Billion Laughs attack

The Billion Laughs attack, also known as the XML bomb attack, is a type of denial of service attack that exploits the way that some XML parsers process external entity references. The attack works by creating a malicious XML document that contains a large number of nested entity references, each of which expands to a small string of text. When an XML parser processes this document, it must expand all of the entity references, which can take a very long time and consume a large amount of memory. As a result, the parser may crash, hang, or run out of resources, effectively denying service to the user.


To prevent Billion Laughs attacks, you can use a number of different strategies:

  1. Use an XML parser that is resistant to expansion attacks. Some XML parsers are designed to be more resistant to this type of attack by limiting the number of entity expansions that they will perform or by using more efficient algorithms for expanding entities.
  2. Use a content-based XML firewall to block malicious XML documents. A content-based firewall can be configured to block documents that contain a large number of nested entity references or that use entity references in a way that is known to be malicious.
  3. Validate input data. Make sure that all input data, including XML documents, is validated to ensure that it is well-formed and does not contain any malicious content.
  4. Use a sanitization library. A sanitization library can be used to filter out potentially malicious content from input data, including nested entity references in XML documents.
  5. Set limits on resource usage. Configure your system to limit the amount of memory and processing time that is available for parsing XML documents. This can help to prevent an attacker from using a Billion Laughs attack to consume all of your system's resources.