CVE-2002-1876 in Exchange
Summary
by MITRE
Microsoft Exchange 2000 allows remote authenticated attackers to cause a denial of service via a large number of rapid requests, which consumes all of the licenses that are granted to Exchange by IIS.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 09/01/2025
Microsoft Exchange 2000 Server contains a critical vulnerability that enables remote authenticated attackers to execute denial of service attacks through excessive request flooding. This flaw specifically targets the licensing mechanism between Exchange 2000 and Internet Information Services IIS, where the system allocates a finite number of licenses that govern concurrent connections and service availability. When an attacker sends a large volume of rapid requests to the Exchange server, these requests consume the allocated IIS licenses at an accelerated rate, effectively exhausting the available license pool and rendering the service unavailable to legitimate users.
The technical nature of this vulnerability stems from insufficient rate limiting and license management controls within the Exchange 2000 implementation. The system does not adequately monitor or throttle incoming requests from authenticated users, allowing a single compromised account or malicious actor to rapidly consume all available licenses. This behavior creates a classic resource exhaustion scenario where legitimate users cannot establish connections to the Exchange server because all available licenses have been consumed by the attacker's rapid request pattern. The vulnerability operates at the application layer and leverages the existing authentication mechanism, making it particularly dangerous as it requires only valid credentials to exploit.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise business continuity and email communication workflows. Organizations relying on Exchange 2000 for email services face significant risks including email delivery failures, user access restrictions, and potential data communication delays that can affect critical business operations. The attack can be executed with minimal resources and technical expertise, making it attractive to threat actors seeking to disrupt organizational communication infrastructure. Furthermore, the vulnerability affects the underlying IIS licensing mechanism rather than Exchange-specific functionality, which means that the impact can be more widespread than initially apparent.
Organizations should implement multiple layers of mitigation to address this vulnerability. Network-level rate limiting and connection throttling should be configured on firewalls and network devices to restrict the number of requests that can be processed within a given time window. Exchange 2000 servers should be configured with appropriate IIS license limits and connection timeouts to prevent rapid license exhaustion. Additionally, monitoring systems should be deployed to detect unusual patterns of request volume from authenticated users, enabling rapid response to potential attacks. The vulnerability aligns with attack patterns documented in the MITRE ATT&CK framework under the denial of service category, specifically targeting service availability through resource exhaustion techniques. Organizations should also consider implementing account lockout policies and monitoring for suspicious authentication patterns to prevent unauthorized users from leveraging this vulnerability. This vulnerability represents a classic example of how insufficient access controls and rate limiting can create exploitable conditions in enterprise messaging systems, highlighting the importance of comprehensive security design principles in critical infrastructure components.