XML External Entities (XXE) Attack

Overview

XML External Entities (XXE) attack is a type of attack where an application that parses XML input is exploited by processing XML input containing a reference to an external entity. This attack is possible when weakly configured XML parsers are used, making most XML processors vulnerable to XXE attacks by default. Preventing XXE attacks is crucial for application security.

Description

XXE attacks are commonly performed by taking advantage of vulnerable XML processors, code, dependencies, and integrations. To prevent XML external entity attacks, it is recommended to use less complex data formats like JSON whenever possible and avoid serialization of sensitive data. Patching or upgrading all XML processors and libraries, as well as using dependency checkers, can help mitigate vulnerabilities. Disabling XML external entity and DTD processing in all XML parsers, implementing server-side input validation and sanitization, and validating incoming XML with XSD validation are additional preventive measures. SAST tools can also help detect XXE vulnerabilities in source code, and manual code review is recommended for large and complex applications. In case these controls aren't feasible, virtual patching, API security gateways, and Web Application Firewalls (WAFs) can be considered to detect, monitor, and block XXE attacks.

How to Prevent ?

- Use less complex data formats like JSON and avoid serialization of sensitive data. - Patch or upgrade all XML processors and libraries in use. - Use dependency checkers. - Disable XML external entity and DTD processing in XML parsers. - Implement server-side input validation and sanitization. - Validate incoming XML using XSD validation. - Utilize SAST tools and manual code review for detecting XXE vulnerabilities. - Consider virtual patching, API security gateways, or WAFs as additional measures.

Example Attack Scenarios:

• Exploiting a Vulnerable XML Processor:  In this scenario, an attacker uploads an XML file containing a reference to a malicious external entity. The vulnerable XML processor used by the application processes the XML input without proper validation, allowing the attacker to manipulate the response or retrieve sensitive information from the server. By exploiting this vulnerability, the attacker gains unauthorized access or performs other malicious actions on the targeted system.

• Exploiting a Vulnerable Dependency:  In this scenario, an attacker identifies a vulnerable XML processing library or dependency used by the target application. By exploiting the vulnerability in the dependency, the attacker can execute arbitrary code, perform unauthorized actions, or access sensitive data within the application's environment. This attack vector highlights the importance of keeping dependencies up to date and regularly applying security patches to prevent XXE attacks.

• Exploiting Vulnerable Integrations:  In this scenario, an attacker identifies a vulnerable XML integration between two systems. By exploiting the vulnerability in the integration, the attacker can manipulate the communication between the systems, inject malicious XML content, or gain unauthorized access to sensitive information. Securing and regularly auditing integrations for XML external entity vulnerabilities is essential to prevent such attacks.