CVE-2016-9439 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-2016-9439 represents a critical denial of service weakness within the w3m web browser implementation developed by Tatsuya Kinoshita. This particular fork of the w3m browser, which is a lightweight text-based web browser designed for terminal environments, contained a flaw that could be exploited by remote attackers to disrupt normal system operations. The issue specifically affects versions prior to 0.5.3-31, indicating that the vulnerability had been present for some time before the patch was implemented. The flaw manifests as an infinite recursion condition that can be triggered through the careful crafting of HTML content, making it particularly dangerous as it requires no special privileges from the attacker beyond the ability to deliver malicious content to a vulnerable system.
The technical nature of this vulnerability stems from improper handling of recursive HTML structures within the w3m parser. When the browser encounters a specially crafted HTML page containing nested elements or circular references that trigger recursive parsing behavior, the application enters an infinite loop that consumes excessive system resources. This recursive behavior occurs at the parsing layer where the browser attempts to process HTML elements without proper bounds checking or recursion depth limitations. The vulnerability maps directly to CWE-674, which describes "Uncontrolled Recursion" in software systems, where the application fails to implement adequate safeguards against recursive operations that could lead to resource exhaustion. The flaw demonstrates a classic example of how seemingly benign parsing operations can become dangerous when proper defensive programming practices are not implemented.
The operational impact of this vulnerability extends beyond simple service disruption to potentially affect system availability and resource allocation. An attacker can cause a denial of service condition that may exhaust memory resources, CPU cycles, or both, depending on how the infinite recursion is implemented within the parsing engine. This type of vulnerability is particularly concerning in environments where w3m is used for automated browsing, web scraping, or as part of larger applications where resource constraints are critical. The attack surface is broad since any system running vulnerable versions of w3m and processing untrusted HTML content could be affected, including servers that might process web content from multiple sources or users who browse the internet through w3m. From an attacker perspective, this vulnerability aligns with ATT&CK technique T1499.004, which covers "Toggle System/Network Monitoring" through resource exhaustion, and represents a straightforward method for achieving system disruption without requiring advanced exploitation techniques.
Mitigation strategies for CVE-2016-9439 primarily focus on updating to the patched version of w3m, specifically version 0.5.3-31 or later. Organizations should implement comprehensive patch management procedures to ensure all instances of the vulnerable software are updated promptly. Additionally, system administrators should consider implementing input validation and content filtering mechanisms that can detect and block potentially malicious HTML content before it reaches the vulnerable parsing engine. The vulnerability highlights the importance of defensive programming practices such as implementing recursion depth limits, stack overflow protection, and proper bounds checking in parsing libraries. Network-level protections such as web application firewalls or content inspection systems can provide additional layers of defense by identifying and blocking suspicious HTML patterns that might trigger the recursive behavior. Regular security assessments and code reviews focusing on parsing routines and recursive operations should be conducted to prevent similar vulnerabilities from emerging in other components of the system architecture.