How to protect against XML External Entity Attacks (XXE) using PHP

How to use PHP to defend against XML external entity attacks (XXE)

In recent years, with the popularity of the Internet and the increase in information exchange, network security issues have also received increasing attention. Among them, XML external entity attack (XXE) is a common security vulnerability. An attacker could exploit this vulnerability to read sensitive information on the server or conduct further attacks. In this article, we will discuss how to use PHP to defend against XML external entity attacks.

XML external entity attacks are usually carried out through maliciously constructed XML files. Attackers use Entity Reference and Entity Declaration in XML to read arbitrary files on the file system, and can even read external resources through remote URLs. This attack is very effective in an insecure XML parser, so we need to take measures to prevent this attack.

Here are some ways to use PHP to defend against XML external entity attacks:

1. Use the option to disable entity parsing:
   In PHP's XML parser, we can pass Set the option to disable entity resolution to prevent XXE attacks. It should be noted that if we use entity references and entity declarations in XML files to represent some predefined entities (such as entities in HTML), disabling entity parsing may cause parsing errors.

The following is an example of using the disabled entity resolution option:

$dom = new DomDocument();
$dom->loadXML($xmlString, LIBXML_NOENT | LIBXML_NOERROR | LIBXML_NOWARNING);

2. Filtering input:
   Input validation is an important step in defending against XXE attacks. We should carefully check whether user-supplied XML files contain malicious entity references or entity declarations. These can be inspected and filtered using regular expressions or other filtering methods.

For example, we can use PHP's preg_replace() function to filter out the <!ENTITY> statement in XML:

$xmlString = preg_replace('/<!ENTITYs+S+s+SYSTEMs+"[^"]*">/', '', $xmlString);

This ensures that before parsing the XML, we filter out any <!ENTITY> statements that may lead to XXE attacks.

3. Use whitelist to verify external entities:
   When we know that a specific external entity needs to be referenced in the XML file, we can use the whitelist mechanism to verify it. That is, we only allow references to external entities that we have predefined, and deny references to other external entities.

For example, we can check if the external file path referenced in the <!ENTITY> declaration is in our whitelist list:

$allowedEntities = [
    'http://example.com/file.xml',
    'file:///path/to/file.xml'
];

$xmlString = preg_replace_callback('/<!ENTITYs+(S+)s+SYSTEMs+"([^"]*)">/', function($matches) use ($allowedEntities) {
    if (!in_array($matches[2], $allowedEntities)) {
        // 非法的外部实体
        return '';
    }
    
    return $matches[0];
}, $xmlString);

The above code Prevent XXE attacks by checking whether external file paths are in the whitelist.

Summary:
In PHP development, defending against XML external entity attacks (XXE) is a key task. We can improve the security of our system by disabling entity resolution options, filtering input, and using whitelist validation. It is important to exercise caution when writing and parsing XML files, and always remain alert for security vulnerabilities.

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