CVE-2015-2937 in MediaWiki
Summary
by MITRE
MediaWiki before 1.19.24, 1.2x before 1.23.9, and 1.24.x before 1.24.2, when using HHVM or Zend PHP, allows remote attackers to cause a denial of service ("quadratic blowup" and memory consumption) via an XML file containing an entity declaration with long replacement text and many references to this entity, a different vulnerability than CVE-2015-2942.
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Analysis
by VulDB Data Team • 05/03/2022
This vulnerability affects MediaWiki versions prior to specific patches and represents a denial of service condition that leverages XML entity processing flaws. The issue manifests when MediaWiki systems process XML files containing entity declarations with extensive replacement text combined with numerous references to those entities. This creates a quadratic blowup scenario where memory consumption grows exponentially with input size rather than linearly, effectively exhausting system resources and rendering the service unavailable to legitimate users. The vulnerability specifically impacts systems using HHVM or Zend PHP environments, making it particularly relevant for web applications that rely on these runtime environments for processing XML content.
The technical flaw stems from inadequate input validation and memory management within MediaWiki's XML parsing routines. When an XML parser encounters an entity declaration with lengthy replacement text, it must store this content in memory for potential expansion. However, when the entity is referenced multiple times throughout the document, the parser processes each reference individually, causing memory allocation to increase disproportionately. This behavior creates a computationally expensive operation where the time and memory required to process the document grows quadratically rather than linearly with document size. The vulnerability operates at the parser level, making it particularly dangerous as it can be triggered through various XML input sources including user uploads, external feeds, or configuration files that MediaWiki processes.
The operational impact of this vulnerability extends beyond simple service disruption, as it can be exploited by attackers to consume excessive system resources and potentially cause cascading failures in web applications. In environments where MediaWiki serves as a core component of content management systems, this vulnerability can lead to complete service outages and may affect other applications sharing the same infrastructure. The quadratic blowup characteristic means that even relatively small malicious XML files can cause significant resource exhaustion, making the attack vector particularly effective for resource-constrained systems. Organizations running vulnerable MediaWiki installations face the risk of sustained denial of service attacks that can be difficult to detect and mitigate due to the gradual nature of memory consumption.
Mitigation strategies for this vulnerability require immediate patching of affected MediaWiki installations to versions that include proper XML parsing safeguards and input validation. System administrators should ensure all instances are updated to MediaWiki 1.19.24, 1.23.9, or 1.24.2 respectively, depending on their current version. Additionally, implementing XML input filtering and size limitations can provide defense-in-depth measures, though these are less effective than proper patching. Network-level protections such as rate limiting and input sanitization should be deployed to reduce the effectiveness of exploitation attempts. Organizations should also consider implementing monitoring for unusual memory consumption patterns and automated alerting for potential exploitation attempts. This vulnerability aligns with CWE-400, which addresses uncontrolled resource consumption, and relates to ATT&CK technique T1499.004 for resource exhaustion attacks, highlighting the need for robust input validation and memory management practices in web applications.
The vulnerability demonstrates the critical importance of proper XML parser security and input validation in web applications. MediaWiki's XML processing code was susceptible to a classic denial of service attack pattern where malformed input can cause exponential resource consumption. This type of vulnerability often stems from insufficient bounds checking and inadequate handling of recursive or repeated elements within XML documents. The fact that this issue affects both HHVM and Zend PHP environments indicates it is rooted in the underlying XML parsing libraries rather than specific runtime implementations, making it a more pervasive concern across different deployment scenarios. Security teams should conduct thorough audits of their XML processing components and implement comprehensive testing procedures to identify similar vulnerabilities in other applications.