An adversary submits data to a target application which contains nested exponential data expansion to produce excessively large output. Many data format languages allow the definition of macro-like structures that can be used to simplify the creation of complex structures. However, this capability can be abused to create excessive demands on a processor's CPU and memory. A small number of nested expansions can result in an exponential growth in demands on memory.

Billion Laughs Attack

XML Bomb

XML Entity Expansion (XEE)

High
Medium

1
Explore
[Survey the target] An adversary determines the input data stream that is being processed by a data parser that supports using subsitituion on the victim's side.
Use an automated tool to record all instances of URLs to process requests.
Use a browser to manually explore the website and analyze how the application processes requests.

2
Experiment
[Craft malicious payload] The adversary crafts a malicious message containing nested exponential expansion that completely uses up available server resources. See the "Example Instances" section for details on how to craft this malicious payload.

3
Exploit
[Send the message] Send the malicious crafted message to the target URL.

This type of attack requires that the target must receive input but either fail to provide an upper limit for entity expansion or provide a limit that is so large that it does not preclude significant resource consumption.

Ability to craft nested data expansion messages.

None: No specialized resources are required to execute this type of attack.

Availability
Unreliable Execution
Resource Consumption
Denial of Service

Design: Use libraries and templates that minimize unfiltered input. Use methods that limit entity expansion and throw exceptions on attempted entity expansion.
Implementation: For XML based data - disable altogether the use of inline DTD schemas when parsing XML objects. If a DTD must be used, normalize, filter and use an allowlist and parse with methods and routines that will detect entity expansion from untrusted sources.

The most common example of this type of attack is the "many laughs" attack (sometimes called the 'billion laughs' attack). For example:
<?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 is well formed and valid XML according to the DTD. Each entity increases the number entities by a factor of 10. The line of XML containing lol9; expands out exponentially to a message with 10^9 entities. A small message of a few KBs in size can easily be expanded into a few GB of memory in the parser. By including 3 more entities similar to the lol9 entity in the above code to the DTD, the program could expand out over a TB as there will now be 10^12 entities. Depending on the robustness of the target machine, this can lead to resource depletion, application crash, or even the execution of arbitrary code through a buffer overflow.

This example is similar, but uses YAML. This was used to attack Kubernetes [REF-686]
a: &a ["lol","lol","lol","lol","lol","lol","lol","lol","lol"]b: &b [*a,*a,*a,*a,*a,*a,*a,*a,*a]c: &c [*b,*b,*b,*b,*b,*b,*b,*b,*b]d: &d [*c,*c,*c,*c,*c,*c,*c,*c,*c]e: &e [*d,*d,*d,*d,*d,*d,*d,*d,*d]f: &f [*e,*e,*e,*e,*e,*e,*e,*e,*e]g: &g [*f,*f,*f,*f,*f,*f,*f,*f,*f]h: &h [*g,*g,*g,*g,*g,*g,*g,*g,*g]i: &i [*h,*h,*h,*h,*h,*h,*h,*h,*h]

44
XML Entity Expansion

CAPEC Content Team
The MITRE Corporation
2014-06-23

CAPEC Content Team
The MITRE Corporation
2017-08-04
Updated Resources_Required

CAPEC Content Team
The MITRE Corporation
2018-07-31
Updated References

CAPEC Content Team
The MITRE Corporation
2020-07-30
Updated Mitigations

CAPEC Content Team
The MITRE Corporation
2020-12-17
Updated Related_Attack_Patterns, Taxonomy_Mappings

CAPEC Content Team
The MITRE Corporation
2021-10-21
Updated @Name, Alternate_Terms, Description, Example_Instances, Execution_Flow, Mitigations, Prerequisites, Related_Weaknesses, Skills_Required

CAPEC Content Team
The MITRE Corporation
2022-02-22
Updated Execution_Flow

CAPEC Content Team
The MITRE Corporation
2022-09-29
Updated Example_Instances

XML Entity Expansion