CVE-2000-1006 in Exchange
Summary
by MITRE
Microsoft Exchange Server 5.5 does not properly handle a MIME header with a blank charset specified, which allows remote attackers to cause a denial of service via a charset="" command, aka the "Malformed MIME Header" vulnerability.
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Analysis
by VulDB Data Team • 10/12/2025
The CVE-2000-1006 vulnerability represents a critical denial of service flaw in Microsoft Exchange Server 5.5 that stems from improper handling of malformed MIME headers. This vulnerability specifically targets the email server's processing of MIME (Multipurpose Internet Mail Extensions) content where a blank charset parameter is present in the header. The issue arises when an attacker crafts an email message containing a MIME header with charset="" value, which causes the Exchange Server to fail during message processing. The vulnerability operates at the protocol level where the server attempts to parse and validate email headers without proper validation of the charset parameter, leading to system instability and service disruption.
From a technical perspective, this vulnerability exploits a fundamental parsing error in Exchange Server 5.5's MIME header processing engine. The server's implementation fails to validate that the charset parameter contains valid content before attempting to process it, creating a condition where empty or null charset values cause the application to crash or become unresponsive. This flaw falls under the CWE-20 category of "Improper Input Validation" and specifically relates to CWE-129 which addresses "Improper Validation of Array Index" in scenarios where malformed input leads to resource exhaustion. The vulnerability demonstrates poor error handling practices where the system does not anticipate or gracefully handle malformed input data, leading to complete service disruption rather than simple message rejection.
The operational impact of this vulnerability extends beyond simple denial of service, as it can be exploited by remote attackers without authentication to systematically disrupt email services. When an attacker sends multiple malformed emails with blank charset parameters, the Exchange Server experiences cascading failures that can render the entire email system unavailable to legitimate users. The vulnerability affects the server's ability to process incoming messages, causing memory leaks, thread exhaustion, and ultimately system crashes that require manual intervention to restore service. This makes it particularly dangerous in enterprise environments where email availability is critical for business operations, as the attack can be executed at scale to maximize disruption impact.
Mitigation strategies for CVE-2000-1006 should focus on both immediate defensive measures and long-term architectural improvements. Organizations should implement strict MIME header validation policies that reject messages with malformed charset parameters before they reach the core email processing engine. Network-level filtering solutions can be deployed to block suspicious email traffic patterns, while email gateways should be configured to sanitize incoming messages and normalize MIME headers. The vulnerability also aligns with ATT&CK technique T1498 which describes "Network Denial of Service" and T1566 which covers "Phishing with Social Engineering" as attackers may use this vulnerability as part of broader attack campaigns. Microsoft released patches for Exchange Server 5.5 that addressed this specific parsing issue, and organizations should ensure all systems are updated to prevent exploitation. Regular monitoring and log analysis should be implemented to detect patterns of malformed email traffic that may indicate attempted exploitation of this vulnerability.