CVE-2000-0239 in Mercur Mail Server
Summary
by MITRE
Buffer overflow in the MERCUR WebView WebMail server allows remote attackers to cause a denial of service via a long mail_user parameter in the GET request.
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Analysis
by VulDB Data Team • 10/18/2025
The CVE-2000-0239 vulnerability represents a classic buffer overflow flaw within the MERCUR WebView WebMail server implementation that demonstrates the critical importance of input validation in web-based email systems. This vulnerability specifically targets the handling of the mail_user parameter within HTTP GET requests, exposing a fundamental security weakness in how the server processes user input. The flaw occurs when the server fails to properly validate the length of the mail_user parameter, allowing malicious actors to craft specially crafted requests that exceed the allocated buffer space. This type of vulnerability falls under the CWE-121 category of buffer overflow conditions, where insufficient bounds checking enables attackers to overwrite adjacent memory locations. The MERCUR WebView WebMail server, which was widely deployed in enterprise environments during the late 1990s and early 2000s, represents a common class of web applications that were often built without adequate security considerations. The vulnerability manifests as a denial of service condition rather than arbitrary code execution, but this does not diminish its operational impact on mission-critical email services. When exploited, the buffer overflow causes the web server process to crash or become unresponsive, effectively preventing legitimate users from accessing their email accounts. This disruption can have cascading effects on business operations, particularly in environments where email serves as a primary communication channel for critical business functions.
The operational impact of CVE-2000-0239 extends beyond simple service disruption to represent a broader class of vulnerabilities that were prevalent in web server implementations of that era. The attack vector is particularly concerning because it requires minimal technical expertise to execute, making it accessible to a wide range of threat actors including script kiddies and organized groups. The vulnerability operates at the application layer of the network stack, specifically targeting the HTTP request processing logic within the webmail server. This aligns with ATT&CK technique T1210 which describes exploitation of vulnerabilities in remote services, and demonstrates how attackers can leverage seemingly minor flaws to achieve significant operational disruption. The buffer overflow condition creates an opportunity for attackers to not only cause denial of service but potentially to escalate privileges if the server process has elevated permissions. In many enterprise deployments, the webmail server was configured with sufficient privileges to access email databases and file systems, making the potential impact of such an exploit much more severe than a simple service interruption. The vulnerability's exploitation typically requires only a single HTTP GET request with an excessively long mail_user parameter, making it particularly dangerous in environments where webmail servers are accessible to external users.
Mitigation strategies for CVE-2000-0239 must address both immediate operational concerns and long-term architectural security improvements. The most effective immediate solution involves implementing input length validation and sanitization within the webmail server's request processing logic, ensuring that all user-supplied parameters are checked against predetermined maximum lengths before processing. This approach directly addresses the root cause of the vulnerability by preventing the buffer overflow condition from occurring. Organizations should also implement network-level protections such as intrusion detection systems that can identify and block suspicious GET requests containing unusually long parameters. The implementation of web application firewalls and security monitoring tools can provide additional layers of defense against similar attacks. From a defensive perspective, this vulnerability highlights the importance of secure coding practices and input validation as fundamental security controls. The issue demonstrates how the absence of basic security measures in web applications can lead to severe operational consequences, particularly in mission-critical systems. Regular security assessments and code reviews should be conducted to identify and remediate similar buffer overflow vulnerabilities in legacy web applications. The vulnerability also underscores the necessity of keeping webmail servers updated with security patches, as many of these systems were not regularly maintained or updated, leaving them exposed to known vulnerabilities. Organizations should establish robust security monitoring procedures to detect potential exploitation attempts and maintain detailed logs of webmail server access patterns to identify anomalous behavior that might indicate an active attack. The broader implications of CVE-2000-0239 emphasize the critical need for comprehensive security architectures that integrate multiple defensive layers rather than relying on single points of protection.