CVE-2009-4873 in Serv-U
Summary
by MITRE
Stack-based buffer overflow in the HTTP server in Rhino Software Serv-U Web Client 9.0.0.5 allows remote attackers to cause a denial of service (server crash) or execute arbitrary code via a long Session cookie.
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Analysis
by VulDB Data Team • 11/17/2025
The vulnerability identified as CVE-2009-4873 represents a critical stack-based buffer overflow flaw within the HTTP server component of Rhino Software Serv-U Web Client version 9.0.0.5. This security weakness resides in the server's handling of Session cookies, specifically when processing excessively long cookie values that exceed the allocated buffer space on the stack. The flaw demonstrates characteristics consistent with CWE-121 Stack-based Buffer Overflow, where insufficient bounds checking allows attackers to overwrite adjacent stack memory locations. The vulnerability operates at the application layer and affects the web server functionality of the Serv-U software, which is widely used for file transfer and web serving operations in enterprise environments. This issue creates a significant risk for organizations relying on Serv-U for their web hosting and file transfer services, as it provides a direct pathway for remote exploitation.
The technical implementation of this vulnerability involves the server's HTTP processing module failing to properly validate the length of Session cookie values before copying them into fixed-size stack buffers. When an attacker submits a Session cookie containing more data than the allocated buffer can accommodate, the excess data overflows into adjacent memory locations, potentially corrupting the stack frame. This overflow can result in two primary attack vectors: denial of service through server crashes or more dangerous arbitrary code execution. The stack corruption may overwrite return addresses, function pointers, or other critical control data, enabling attackers to redirect program execution flow. The vulnerability is particularly concerning because it requires no authentication to exploit, making it accessible to any remote attacker who can send HTTP requests to the affected server. This characteristic aligns with ATT&CK technique T1210 Exploitation of Remote Services, where adversaries leverage vulnerabilities in network services to gain unauthorized access or cause system disruption.
The operational impact of this vulnerability extends beyond simple server instability to encompass potential complete system compromise. A successful exploitation could allow attackers to execute malicious code with the privileges of the web server process, potentially leading to full system compromise if the server runs with elevated permissions. The denial of service aspect alone creates significant business disruption, as the affected web server would become unavailable to legitimate users, impacting service continuity and potentially causing revenue loss for organizations relying on the affected infrastructure. Organizations using Serv-U Web Client 9.0.0.5 face substantial risk from this vulnerability, particularly those with internet-facing web servers or those that do not implement proper network segmentation. The vulnerability affects systems where Session cookies are processed without adequate input validation, making it relevant to any web application that relies on cookie-based session management without proper boundary checking. The attack surface is broad given Serv-U's usage in enterprise environments for hosting web content and managing file transfers, where the combination of web serving capabilities and session management creates multiple potential entry points for exploitation. Security teams must consider this vulnerability as part of their broader threat landscape assessment, particularly when evaluating legacy systems that may not receive regular security updates or patches.