CVE-2016-9431 in w3m
Summary
by MITRE
An issue was discovered in the Tatsuya Kinoshita w3m fork before 0.5.3-31. Infinite recursion vulnerability in w3m allows remote attackers to cause a denial of service via a crafted HTML page.
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Analysis
by VulDB Data Team • 10/05/2022
The vulnerability identified as CVE-2016-9431 represents a critical denial of service flaw within the w3m web browser implementation developed by Tatsuya Kinoshita. This issue affects versions prior to 0.5.3-31 and demonstrates a classic infinite recursion vulnerability that can be exploited remotely through specially crafted HTML content. The w3m browser, being a text-based web client designed for terminal environments, processes HTML documents in a manner that makes it susceptible to recursive parsing behaviors that can lead to system resource exhaustion.
The technical root cause of this vulnerability lies in the HTML parser's handling of nested or malformed HTML structures within the w3m fork. When processing a maliciously crafted HTML page, the parser enters an infinite recursive loop where it continuously processes elements that reference themselves or create circular dependencies. This recursive behavior consumes system resources without bounds, ultimately leading to process termination or system instability. The vulnerability operates at the parsing layer of the application, where HTML elements are processed and rendered, making it particularly dangerous as it requires no user interaction beyond visiting the malicious page.
From an operational perspective, this vulnerability presents significant risk to systems running affected versions of w3m, particularly in environments where automated browsing or web content processing occurs. The remote exploitation capability means that attackers can trigger the denial of service condition without requiring local access or user interaction beyond navigation to the malicious page. This makes the vulnerability particularly concerning for web services, automated monitoring systems, or any application that relies on w3m for web content retrieval. The impact extends beyond simple service disruption as the infinite recursion can cause memory exhaustion, CPU starvation, and potentially system crashes in resource-constrained environments.
The vulnerability aligns with CWE-674, which addresses the issue of uncontrolled recursion in software implementations. This classification indicates that the flaw stems from inadequate recursion bounds checking and proper termination conditions within the parsing logic. From an attack framework perspective, this vulnerability would be categorized under the denial of service category within the MITRE ATT&CK framework, specifically mapping to techniques involving resource exhaustion and process manipulation. The attack surface is particularly broad given that w3m is commonly used in terminal-based environments, embedded systems, and automated processes where the application might be invoked without user awareness.
Mitigation strategies for this vulnerability primarily focus on immediate version updates to w3m 0.5.3-31 or later, which contain patches addressing the recursive parsing issue. System administrators should implement network-level controls to prevent access to known malicious content and consider implementing web filtering solutions that can detect and block potentially harmful HTML constructs. Additionally, organizations should conduct thorough testing of their w3m implementations to ensure proper patching and monitor for any anomalous resource usage patterns that might indicate exploitation attempts. The vulnerability highlights the importance of input validation and robust parsing mechanisms in text-based web browsers, particularly those designed for automated or embedded use cases where resource constraints and security considerations are paramount.