CWE-776: Improper Restriction of Recursive Entity References in DTDs ('XML Entity Expansion')
Official CWE-776 CWE context with Glexia analysis, remediation guidance, related CVEs, and ATT&CK context.
Glexia's Take
CWE-776: XEE
Improper Restriction of Recursive Entity References in DTDs ('XML Entity Expansion') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.
Executive Impact
- Availability: DoS: Resource Consumption (Other): If parsed, recursive entity references allow the attacker to expand data exponentially, quickly consuming all system resources.
Developer Pattern
CWE-776 is the kind of defect developers can usually prevent with explicit validation, safer framework defaults, and tests that exercise hostile input or unsafe state transitions.
Confidence
high confidence from CWE-776, 4.20.
Official CWE Definition
CWE-776: Improper Restriction of Recursive Entity References in DTDs ('XML Entity Expansion')
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.
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.
Developer And Remediation Guidance
How teams prevent and detect this weakness
Causes
- 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.
Remediation
- Operation: If possible, prohibit the use of DTDs or use an XML parser that limits the expansion of recursive DTD entities.
- Implementation: Before parsing XML files with associated DTDs, scan for recursive entity declarations and do not continue parsing potentially explosive content.
Detection
- Automated Static Analysis: Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
Mappings
Related CVEs, CWEs, and ATT&CK context
Related CWEs
ATT&CK Relevance
ATT&CK relevance is shown only when reviewed or responsibly inferred.