XML Bombs: Understanding the Billion Laughs Attack and Its Impact

 XML Bomb

An XML bomb, also known as a billion laughs attack, is a denial-of-service (DoS) attack targeting XML parsers. This attack involves sending a small, malicious XML file to a server. When the server's XML parser processes this file, the nested data entities within the file expand exponentially, consuming excessive resources and leading to a server crash.

How XML Bombs Work:

• Recursive Entity Expansion: XML bombs exploit XML parsers' recursive entity expansion feature. When an XML parser encounters a document with nested entities, it attempts to resolve each entity by expanding it into its defined value. This process can lead to exponential growth in the amount of data being processed.

Example of a Billion Laughs Attack:

• A classic example of an XML bomb is the "billion laughs" attack. In this attack, a small XML document defines multiple nested entities that expand exponentially. For instance, an entity named "lol" is defined and referenced repeatedly within other entities, causing a massive expansion when parsed.

xml

<?xml version="1.0"?>

<!DOCTYPE lolz [

  <!ENTITY lol "lol">

  <!ENTITY lol2 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;">

  <!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;">

  <!ENTITY lol4 "&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;">

  <!ENTITY lol5 "&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;">

  <!ENTITY lol6 "&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;">

  <!ENTITY lol7 "&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;">

  <!ENTITY lol8 "&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;">

  <!ENTITY lol9 "&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;">

]>

<lolz>&lol9;</lolz>

In this example, there are 10 different XML entities, lol to lol9. The first entity, lol, is the string "lol." Each subsequent entity is defined as 10 of the previous entity. When the parser processes lol9, it expands into 10 lol8s, each of which expands into 10 lol7s, and so on. By the time everything is expanded, there are 1,000,000,000 instances of the string "lol," consuming an exponential amount of resources.

Potential Risks of XML Bombs:

• System Crashes: An XML bomb can cause a server to crash by overwhelming it with exponentially growing nested data entities.
• Service Disruption: The primary goal of an XML bomb is to cause a denial of service, making the affected application or service unavailable.

Defenses Against XML Bombs:

• Limit Entity Expansion: Configure XML parsers to limit the number of entity expansions allowed.
• Disable External Entities: Disable the processing of external entities in XML parsers to prevent external XML bomb attacks.
• Use Secure Parsers: XML parsers are designed to handle entity references securely and efficiently.

Conclusion:

XML bombs are a serious threat to systems that rely on XML parsers. By understanding how these attacks work and implementing appropriate defenses, organizations can protect their systems from being overwhelmed by malicious XML documents.

This is covered in CompTIA CySA+.