Improper Restriction of Recursive Entity References in DTDs ('XML Entity Expansion')

The software 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.


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.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

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>

See Also

OWASP Top Ten 2017 Category A4 - XML External Entities (XXE)

Weaknesses in this category are related to the A4 category in the OWASP Top Ten 2017.

Data Processing Errors

Weaknesses in this category are typically found in functionality that processes data. Data processing is the manipulation of input to retrieve or save information.

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