XML External Entity (XXE) Vulnerability and Billion Laughs Hack

XML External Entity (XXE) Vulnerability and Billion Laughs Hack

XML, a common data exchange format used by websites for transferring data between users and servers, can be vulnerable to attacks if not properly secured. One such threat is the XML External Entity (XXE) attack, which exploits unsafe XML parser features to potentially read local files, cause a Denial of Service (DoS), or perform Server-Side Request Forgery (SSRF).

In an XXE attack, an attacker can disclose local files within a website's file system by passing an external XML entity in user-controlled data. For example, an entity might be defined as , which refers to the sensitive file on the website's application server.

Attackers use various methods to carry out XXE attacks, including Entity Expansion, XXE via File Upload, XPath Injection, XXE in SOAP Web Services, and Blind XXE.

Entity Expansion involves creating recursive nested XML entities that consume excessive parser memory, potentially causing a DoS by exhausting system resources. An example of such an entity is .

XXE via File Upload occurs when applications accept XML file uploads without validation. Attackers can embed malicious XML payloads that reference local files, causing sensitive file disclosure upon XML processing.

XPath Injection involves injecting malicious XPath queries into XML data to extract sensitive information or execute unauthorized commands within XML-based systems.

XXE in SOAP Web Services exploits SOAP requests that rely on XML parsing. Attackers insert malicious external entity references in the XML payload to read local files or perform SSRF and data exfiltration.

Blind XXE is carried out by trying out malformed user inputs to make the system fail and check if it throws out sensitive information in the error response.

To protect websites and applications from XXE attacks, several measures are crucial:

1. Disable External Entity Processing: Configuring XML parsers to disable support for external entities and DTDs where possible is the most effective way to prevent XXE vulnerabilities.
2. Validate and Sanitize XML Input: Rigorously validate and sanitize all incoming XML content, especially files uploaded by users, to ensure no malicious entities are included.
3. Use Less-Complex Data Formats: Prefer simpler data formats like JSON over XML since JSON parsers typically do not support external entities.
4. Keep Libraries Updated: Use up-to-date XML processing libraries and frameworks patched against known XXE vulnerabilities.
5. Apply Web Application Firewalls (WAFs): WAFs can detect and block suspicious XML payloads with malicious entity declarations or external references.
6. Limit File Access and Network Calls: Restrict the XML parser’s access to the file system and network resources to prevent unwanted file disclosure or SSRF exploits.
7. Error Handling: Avoid detailed XML error messages leaking internal file contents or system information to users, preventing blind XXE data leaks.

Another common vulnerability associated with XML parsing is the A Billion Laughs Attack. This attack uses an entity to resolve itself cyclically, causing a DoS. Disabling DOCTYPE or setting a maximum limit on the evaluation of entities can prevent A Billion Laughs attacks.

To protect against XXE attacks, websites should disable entities in user-generated XML content before parsing them. It's also essential to be vigilant and stay updated on the latest XML security threats and best practices to ensure the safety of your website and its users.

In the realm of cybersecurity, disabling external entity processing in XML parsers is a significant measure to prevent XXE attacks that can disclose local files and cause a Denial of Service (DoS). Regularly updating XML processing libraries and applying Web Application Firewalls (WAFs) can also help protect against such threats.

In addition, data-and-cloud-computing systems should consider using less-complex data formats like JSON instead of XML, as JSON parsers typically do not support external entities that can lead to potential XXE attacks. Employing rigorous validation and sanitization of all XML input, especially in user-controlled data, is also crucial for maintaining cybersecurity.

Read also: