COMMON ATTACK PATTERN ENUMERATION AND CLASSIFICATIONLive MITRE record
CAPEC-197
Exponential Data Expansion
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.
Abstraction
Detailed
Typical severity
Medium
Likelihood of attack
High
Understand the attack
What is this attack pattern?
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.
Execution flow
How does the attack proceed, step by step?
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.
Techniques used
- 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.
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.
Exploit
Send the message
Send the malicious crafted message to the target URL.
Requirements
What does the attack require?
Prerequisites
- 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.
Skills required
- Low: Ability to craft nested data expansion messages.
Resources required
- None: No specialized resources are required to execute this type of attack.
Impact
What does a successful attack lead to?
Unreliable ExecutionResource Consumption
Affected scopes: Availability
Defence
How is it prevented and mitigated?
1
Design: Use libraries and templates that minimize unfiltered input. Use methods that limit entity expansion and throw exceptions on attempted entity expansion.
2
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.
Real world
MITRE example instances
Example instance 1Detail
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.
Example instance 2Detail
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]