CVE-2016-9626 in w3m
Summary
by MITRE
An issue was discovered in the Tatsuya Kinoshita w3m fork before 0.5.3-33. 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-2016-9626 represents a critical denial of service flaw within the w3m web browser implementation developed by Tatsuya Kinoshita. This issue affects versions prior to 0.5.3-33 and demonstrates a classic infinite recursion pattern that can be exploited remotely through carefully crafted HTML content. The w3m browser, known for its lightweight design and terminal-based interface, is widely used in environments where minimal resource consumption is essential, making this vulnerability particularly concerning for system administrators and security professionals managing such deployments. The flaw manifests when the browser processes specific HTML constructs that trigger recursive parsing behaviors, leading to uncontrolled memory consumption and eventual system resource exhaustion.
The technical root cause of this vulnerability stems from inadequate input validation and recursive parsing logic within the HTML parser component of the w3m fork. When encountering malformed HTML structures, particularly those involving nested or self-referencing elements, the parser enters an infinite loop where it repeatedly processes the same HTML fragments without proper termination conditions. This recursive behavior is not properly bounded by stack limits or recursion depth counters, allowing malicious actors to craft HTML pages that contain recursive references or deeply nested structures. The vulnerability maps directly to CWE-674, which catalogs "Uncontrolled Recursion" as a common weakness pattern, specifically addressing the lack of safeguards against infinite recursive calls in parsing routines. The flaw exists at the intersection of parsing logic and memory management, where the browser's inability to detect and terminate recursive parsing sequences creates an exploitable condition.
The operational impact of this vulnerability extends beyond simple service disruption, potentially affecting system availability in critical infrastructure environments where w3m is deployed. Remote attackers can leverage this flaw to consume excessive system resources, leading to complete system hangs or crashes, particularly in resource-constrained environments such as embedded systems or containers. The denial of service condition can be triggered without any authentication requirements, making it highly attractive to malicious actors seeking to disrupt services. Organizations relying on w3m for web browsing in terminal environments, automated systems, or embedded applications face significant risk from this vulnerability. The attack surface is broad since any system that processes HTML content through this vulnerable version of w3m could be affected, including web proxies, automated testing environments, and security scanning tools that utilize w3m for content analysis.
Mitigation strategies for CVE-2016-9626 primarily focus on immediate version updates to w3m 0.5.3-33 or later, which contain the necessary patches to address the recursive parsing issue. System administrators should prioritize patching affected installations across all environments where w3m is deployed, particularly in production systems where availability is critical. Additional defensive measures include implementing content filtering mechanisms that can detect and block suspicious HTML patterns before they reach the vulnerable parser, as well as monitoring system resource consumption for unusual spikes that might indicate exploitation attempts. Network-level protections such as web application firewalls can help filter malicious HTML content, while application-level sandboxing techniques can limit the impact of successful exploitation attempts. The vulnerability also highlights the importance of proper input validation and recursion bounds in parsing components, aligning with ATT&CK technique T1499.004 for Network Denial of Service and emphasizing the need for robust defensive coding practices. Organizations should also consider implementing automated vulnerability scanning processes that can identify and remediate affected w3m installations across their infrastructure.