Source: cve@mitre.org
In Sendmail, attackers can gain root privileges via SMTP by specifying an improper "mail from" address and an invalid "rcpt to" address that would cause the mail to bounce to a program.
Sendmail, a widely used email server, is vulnerable to a critical flaw that allows attackers to gain root privileges on the affected system. By crafting a malicious email with specific 'mail from' and 'rcpt to' addresses, attackers can trigger a mail bounce that executes arbitrary code with elevated privileges, potentially leading to complete system compromise and data exfiltration.
Step 1: Malicious Email Creation: The attacker crafts an SMTP email with a specially crafted 'mail from' address and an invalid 'rcpt to' address. The 'mail from' address is designed to appear legitimate, while the invalid 'rcpt to' address ensures the email will bounce.
Step 2: Email Delivery: The attacker sends the malicious email to the vulnerable Sendmail server.
Step 3: Bounce Trigger: The Sendmail server attempts to deliver the email to the invalid 'rcpt to' address. Since the address is invalid, the delivery fails, and a bounce message is generated.
Step 4: Command Injection: The bounce message, intended to notify the sender, is processed by Sendmail. The attacker's crafted 'mail from' address, which contains a malicious command, is then executed by the system, often with root privileges, due to the context in which Sendmail operates.
Step 5: Privilege Escalation: The malicious command executes, granting the attacker root access to the system, allowing them to perform actions such as installing backdoors, stealing data, or further compromising the network.
The vulnerability stems from insufficient input validation and improper handling of bounced email messages within Sendmail. Specifically, when a mail delivery fails, Sendmail attempts to notify the sender. This notification process, if misconfigured or exploited, can be manipulated to execute arbitrary commands. The root cause lies in the lack of sanitization of the 'mail from' and 'rcpt to' addresses, allowing attackers to inject commands that are then executed by the mail server with root privileges. This is a classic example of a command injection vulnerability, where user-controlled input is directly used in a system call without proper validation.
This vulnerability is a prime example of a historical, but still dangerous, attack vector. While specific APT groups are not directly linked to this specific CVE, the techniques used are common and could be employed by any attacker. CISA KEV status is unlikely, given the age of the vulnerability, but the underlying principles of command injection are relevant to modern attacks.
Monitor SMTP traffic for unusual 'mail from' and 'rcpt to' address patterns, especially those containing shell metacharacters or command injection attempts.
Analyze Sendmail logs for failed delivery attempts and bounce messages, looking for suspicious activity or error messages.
Implement intrusion detection systems (IDS) with rules specifically designed to detect command injection attempts within SMTP traffic.
Monitor system process activity for unexpected processes spawned by the Sendmail daemon (e.g., sendmail).
Review email headers for suspicious patterns in the 'Return-Path' or 'From' fields.
Upgrade Sendmail to a patched version that addresses the vulnerability. This is the primary and most effective remediation step.
Implement input validation and sanitization for all user-supplied data, including email addresses, to prevent command injection.
Configure Sendmail to restrict the use of dangerous features or commands.
Apply the principle of least privilege, ensuring that Sendmail runs with the minimum necessary privileges.
Regularly audit and monitor Sendmail configuration files for any unauthorized changes.
Implement a Web Application Firewall (WAF) to filter malicious requests.