CVE-2012-0877 in PyXML
Summary
by MITRE
PyXML: Hash table collisions CPU usage Denial of Service
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Analysis
by VulDB Data Team • 03/04/2025
The vulnerability identified as CVE-2012-0877 represents a significant denial of service weakness affecting PyXML implementations that stems from inadequate handling of hash table collisions within the XML parsing library. This flaw specifically targets the Python XML processing framework where hash table collisions can be exploited to cause excessive CPU consumption, ultimately leading to system resource exhaustion and service unavailability.
The technical root cause of this vulnerability lies in the hash table implementation used by PyXML's internal data structures. When maliciously crafted XML documents are processed, attackers can construct input data that deliberately triggers hash collisions within the hash table structures used for parsing XML elements and attributes. This collision behavior forces the hash table to degrade from optimal O(1) lookup performance to O(n) behavior, where n represents the number of colliding elements. As the number of collisions increases, the CPU usage spikes dramatically, consuming system resources at an exponential rate proportional to the number of colliding hash values.
This vulnerability operates under the broader category of hash collision attacks that fall under the Common Weakness Enumeration CWE-327, which specifically addresses weaknesses related to the use of insecure or weak cryptographic algorithms and hash functions. The attack vector is particularly insidious because it requires no authentication or special privileges to execute, making it a low-effort, high-impact method for disrupting XML processing services. The operational impact extends beyond simple resource exhaustion, as systems may become unresponsive to legitimate requests, leading to cascading failures in applications that depend on XML parsing capabilities.
The attack mechanism leverages the fundamental design flaw in how PyXML manages hash table collisions, where the system fails to implement proper collision handling or load balancing techniques. When an attacker crafts XML input containing specially designed elements that map to the same hash value, the system's hash table implementation becomes overwhelmed with collision resolution operations. This creates a denial of service condition where legitimate processing of XML documents becomes extremely slow or impossible, effectively rendering the service unavailable to legitimate users.
Systems utilizing PyXML for processing external XML data are particularly vulnerable to this attack, especially those that process untrusted input from web applications, APIs, or file uploads. The vulnerability affects any application that relies on PyXML's parsing capabilities for XML document processing, including web services, content management systems, and enterprise applications. The attack can be executed through various means including HTTP requests containing malicious XML payloads, file upload processing, or any other scenario where external XML input is parsed without proper validation or rate limiting.
Mitigation strategies for CVE-2012-0877 should focus on implementing proper input validation and rate limiting mechanisms to prevent excessive hash table collision scenarios. Organizations should consider upgrading to more recent XML processing libraries that implement robust hash table collision handling techniques or employ hash function randomization to prevent predictable collision patterns. Additionally, implementing resource limits and monitoring for unusual CPU usage patterns can help detect and respond to active exploitation attempts. The mitigation approach aligns with ATT&CK technique T1499.004 which addresses network denial of service attacks and emphasizes the importance of resource exhaustion prevention in maintaining system availability and service integrity.