CVE-2016-9625 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-2016-9625 represents a critical denial of service flaw 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, contains a flaw that manifests as infinite recursion when processing specially crafted HTML content. The vulnerability affects versions prior to 0.5.3-33, indicating that this issue has been present for several years within the software ecosystem. The w3m browser is commonly used in environments where graphical interfaces are unavailable or impractical, making it a target for attackers seeking to disrupt services in constrained computing environments.
The technical nature of this vulnerability stems from improper handling of recursive HTML structures within the w3m parser. When the browser encounters a crafted HTML page containing nested elements or circular references that trigger recursive parsing behavior, the parser enters an infinite loop that consumes system resources and ultimately causes the application to become unresponsive. This type of vulnerability falls under the category of CWE-674, which specifically addresses "Uncontrolled Recursion" in software implementations. The flaw demonstrates poor input validation and lacks proper recursion depth limiting mechanisms that would normally prevent such scenarios from occurring in well-designed applications.
The operational impact of this vulnerability extends beyond simple service disruption as it can be exploited by remote attackers to cause persistent denial of service conditions. Attackers can craft malicious HTML pages that, when loaded by an affected w3m instance, will trigger the recursive parsing behavior and consume excessive CPU and memory resources until the system becomes unresponsive or the process terminates. This vulnerability is particularly concerning in environments where w3m is used as part of automated systems, web scraping tools, or in embedded systems where service availability is critical. The attack vector is straightforward requiring only the delivery of a specially crafted HTML page to the vulnerable browser instance, making it an attractive target for malicious actors seeking to disrupt services.
Mitigation strategies for this vulnerability should prioritize immediate patching of affected systems to version 0.5.3-33 or later, which contains the necessary fixes for the recursive parsing issue. System administrators should also implement network-level controls to prevent access to untrusted HTML content when w3m is used in production environments. Additionally, organizations should consider implementing input validation and content filtering mechanisms that can detect and block potentially malicious HTML constructs before they reach the browser parser. From a threat modeling perspective, this vulnerability aligns with ATT&CK technique T1499.004, which covers "Elevated Execution with Systemd Service" and related denial of service techniques that can be leveraged to compromise availability. The vulnerability demonstrates the importance of proper recursion handling in parsing libraries and serves as a reminder of the need for comprehensive input validation in all software components that process external data.