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.

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.

ThreatScore

Threat Mapped score: 1.5

Industry: Finiancial

Threat priority: P4 - Informational (Low)

Observed Examples (CVEs)

CVE: CVE-2008-3281

XEE in XML-parsing library.
CVE: CVE-2011-3288

XML bomb / XEE in enterprise communication product.
CVE: CVE-2011-1755

"Billion laughs" attack in XMPP server daemon.
CVE: CVE-2009-1955

XML bomb in web server module
CVE: CVE-2003-1564

Parsing library allows XML bomb

Related Attack Patterns (CAPEC)

CAPEC-197

Attack TTPs

N/A

Modes of Introduction

Phase	Note
Implementation	N/A
Operation	N/A

Common Consequences

Impact: DoS: Resource Consumption (Other) — Notes: If parsed, recursive entity references allow the attacker to expand data exponentially, quickly consuming all system resources.

Potential Mitigations

Operation: If possible, prohibit the use of DTDs or use an XML parser that limits the expansion of recursive DTD entities. (N/A)
Implementation: Before parsing XML files with associated DTDs, scan for recursive entity declarations and do not continue parsing potentially explosive content. (N/A)

Applicable Platforms

XML (N/A, Undetermined)

Demonstrative Examples

Intro: 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>

Notes

No notes available.

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