CVE-2007-4565 in fetchmail
Summary
by MITRE
sink.c in fetchmail before 6.3.9 allows context-dependent attackers to cause a denial of service (NULL dereference and application crash) by refusing certain warning messages that are sent over SMTP.
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Analysis
by VulDB Data Team • 07/25/2019
The vulnerability described in CVE-2007-4565 affects the fetchmail email retrieval utility version 6.3.8 and earlier, representing a critical denial of service flaw that can be exploited by context-dependent attackers. This vulnerability specifically resides within the sink.c component of fetchmail's source code, which handles the processing of incoming email messages through SMTP connections. The flaw manifests when fetchmail encounters certain warning messages during the SMTP communication process and subsequently fails to properly handle these conditions, leading to a NULL pointer dereference that ultimately causes the application to crash and become unavailable to legitimate users.
The technical implementation of this vulnerability stems from inadequate error handling within the SMTP warning message processing logic. When fetchmail receives specific types of warning messages over SMTP connections, the application fails to properly validate or handle the context of these messages before attempting to dereference a NULL pointer. This condition occurs because the software does not perform proper null checks on critical data structures that should contain valid message information when processing SMTP warnings. The vulnerability is classified as context-dependent because attackers must be positioned to send specific warning messages through the SMTP channel that will trigger this particular code path, making it less exploitable than a fully remote vulnerability but still highly concerning for systems that process email from untrusted sources.
The operational impact of this vulnerability extends beyond simple service disruption, as it can be leveraged to create persistent availability issues for email servers and clients that rely on fetchmail for message retrieval. Organizations using fetchmail in production environments may experience unexpected service outages whenever malicious actors send crafted warning messages through SMTP connections, potentially disrupting legitimate email processing workflows and requiring system administrators to manually restart services. This vulnerability directly impacts the reliability and stability of email infrastructure, particularly in environments where fetchmail is used to retrieve messages from multiple sources, as a single malicious SMTP warning can bring down the entire email retrieval process.
Mitigation strategies for CVE-2007-4565 should prioritize immediate patching of fetchmail installations to version 6.3.9 or later, which contains the necessary fixes for the NULL pointer dereference issue. System administrators should also implement network-level filtering to block or sanitize SMTP warning messages that may trigger this vulnerability, particularly in environments where fetchmail processes email from untrusted sources. The vulnerability aligns with CWE-476 which describes NULL pointer dereference conditions, and could be categorized under ATT&CK technique T1499.1 for network denial of service attacks. Additionally, organizations should consider implementing monitoring and alerting mechanisms to detect unusual patterns of fetchmail crashes or restarts that may indicate exploitation attempts. The fix implemented in fetchmail 6.3.9 involved adding proper null pointer validation and error handling to ensure that warning messages are processed safely without causing application crashes, thereby addressing the root cause of the vulnerability through defensive programming practices that prevent the execution of dangerous code paths.