CVE-1999-0393 in Sendmail
Summary
by MITRE
Remote attackers can cause a denial of service in Sendmail 8.8.x and 8.9.2 by sending messages with a large number of headers.
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Analysis
by VulDB Data Team • 08/19/2024
The vulnerability described in CVE-1999-0393 represents a classic denial of service attack vector targeting the sendmail mail transfer agent version 8.8.x and 8.9.2. This issue stems from the software's inadequate handling of message headers during the email processing pipeline, creating a condition where malicious actors can exploit the system's resource consumption patterns to render it unavailable to legitimate users. The vulnerability operates through a straightforward yet effective mechanism that demonstrates how seemingly benign email formatting can be weaponized to compromise system availability.
The technical flaw manifests in sendmail's header parsing routine where the software fails to properly limit or validate the number of headers that can be processed within a single email message. When an attacker crafts a message containing an excessive number of headers, the sendmail daemon begins to consume disproportionate system resources including memory and processing cycles as it attempts to parse and handle each header. This resource exhaustion occurs because the software lacks proper bounds checking and input validation mechanisms that would normally prevent such malformed input from causing system instability. The vulnerability is particularly dangerous because it can be exploited remotely without requiring authentication or special privileges, making it accessible to any attacker who can send email to the target system.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise the entire email infrastructure of affected systems. When exploited successfully, the denial of service condition can cause sendmail processes to consume all available memory or CPU resources, leading to system crashes or severe performance degradation that prevents legitimate email traffic from being processed. This type of attack can be particularly devastating in environments where email is critical for business operations, as it can effectively shut down communication channels and potentially impact other services that depend on the email system. The vulnerability affects systems running sendmail versions that are particularly common in unix-based environments during the late 1990s, making it a widespread concern for organizations with legacy email infrastructure.
Organizations should implement immediate mitigations including configuring sendmail to limit the maximum number of headers allowed in incoming messages, setting appropriate resource limits on sendmail processes, and implementing network-level filtering to block suspicious email traffic patterns. The vulnerability aligns with CWE-129, which addresses issues related to insufficient validation of header counts, and represents a classic example of how improper input validation can lead to resource exhaustion attacks. From an ATT&CK framework perspective, this vulnerability maps to the denial of service tactic and can be classified under the technique of resource exhaustion, where attackers leverage software weaknesses to consume system resources. System administrators should also consider deploying intrusion detection systems to monitor for unusual header patterns and implement proper logging to track exploitation attempts, as the vulnerability demonstrates the importance of input sanitization in preventing remote exploitation of core system services.