What is the billion laughs attack?

by | Mar 7, 2017 | CCNA, CCNP, CompTIA, DoS and DDoS Prevention, Most Common Vulnerabilities, Network Security

What is the billion laughs attack?

The billion laughs attack is a Denial of Service or DoS attack that targets parsers of XML documents. The attack is also known as an XML Bomb, Exponential Entity Expansion Attack, or XEE Attack.ย Let’s understand how the attack works in detail.

How does the billion laughs attack work?

An XML entity is a symbolic representation of information like a variable in a computer program. It is declared in Document Type Definition or DTD with the following syntax :

<!ENTITY entityName “The text you want to appear when the entity is used”>

Now, let’s consider the following lines of code :

<?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>

This example consists of ten entities. Each entity consists of ten of the previous entity. And, the document will consist of one billion copies of the first entity.

In most of the examples, the first entity is represented as the string โ€œlol,โ€ and the document consists of one billion such “lol.” And hence, the name the billion laughs attack.

As the document consists of one billion copies of the first entity, the computer memory used will exceed the memory available to the process that is parsing the XML. As a result, the parser will consume an exponential amount of resources and cause a Denial of Service attack.

The attack was first reported in 2002. But, it began to be widely addressed later.

How to prevent the billion laughs attack?

There are a couple of steps that can be taken to prevent the billion laughs attack.

  • We can limit the number of Entity Reference Nodes that the parser can expand.
  • We can limit the number of charactersย up toย which the entity can expand.

So, beware of all the vulnerabilities. And stay safe and secure.

 

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Amrita Mitra

Author

Ms. Amrita Mitra is an author, who has authored the books “Cryptography And Public Key Infrastructure“, “Web Application Vulnerabilities And Prevention“, “A Guide To Cyber Security” and “Phishing: Detection, Analysis And Prevention“. She is also the founder of Asigosec Technologies, the company that owns The Security Buddy.

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