CVE-1999-0565 in Sendmail
Summary
by MITRE
A Sendmail alias allows input to be piped to a program.
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Analysis
by VulDB Data Team • 04/18/2026
The vulnerability described in CVE-1999-0565 represents a critical security flaw in the Sendmail mail transfer agent that enables malicious actors to execute arbitrary commands through email alias configurations. This issue stems from the improper handling of input within Sendmail's alias processing mechanism, where user-supplied data can be directly piped to shell commands without adequate sanitization or validation. The vulnerability exists at the core of Sendmail's alias expansion functionality, which allows administrators to define email aliases that redirect messages to external programs or scripts. When an attacker crafts a malicious alias entry or exploits an existing alias with unsafe input handling, the system processes the input and executes commands in the context of the Sendmail process, potentially leading to complete system compromise.
The technical implementation of this vulnerability aligns with CWE-78, which describes improper neutralization of special elements used in OS commands, and CWE-22, addressing improper limitation of a pathname to a restricted directory. The flaw manifests when Sendmail processes alias definitions that contain shell metacharacters or command injection sequences, allowing attackers to append arbitrary commands that get executed by the underlying shell. This represents a classic command injection vulnerability where the application fails to properly escape or filter user input before incorporating it into system commands. The vulnerability is particularly dangerous because Sendmail typically runs with elevated privileges, often as root or a system user with significant access rights, amplifying the potential impact of successful exploitation.
The operational impact of this vulnerability extends far beyond simple email redirection, as it provides attackers with a potential pathway to gain unauthorized access to the underlying system. An attacker who can influence Sendmail alias configurations or find a way to inject malicious input into existing aliases can execute commands with the privileges of the Sendmail process, potentially leading to privilege escalation, data exfiltration, or system compromise. The vulnerability affects systems where Sendmail is used for email routing and alias management, which includes virtually all Unix-like systems that rely on Sendmail for mail handling. This makes it particularly widespread and dangerous in enterprise environments where Sendmail has been the default mail transfer agent for decades.
Mitigation strategies for CVE-1999-0565 require immediate attention and should include comprehensive input validation and sanitization of all alias configurations, ensuring that special shell characters are properly escaped or filtered before processing. System administrators should implement strict access controls on Sendmail configuration files and alias databases, limiting write permissions to trusted users only. The recommended approach involves upgrading to newer versions of Sendmail that have addressed this vulnerability through improved input handling and command execution mechanisms. Additionally, organizations should consider implementing proper privilege separation, running Sendmail with reduced privileges, and employing input validation frameworks that prevent command injection attacks. Security monitoring should include detection of unusual alias modifications and suspicious command execution patterns, as outlined in the ATT&CK framework under T1059 for command and script injection techniques. Regular security audits of alias configurations and comprehensive testing of input handling mechanisms remain essential defensive measures against this and similar vulnerabilities.