How to use Nginx to protect against XML external entity attacks (XXE)

With the rapid development of Internet technology, network security has attracted more and more attention. Among them, a common network security problem is XML external entity attack (XXE). This attack method allows attackers to obtain sensitive information or execute remote code through malicious XML documents. This article will introduce how to use Nginx to prevent XXE attacks.

1. What is an XXE attack

XML external entity attack is a web vulnerability that an attacker can use to access sensitive data on the server or perform unauthorized operations. This attack is achieved by constructing a malicious XML document and then passing it to an open XML parser. An attacker can define entities in an XML document and then reference external files into the entities. The XML parser loads data from an external file and inserts it into the XML document, resulting in a successful attack.

For example, an attacker could pass the following malicious XML document to an XML parser:

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [
  <!ELEMENT foo ANY >
  <!ENTITY xxe SYSTEM "file:///etc/passwd" >]>
<foo>&xxe;</foo>

In the above XML document, we define an external entity named "xxe" and Reference it into the "foo" element in the XML document. This external entity is actually a reference to the "/etc/passwd" file, which an attacker can parse to obtain sensitive information.

2. Use Nginx to prevent XXE attacks

In order to effectively prevent XXE attacks, we can use Nginx to filter all incoming XML requests. Nginx provides some powerful directives to scan requests and filter them for malicious XML entities. The following are some possible measures:

1. Disable external entities

You can use XML declarations to disable external entities. In Nginx, we can use the following directive to achieve this:

xml_disable_external_entities on;

This directive will disable the parsing of all external entities.

2. Limit internal entity size

An attacker may define a large number of internal entities in an XML document, thereby consuming server resources. Therefore, we can use the following directive to limit the size of internal entities:

xml_max_entity_size size;

Where "size" can be set to the size in bytes. If the size of any internal entity exceeds this limit, the request is rejected.

3. Disable DTD parsing

An attacker can define the structure of an XML document through a DTD (Document Type Definition). To prevent XXE attacks, we can disable DTD parsing using the following directive:

xml_disallow_doctype yes;

If the parser attempts to load a DTD, the request will be rejected.

4. Limit XML file size

You can use the following instructions to limit the size of XML files:

client_max_body_size size;

Where "size" can be set to bytes is the size of the unit. If the size of the request body exceeds this limit, the request is rejected.

In addition to the above measures, we can also use Nginx's "if" judgment statement to check whether there is a malicious entity in the request. For example, the following configuration can be added to check for the "xxe" entity in the request:

if ($request_body ~ "xxe") {
    return 403;
}

The above configuration will block any request containing the "xxe" entity.

3. Summary

XML external entity attack is a common network security problem. To protect against this kind of attack, we can use Nginx to inspect all incoming XML requests and filter them for malicious entities. The above measures can help us effectively protect web applications from XXE attacks.

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