CVE-2008-5006 in IMAP Toolkit
Summary
by MITRE
smtp.c in the c-client library in University of Washington IMAP Toolkit 2007b allows remote SMTP servers to cause a denial of service (NULL pointer dereference and application crash) by responding to the QUIT command with a close of the TCP connection instead of the expected 221 response code.
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Analysis
by VulDB Data Team • 05/29/2025
The vulnerability described in CVE-2008-5006 represents a critical denial of service weakness within the University of Washington IMAP Toolkit's c-client library version 2007b. This flaw specifically manifests in the smtp.c component which handles SMTP protocol interactions, creating a scenario where remote malicious or compromised SMTP servers can deliberately trigger application instability. The issue stems from the library's inadequate handling of unexpected network conditions during the SMTP session termination process, particularly when the QUIT command is issued by the client.
The technical mechanism of this vulnerability operates through a NULL pointer dereference condition that occurs when the c-client library anticipates a standard 221 response code from the SMTP server upon receiving a QUIT command but instead receives an immediate TCP connection closure. This behavior violates the expected SMTP protocol sequence where servers should respond with a proper 221 code indicating successful session termination before closing the connection. When the library attempts to process what it believes to be a normal response but encounters an abrupt connection close, it triggers a NULL pointer dereference that results in an application crash and subsequent denial of service condition.
From an operational impact perspective, this vulnerability creates significant security concerns for systems relying on the IMAP Toolkit for email processing and communication. The denial of service condition affects not only the immediate application but can potentially disrupt email services, authentication mechanisms, and other systems that depend on proper SMTP communication. The vulnerability is particularly dangerous because it requires no authentication or special privileges from the attacking server, making it an attractive target for attackers seeking to disrupt email services or create service availability issues.
The flaw aligns with CWE-476 which describes NULL pointer dereference conditions in software implementations, representing a fundamental programming error that leads to application instability. From an ATT&CK framework perspective, this vulnerability maps to the T1499.004 technique related to network denial of service, where adversaries exploit weaknesses in network protocols or applications to disrupt service availability. The vulnerability also demonstrates characteristics of T1595.001 which involves reconnaissance to identify potential attack vectors through understanding application behavior and protocol implementations.
Mitigation strategies for this vulnerability should focus on immediate library updates to patched versions of the University of Washington IMAP Toolkit where the NULL pointer dereference has been addressed through proper error handling mechanisms. System administrators should implement network monitoring to detect unusual connection patterns and implement connection timeout configurations to prevent prolonged exposure. Additionally, network segmentation and firewall rules can be configured to limit SMTP communication to trusted sources only, reducing the attack surface. The most effective long-term solution involves upgrading to newer versions of the IMAP Toolkit that have properly implemented protocol compliance and robust error handling for unexpected network conditions, ensuring that all SMTP communication sequences are properly validated before proceeding with application operations.