CVE-2012-6051 in CityHash
Summary
by MITRE
Google CityHash computes hash values without properly restricting the ability to trigger hash collisions predictably, which allows context-dependent attackers to cause a denial of service (CPU consumption) via crafted input to an application that maintains a hash table, as demonstrated by a universal multicollision attack.
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Analysis
by VulDB Data Team • 03/16/2019
The vulnerability identified as CVE-2012-6051 represents a critical weakness in Google CityHash cryptographic hash function implementation that has significant implications for system security and availability. This flaw resides in the hash function's inability to properly constrain collision resistance properties, creating predictable pathways for attackers to manipulate hash table operations. The vulnerability specifically affects applications that rely on hash tables for data structure management, where the hash function serves as the foundation for key distribution and lookup mechanisms.
The technical implementation flaw stems from CityHash's design characteristics that fail to adequately address the mathematical properties required for robust hash collision resistance. When attackers craft specific input patterns, they can exploit predictable collision patterns within the hash function's output space, enabling them to force hash table implementations into degenerate states where performance degrades dramatically. This occurs because hash tables typically rely on uniform distribution of hash values to maintain O(1) average lookup times, but predictable collisions force the system into O(n) worst-case scenarios where every element must be examined sequentially.
The operational impact of this vulnerability manifests as significant CPU consumption and potential denial of service conditions across affected systems. Attackers can craft input data that causes hash table operations to consume excessive computational resources, effectively creating a resource exhaustion attack vector that impacts system availability. This vulnerability is particularly dangerous because it operates in a context-dependent manner, meaning the attack effectiveness varies based on the specific hash table implementation, data characteristics, and system configuration. The universal multicollision attack demonstrated in this vulnerability allows adversaries to create multiple inputs that map to the same hash value, amplifying the impact of the attack and making it more difficult to detect and mitigate.
From a cybersecurity perspective, this vulnerability aligns with CWE-327, which addresses broken cryptographic algorithms and improper implementation of cryptographic functions. The weakness also maps to ATT&CK technique T1499.004, which covers network denial of service through resource exhaustion. The attack vector specifically targets the fundamental data structures used in many applications, making it a widespread concern across software systems that implement hash-based data structures. Organizations using affected applications may experience complete service unavailability as hash table operations become computationally prohibitive, requiring immediate remediation to prevent operational disruption.
Mitigation strategies for this vulnerability include updating to patched versions of affected software libraries, implementing hash function selection criteria that favor collision-resistant algorithms, and deploying input validation mechanisms that detect and reject suspicious data patterns. System administrators should consider implementing rate limiting and resource monitoring to detect anomalous hash table behavior, while developers need to evaluate their hash table implementations for susceptibility to similar attacks. The vulnerability underscores the importance of cryptographic algorithm selection and the need for comprehensive security testing of fundamental system components, particularly those involving hash functions and data structures that handle user-provided input.