CVE-2011-4858 in Tomcat
Summary
by MITRE
Apache Tomcat before 5.5.35, 6.x before 6.0.35, and 7.x before 7.0.23 computes hash values for form parameters without restricting the ability to trigger hash collisions predictably, which allows remote attackers to cause a denial of service (CPU consumption) by sending many crafted parameters.
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Analysis
by VulDB Data Team • 12/23/2024
The vulnerability identified as CVE-2011-4858 represents a critical hash collision attack vector within the Apache Tomcat web application server that affects multiple version ranges including 5.5.x before 5.5.35, 6.x before 6.0.35, and 7.x before 7.0.23. This weakness stems from the application server's improper handling of hash computation for form parameters during HTTP request processing, creating an environment where malicious actors can exploit predictable hash collisions to consume excessive CPU resources. The vulnerability operates at the core of Tomcat's parameter processing mechanism, specifically within the hash table implementation used to store and manage form data submitted through web applications. This flaw falls under the category of CWE-400, which addresses Uncontrolled Resource Consumption, and represents a classic example of a denial of service attack leveraging algorithmic complexity weaknesses in data structures. The attack exploits the fundamental hash table collision behavior by crafting HTTP parameters that deliberately produce the same hash values, causing the underlying hash table implementation to degrade from optimal O(1) lookup time to O(n) performance where n represents the number of collisions.
The operational impact of this vulnerability manifests as a significant degradation in system performance and potential complete service disruption for affected Apache Tomcat installations. When remote attackers submit multiple crafted form parameters designed to trigger hash collisions, the server's CPU utilization spikes dramatically as it struggles to process the increased computational overhead required to maintain the hash table structure. This type of attack represents a sophisticated denial of service vector that can be executed with minimal resources and can effectively bring down web applications that rely on Tomcat for processing user input. The vulnerability specifically targets the parameter processing pipeline within Tomcat's request handling architecture, where form parameters are parsed and stored in hash-based data structures for subsequent application processing. Attackers can leverage this weakness to perform resource exhaustion attacks against web applications without requiring authentication or privileged access, making it particularly dangerous for publicly accessible services.
From a cybersecurity perspective, this vulnerability aligns with ATT&CK technique T1499.004, which covers "Cloud Compute Infrastructure Abuse" and represents a form of resource exhaustion attack that consumes computational resources to deny service to legitimate users. The vulnerability demonstrates the importance of proper input validation and hash table implementation security within web application servers, as it exploits the fundamental assumption that hash-based data structures provide efficient constant-time operations. Organizations running affected versions of Apache Tomcat face substantial risk of service disruption and potential business impact when this vulnerability is exploited. The attack requires minimal sophistication to execute, as it only involves crafting HTTP requests with specific parameter names that will generate hash collisions, making it accessible to attackers with basic web application knowledge. The computational overhead increases exponentially with the number of colliding parameters, allowing attackers to consume significant CPU cycles with relatively few requests, thereby maximizing the impact of their attack while minimizing the resources required to execute it.
The recommended mitigation strategy involves upgrading to patched versions of Apache Tomcat where the hash collision vulnerability has been addressed through improved hash function selection and parameter processing algorithms. Organizations should prioritize immediate deployment of security patches for versions 5.5.35, 6.0.35, and 7.0.23 or later, as these releases contain fixes that prevent predictable hash collisions from occurring during parameter processing. Additionally, implementing rate limiting and request validation mechanisms at the application level can provide additional defense in depth against this specific attack vector. Network-level protections such as web application firewalls and intrusion detection systems can help identify and block malicious parameter patterns that attempt to trigger hash collisions, though these solutions should not be considered replacements for proper software patching. The vulnerability highlights the critical importance of keeping web application server software up to date and demonstrates how seemingly minor implementation details in core data structures can have significant security implications when exposed to malicious input processing. Security teams should also consider implementing monitoring and alerting for unusual CPU consumption patterns that might indicate exploitation attempts, as this vulnerability can be difficult to detect through traditional security scanning methods due to its reliance on legitimate HTTP parameter processing behavior.