CWE-776: Improper Restriction of Recursive Entity References in DTDs ('XML Entity Expansion')
Weakness ID: 776
Abstraction: Variant Structure: Simple
Status: Draft
Presentation Filter:
Description
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.
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.
Alternate Terms
XEE:
XEE is the acronym commonly used for XML Entity Expansion.
Billion Laughs Attack
XML Bomb:
While the "XML Bomb" term was used in the early years of knowledge of this issue, the XEE term seems to be more commonly used.
Relationships
The table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
Relevant to the view "Research Concepts" (CWE-1000)
Nature
Type
ID
Name
ChildOf
Base - a weakness that is described in an abstract fashion, but with sufficient details to infer specific methods for detection and prevention. More general than a Variant weakness, but more specific than a Class weakness.
Base - a weakness that is described in an abstract fashion, but with sufficient details to infer specific methods for detection and prevention. More general than a Variant weakness, but more specific than a Class weakness.
Base - a weakness that is described in an abstract fashion, but with sufficient details to infer specific methods for detection and prevention. More general than a Variant weakness, but more specific than a Class weakness.
Base - a weakness that is described in an abstract fashion, but with sufficient details to infer specific methods for detection and prevention. More general than a Variant weakness, but more specific than a Class weakness.
Base - a weakness that is described in an abstract fashion, but with sufficient details to infer specific methods for detection and prevention. More general than a Variant weakness, but more specific than a Class weakness.
The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the software life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
Phase
Note
Implementation
Operation
Applicable Platforms
The listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.
Languages
XML (Undetermined Prevalence)
Paradigms
Web Based (Undetermined Prevalence)
Common Consequences
The table below specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
If parsed, recursive entity references allow the attacker to expand data exponentially, quickly consuming all system resources.
Likelihood Of Exploit
Medium
Demonstrative Examples
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.
(attack code)
Example Language: XML
<?xml version="1.0"?> <!DOCTYPE MaliciousDTD [ <!ENTITY ZERO "A"> <!ENTITY ONE "&ZERO;&ZERO;"> <!ENTITY TWO "&ONE;&ONE;"> ... <!ENTITY THIRTYTWO "&THIRTYONE;&THIRTYONE;"> ]> <data>&THIRTYTWO;</data>
If possible, prohibit the use of DTDs or use an XML parser that limits the expansion of recursive DTD entities.
Phase: Implementation
Before parsing XML files with associated DTDs, scan for recursive entity declarations and do not continue parsing potentially explosive content.
Memberships
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
Nature
Type
ID
Name
MemberOf
Category - a CWE entry that contains a set of other entries that share a common characteristic.
Unrestricted Recursive Entity References in DTDs ('XML Bomb')
More information is available — Please select a different filter.
Page Last Updated:
March 29, 2018
Use of the Common Weakness Enumeration and the associated references from this website are subject to the
Terms of Use. For more information, please email
cwe@mitre.org.
Description
Relevant to the view "Research Concepts" (CWE-1000)
