CVE-2018-6196 in w3m
Summary
by MITRE
w3m through 0.5.3 is prone to an infinite recursion flaw in HTMLlineproc0 because the feed_table_block_tag function in table.c does not prevent a negative indent value.
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Analysis
by VulDB Data Team • 02/02/2023
The vulnerability identified as CVE-2018-6196 affects the w3m web browser version 0.5.3 and earlier, presenting a critical infinite recursion flaw within its HTML processing engine. This issue resides in the HTMLlineproc0 function specifically within the table.c source file where the feed_table_block_tag function fails to properly validate input parameters, particularly allowing negative indent values that can trigger recursive processing loops. The flaw represents a classic example of inadequate input validation and control flow management that can be exploited to cause denial of service conditions.
The technical implementation of this vulnerability stems from the absence of proper boundary checking within the table processing logic. When w3m encounters HTML content containing malformed table structures with negative indentation values, the feed_table_block_tag function enters an infinite recursion loop where it repeatedly processes the same table elements without proper termination conditions. This recursive behavior consumes system resources rapidly and can lead to complete system exhaustion, making the application unavailable to legitimate users. The vulnerability falls under CWE-674 which specifically addresses uncontrolled recursion in software systems, and aligns with ATT&CK technique T1499.100 related to resource exhaustion attacks.
The operational impact of this vulnerability extends beyond simple denial of service scenarios, as it can be leveraged by attackers to disrupt services on systems running vulnerable versions of w3m. Given that w3m is commonly used as a text-based web browser in terminal environments, server configurations, and embedded systems, exploitation could affect a wide range of deployments including web servers, automated systems, and network appliances. The recursive nature of the flaw means that even a single malicious HTML document can cause significant system instability, potentially affecting multiple concurrent users or processes depending on the deployment architecture.
Mitigation strategies for CVE-2018-6196 require immediate patching of affected w3m installations to version 0.5.4 or later where the recursion limits have been properly implemented. System administrators should also consider implementing HTML content filtering mechanisms to prevent processing of malformed table structures, particularly in environments where users can submit arbitrary web content. Additionally, monitoring for unusual CPU usage patterns or process spawning behaviors can help detect exploitation attempts. The fix implemented in newer versions typically involves adding proper input validation checks for indent values and implementing maximum recursion depth limits to prevent the infinite loop condition from occurring. Organizations should also review their deployment configurations to ensure that w3m instances are not running with elevated privileges that could amplify the impact of such denial of service conditions.