CWE-776

Base Abstraction Level
Pillar — Highest-level weakness category
Class — Abstract, language-independent
Base — Specific enough to detect
Variant — Tied to specific technology
Compound — Requires multiple weaknesses Draft MITRE CWE Status
Stable — Fully reviewed and complete
Draft — Under development, may change
Incomplete — Partially defined by MITRE
Deprecated — No longer recommended
Obsolete — Replaced by another CWE Exploit: Medium
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.

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.

Consequences

Availability — DoS: Resource Consumption (Other)

If parsed, recursive entity references allow the attacker to expand data exponentially, quickly consuming all system resources.

Mitigations

Phase: Operation

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.

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.)