CWE-776: Improper Restriction of Recursive Entity References in DTDs ('XML Entity Expansion')
Description
The product uses XML documents and allows their structure to be defined with a Document Type Definition (DTD), but it does not properly control the number of recursive definitions of entities.
Submission Date :
June 30, 2009, midnight
Modification Date :
2023-06-29 00:00:00+00:00
Organization :
MITRE
Extended Description
If the DTD contains a large number of nested or recursive entities, this can lead to explosive growth of data when parsed, causing a denial of service.
Example - 1
The DTD and the very brief XML below illustrate what is meant by an XML bomb. The ZERO entity contains one character, the letter A. The choice of entity name ZERO is being used to indicate length equivalent to that exponent on two, that is, the length of ZERO is 2^0. Similarly, ONE refers to ZERO twice, therefore the XML parser will expand ONE to a length of 2, or 2^1. Ultimately, we reach entity THIRTYTWO, which will expand to 2^32 characters in length, or 4 GB, probably consuming far more data than expected.
<?xml version="1.0"?><!DOCTYPE MaliciousDTD [<!ENTITY ZERO "A"><!ENTITY ONE "&ZERO;&ZERO;"><!ENTITY TWO "&ONE;&ONE;">...<!ENTITY THIRTYTWO "&THIRTYONE;&THIRTYONE;">]><data>&THIRTYTWO;</data>
Related Weaknesses
This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined to give an overview of the different insight to similar items that may exist at higher and lower levels of abstraction.
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