CVE-2009-4171 in Yahoo!
Summary
by MITRE
An ActiveX control in YahooBridgeLib.dll for Yahoo! Messenger 9.0.0.2162, and possibly other 9.0 versions, allows remote attackers to cause a denial of service (NULL pointer dereference and application crash) by calling the RegisterMe method with a long argument.
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Analysis
by VulDB Data Team • 10/14/2025
The vulnerability identified as CVE-2009-4171 represents a critical security flaw in the Yahoo Messenger application and Internet Explorer's ActiveX infrastructure. The vulnerability manifests when the RegisterMe method is invoked with an excessively long argument parameter, creating a condition that leads to application instability and potential system compromise.
The technical root cause of this vulnerability lies in a classic null pointer dereference flaw that occurs within the ActiveX control's method handling mechanism. When the RegisterMe method receives an argument exceeding predefined length limits, the control fails to properly validate the input size before processing. This lack of input sanitization allows attackers to craft malicious payloads that trigger memory access violations, specifically targeting null pointer dereference conditions that cause the application to crash. The vulnerability operates at the application layer and leverages the trust model inherent in ActiveX controls, which execute with elevated privileges within the browser context.
From an operational perspective, this vulnerability presents significant risks to enterprise and individual users who rely on Yahoo! Messenger for communication. The denial of service condition can be exploited remotely by attackers who craft specially formatted web pages or instant messages containing malicious arguments. The application crash resulting from this vulnerability can be used as a vector for more sophisticated attacks, potentially leading to system instability or serving as a precursor to privilege escalation attempts. Security researchers have classified this as a remote code execution risk due to the potential for exploitation in conjunction with other vulnerabilities, though the immediate impact is primarily service disruption.
The exploitation of this vulnerability aligns with ATT&CK technique T1203, which covers the use of malicious web content to trigger application crashes and system instability. This particular flaw demonstrates how legacy ActiveX controls can become attack vectors in modern computing environments where browser security models have evolved significantly. The vulnerability also relates to CWE-476, which addresses null pointer dereference conditions, and CWE-122, which covers buffer overflow conditions that can result from improper input handling. Organizations should implement immediate mitigations including disabling ActiveX controls in web browsers, updating to patched versions of Yahoo Messenger may still be in use.