5. Examining the application's responses for any sensitive information leaks or access to unauthorized files

6. Testing different inputs to see if the application is vulnerable to various XXE attack types.

Ways to stop XXE vulnerabilities

To avoid XXE vulnerabilities, there are some practices that can be implemented, such as:

1. Disabling the usage of external entities

2. Restricting permissions or implementing a separate user account for XML processing

3. Validating XML input data to ensure it is formatted correctly and not malicious

4. Sanitizing input data by removing any external entities and validating against an agreed-upon schema

5. Implementing a secure XML parser configuration that removes support for external entities

6. Using virtual patching to limit the impact of XXE vulnerabilities until a proper fix can be implemented.

How to fix the XXE

The XXE flaw can be addressed through a variety of techniques, including:

1. Disabling external entities – removing support for external entity expansion at the application level

2. Implementing secure parsing - ensuring only file types and formats specified in the application are allowed for parsing

3. Input sanitization – verifying and removing any external entities and validating against an agreed-upon schema

4. Restricting user input – limiting user input data to only XML that fits the predefined tag specifications in the parser

5. Implementing proper error handling – ensuring that the application does not crash and sensitive information is not leaked even after an XXE attack.

6. Utilizing patch releases – implementing vendor-released patches that address XXE vulnerabilities in the system.

Lab experiment

In this project, a lab experiment is performed to induce and detect XXE vulnerabilities. It involves creating and testing an XXE attack that allows attackers to access confidential files on the application server.

Conclusion

This project aimed to define and provide an overview of XML external entity injection, its types, vulnerabilities, and risk factors. This document discussed how to identify, analyze, and mitigate XXE vulnerabilities and how to prevent XXE injection attacks in mobile application security assessments. Through laboratory experiments and practical examples, we have demonstrated the importance of staying vigilant and proactive in preventing and addressing XXE flaws within mobile applications.

References

1. OWASP. (2017). XML External Entity (XXE) Processing. Retrieved from https://owasp.org/www-project-top-ten/OWASP_Top_Ten_2017/Top_10-2017_A4-XML_External_Entity_(XXE)_Processing.html

2. Techbeacon. (2019). 5 common questions about XXE injections in XML: What developers need to know. Retrieved from https://techbeacon.com/security/5-common-questions-about-xxe-injections-xml-what-developers-need-know

To test for file retrieval, one can create an external object based on the URL of a system under their control and analyze interactions with it. This is particularly useful for detecting blind XXE vulnerabilities. The Burp Collaborator client is a recommended tool for this task. Another test is to use an XInclude attack on a server-side XML document to verify the inclusion of user-supplied non-XML data.

The primary strategy to prevent XXE attacks is to disable potentially harmful XML capabilities that the application doesn't require or intend to use. Disabling support for XInclude and external entity resolution is usually adequate, and this can be changed through configuration options or by programmatically altering the default settings. The user-defined DTD, attributes, and (external) entities should not be permitted. Input validation should be performed on user data before it is processed by the XML parser. For example, OWASP provides comprehensive guidance on how to fix XXE flaws.

An XXE attack can be demonstrated through a lab experiment by modifying the submitted XML to retrieve a file from the server's filesystem. This is achieved by adding or modifying the DOCTYPE element and then using a specified external entity to modify a data value in the XML returned in the application's response.

In conclusion, proactive measures, such as penetration testing, should be taken by enterprises to identify potential points of vulnerability on their web applications. And finally, exploiting XXE to receive files is possible, and it's essential to be aware of this security risk to prevent possible data breaches.

Sources

XML vulnerabilities are still a preferred target for attackers as they can be used to exploit various applications and obtain sensitive data. More information on the matter can be found at infosecinstitute.com.

Spirent highlights how XXE (XML External Entity) is an often overlooked yet highly dangerous threat that can lead to severe consequences if not remediated. Check out their blog for further insights.

PortSwigger, a web security company, offers an in-depth explanation of XXE and how to prevent attacks on their website.

For those interested in learning about XXE attacks, web-security-academy.net provides an interactive platform where users can practice identifying weaknesses and implementing solutions.