CVE-2012-0841 in iOS
Summary
by MITRE
libxml2 before 2.8.0 computes hash values without restricting the ability to trigger hash collisions predictably, which allows context-dependent attackers to cause a denial of service (CPU consumption) via crafted XML data.
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Analysis
by VulDB Data Team • 02/05/2019
The vulnerability identified as CVE-2012-0841 affects libxml2 versions prior to 2.8.0 and represents a significant hash collision attack vector that can lead to substantial system resource exhaustion. This weakness specifically targets the hash table implementation within the XML parsing library, where the algorithm fails to properly restrict hash value computation, allowing malicious actors to craft XML data that deliberately triggers hash collisions. The vulnerability operates at the core of how libxml2 processes XML documents, making it particularly dangerous as it can be exploited through standard XML parsing operations without requiring special privileges or complex attack chains.
The technical flaw manifests in the hash table collision handling mechanism within libxml2's internal data structures. When processing XML data, the library uses hash tables to store and retrieve element names, attributes, and other identifiers. The vulnerability occurs because the hash function implementation does not adequately prevent predictable collision patterns, allowing attackers to construct XML documents with specifically crafted element names and attributes that force hash table entries to collide. This creates a scenario where the hash table degenerates into a linked list structure, causing the time complexity of hash operations to increase from O(1) to O(n) where n represents the number of colliding elements. This degradation leads to exponential CPU consumption as the parser must sequentially process each colliding entry rather than performing constant-time lookups.
The operational impact of this vulnerability extends beyond simple denial of service, as it can effectively render systems unusable by consuming excessive CPU resources. Attackers can craft XML documents that trigger hash collisions with minimal input, causing parsers to spend disproportionate amounts of time processing what should be simple XML structures. This vulnerability is particularly concerning in web applications and services that process untrusted XML data, as it can be exploited through common attack vectors such as XML-based web services, file uploads, or API endpoints that accept XML input. The resource consumption pattern makes detection difficult as the attack may appear as normal CPU usage spikes rather than obvious malicious behavior, and the impact can be sustained over extended periods if not properly mitigated.
Organizations should implement immediate mitigations including updating to libxml2 version 2.8.0 or later, which contains fixes for the hash collision vulnerability. Additional protective measures include implementing XML size limits and processing timeouts, deploying input validation and sanitization mechanisms, and monitoring for unusual CPU consumption patterns that may indicate exploitation attempts. This vulnerability aligns with CWE-327, which addresses weak cryptographic hash functions, and relates to ATT&CK technique T1498, specifically focusing on resource exhaustion attacks. System administrators should also consider implementing network segmentation and rate limiting for XML processing endpoints to reduce the potential impact of successful exploitation attempts. The fix implemented in libxml2 2.8.0 involves strengthening the hash function to prevent predictable collision patterns while maintaining performance characteristics for legitimate XML processing operations.