CVE-2000-0216 in Exchange
Summary
by MITRE
Microsoft email clients in Outlook, Exchange, and Windows Messaging automatically respond to Read Receipt and Delivery Receipt tags, which could allow an attacker to flood a mail system with responses by forging a Read Receipt request that is redirected to a large distribution list.
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Analysis
by VulDB Data Team • 04/21/2026
This vulnerability exists in Microsoft email clients including Outlook, Exchange, and Windows Messaging where the software automatically processes and responds to read receipt and delivery receipt requests without proper validation. The flaw stems from the automatic handling of these specific email headers that indicate when a message has been read or delivered, creating an inherent security weakness in the email processing pipeline. When an attacker crafts a malicious email with forged read receipt requests, the vulnerable clients automatically generate and send back confirmation responses to the specified return addresses, effectively enabling a form of automated response flooding.
The technical implementation of this vulnerability operates through the standard email protocol handling mechanisms within Microsoft's messaging clients. When an email client encounters a message containing read receipt or delivery receipt headers, it automatically generates and dispatches a response message back to the sender or specified return address. This automatic behavior occurs regardless of the legitimacy of the request or the source of the original message, creating a vector for abuse. The vulnerability is particularly dangerous because it leverages the normal functioning of email clients to create an automated response system that can be easily exploited for denial of service attacks.
The operational impact of CVE-2000-0216 extends beyond simple spam flooding to potentially overwhelm network resources and disrupt legitimate email services. Attackers can construct malicious emails with forged read receipt requests that target large distribution lists or specific users within an organization, causing cascading responses that flood mail servers and consume significant bandwidth and processing resources. This creates a substantial burden on email infrastructure and can lead to service degradation or complete disruption of email communications within affected organizations, particularly those with large user bases or limited network capacity.
This vulnerability aligns with CWE-829, which addresses the inclusion of a security-relevant operation in a system that does not properly validate user inputs or requests. The automatic processing of email headers without proper authentication or validation represents a classic example of insecure input handling in communication protocols. From an ATT&CK framework perspective, this vulnerability maps to techniques involving denial of service through resource exhaustion and abuse of legitimate system functionality for malicious purposes. The attack pattern demonstrates how legitimate email features can be exploited to create automated response loops that overwhelm target systems.
Organizations should implement immediate mitigations including configuring email clients to disable automatic read receipt and delivery receipt processing, implementing email filtering rules to identify and block messages with suspicious receipt headers, and establishing network-level controls to limit the volume of automated responses. Security administrators should also consider implementing rate limiting on email server responses and configuring email clients to require explicit user confirmation before sending any receipt notifications. The vulnerability highlights the importance of proper input validation and the need for security-conscious design of email client functionality that does not automatically process potentially malicious requests without verification.