-in SimpleXML and DOM extensions. These parsers are tested and have known vulnerabilities fixed.
2. Restrict entity parsing of XML parser: Disable entity parsing function before parsing XML. This can be achieved using a code snippet like the following:

libxml_disable_entity_loader(true);

This will prevent the XML parser from loading external entities, thereby reducing the risk of XXE attacks.

3. Input validation and filtering: For XML data obtained from user input, strict validation and filtering are required. Make sure to only accept legal data formats and escape special characters in the input to prevent the injection of malicious data.
4. Strict file access control: restrict access permissions to XML documents to ensure that only legal users or roles can access. This can be accomplished through the file system's access control list (ACL) or using PHP's file permissions functionality.

2. XML External Entity (XXE) attack defense
XML external entity attack is an attack that uses the characteristics of the XML parser to read system files or make remote requests. Here are some defenses:

1. Disable external entity parsing: Before parsing the XML, you can use a code snippet like this to disable external entity parsing:

libxml_disable_entity_loader(true);

This will block the XML The parser loads external entities, thus protecting against XXE attacks.

2. Use whitelist: Limit the external entities that the resolver can access. You can use the following code snippet to achieve this:

$dom = new DOMDocument();
$dom->loadXML($xml);

$allowedExternalEntities = [
    'http://example1.com',
    'http://example2.com'
];

$dom->doctype->entities = null;
foreach ($dom->getElementsByTagNameNS('*', '*') as $element) {
    if ($element->isEntityNode()) {
        $systemId = $element->systemId;
        if (!in_array($systemId, $allowedExternalEntities)) {
            $element->parentNode->removeChild($element);
        }
    }
}

The above code will use the whitelist to check the entities in the XML and remove the entity nodes that are not in the whitelist.

3. Use XML verification: Use XML Schema (XSD) or DTD (Document Type Definition) to verify the input XML data structure. By verifying the structure of XML, some malicious XML codes can be eliminated.

Conclusion:
It is very important to protect web applications from malicious XML parsing attacks and XML external entity attacks. Application security can be strengthened by using secure XML parsers, disabling entity parsing, input validation and filtering, and strict file access controls. In addition, the use of whitelists and XML verification are also effective ways to defend against XXE attacks. In summary, through reasonable security measures, the risks of malicious XML parsing and XXE attacks can be effectively defended.

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