CVE-2011-2205 in Prosody
Summary
by MITRE
Prosody before 0.8.1 does not properly detect recursion during entity expansion, which allows remote attackers to cause a denial of service (memory and CPU consumption) via a crafted XML document containing a large number of nested entity references, a similar issue to CVE-2003-1564.
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Analysis
by VulDB Data Team • 02/09/2019
The vulnerability identified as CVE-2011-2205 affects Prosody versions prior to 0.8.1 and represents a classic example of a denial of service attack through XML entity expansion recursion. This flaw resides in the XML processing capabilities of the Prosody instant messaging server, which is widely used for Jabber/XMPP communications. The vulnerability stems from inadequate validation mechanisms that fail to properly detect when XML entities reference themselves or create recursive expansion chains. When a malicious user submits a crafted XML document containing deeply nested entity references, the server attempts to resolve each entity reference recursively, leading to exponential resource consumption patterns that can quickly exhaust available memory and CPU resources. This issue is particularly concerning because it operates at the core parsing layer of the application, making it difficult to mitigate through application-level defenses alone. The vulnerability demonstrates characteristics consistent with CWE-617, which describes reachable assertion conditions that can lead to denial of service through resource exhaustion.
The technical exploitation of this vulnerability relies on the XML parser's failure to implement proper recursion depth limits or detection mechanisms during entity expansion processes. In XML processing, entities can reference other entities, creating a hierarchical structure that should be bounded to prevent infinite expansion. However, Prosody's XML parser lacks adequate safeguards to detect when entity references create circular or deeply nested structures that would cause the parser to consume excessive computational resources. When such malformed XML is processed, the parser enters a recursive expansion loop where each entity reference triggers further resolution of nested references, resulting in exponential growth of processing time and memory usage. This behavior is similar to the well-known CVE-2003-1564 vulnerability that affected other XML parsers, demonstrating that this is a persistent issue in XML processing implementations. The attack vector requires only a remote attacker to send a specially crafted XML document, making it particularly dangerous in networked environments where the server processes untrusted input from external sources.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise the entire messaging infrastructure. When exploited successfully, the vulnerability can cause the Prosody server to consume all available system resources, resulting in complete denial of service for legitimate users attempting to access the messaging service. The resource exhaustion occurs gradually but can escalate rapidly once the recursive expansion begins, making it difficult to distinguish between legitimate high-volume usage and malicious attack patterns. Network administrators may find that the server becomes unresponsive, crashes, or requires manual intervention to restore normal operations. Additionally, the vulnerability can be leveraged as part of broader attack strategies where attackers use multiple simultaneous connections to amplify the resource consumption effects, potentially causing cascading failures in larger network environments. The attack requires minimal technical expertise to execute, making it particularly dangerous as it can be exploited by attackers with basic knowledge of XML processing concepts.
Mitigation strategies for CVE-2011-2205 focus primarily on updating the Prosody server to version 0.8.1 or later, which includes proper recursion detection and limiting mechanisms for entity expansion. System administrators should implement comprehensive patch management processes to ensure all instances of Prosody are updated promptly. Additional defensive measures include configuring XML parser settings to limit entity expansion depth, implementing rate limiting on XML processing requests, and deploying network-level monitoring to detect unusual resource consumption patterns. Organizations should also consider implementing input validation layers that filter or sanitize XML content before it reaches the core parsing components. From a security architecture perspective, this vulnerability highlights the importance of implementing defense-in-depth strategies that protect against both known and unknown vulnerabilities in parsing components. The ATT&CK framework categorizes this vulnerability under the T1499.004 technique for network denial of service, emphasizing the need for proper input validation and resource management controls to prevent exploitation. Regular security assessments and penetration testing should include validation of XML processing capabilities to identify similar vulnerabilities in other applications and systems.