Source: cve@mitre.org
Sendmail decode alias can be used to overwrite sensitive files.
Sendmail's decode alias, a legacy feature, allows attackers to overwrite arbitrary files on a vulnerable server, leading to complete system compromise. This vulnerability, dating back to the late 1990s, enables attackers to gain remote code execution and control of the affected system. Successful exploitation can result in data theft, service disruption, and the deployment of persistent malware.
Step 1: Craft Malicious Email: The attacker constructs a specially crafted email message. This email includes a decode alias directive in the recipient address or in the email body.
Step 2: Alias Processing: Sendmail processes the email and identifies the decode alias. This alias is designed to decode a base64 encoded attachment.
Step 3: File Specification: The attacker includes instructions within the email to specify the target file path and the base64 encoded content to write to that file. This is often achieved by manipulating the email headers or body.
Step 4: Decoding and Overwrite: Sendmail's decode alias decodes the base64 encoded content and attempts to write it to the specified file path. Due to the lack of proper input validation, Sendmail does not verify the destination file or its permissions.
Step 5: Privilege Escalation: The attacker can overwrite critical system files, such as /etc/passwd or /etc/shadow, with malicious content. This allows the attacker to create a new user account with elevated privileges or gain access to existing privileged accounts.
The vulnerability stems from a design flaw in Sendmail's handling of the decode alias. Specifically, the decode alias, when used in conjunction with certain crafted email messages, allows an attacker to specify the destination file and its contents. This bypasses security checks and enables the attacker to overwrite critical system files, such as /etc/passwd or /etc/shadow, leading to privilege escalation. The root cause is a lack of proper input validation and sanitization of user-controlled data within the decode alias processing logic. The absence of robust access controls further exacerbates the issue. This is not a buffer overflow vulnerability in the traditional sense, but rather a file overwrite vulnerability due to insufficient input validation.
While no specific APTs are exclusively known to leverage this vulnerability, it's a common attack vector used by various threat actors due to its simplicity and effectiveness. This vulnerability is a prime candidate for inclusion in botnets and other automated attack tools. CISA KEV status is highly probable if the vulnerable software is still in use.
Monitor Sendmail logs (e.g., /var/log/mail.log) for suspicious activity, such as unusual decode alias usage or attempts to write to sensitive files.
Analyze email headers for unusual patterns or directives related to the decode alias, especially those specifying file paths.
Implement file integrity monitoring (FIM) to detect unauthorized modifications to critical system files like /etc/passwd, /etc/shadow, and Sendmail configuration files.
Network Intrusion Detection Systems (NIDS) can be configured to detect malicious email traffic containing patterns associated with the exploit, such as specific email headers or body content.
Upgrade Sendmail: The most effective remediation is to upgrade to a patched version of Sendmail that addresses the vulnerability. This is the primary and most important step.
Disable the decode alias: If upgrading is not immediately feasible, disable the decode alias in Sendmail's configuration. This will prevent exploitation.
Implement Strong Access Controls: Restrict access to Sendmail configuration files and directories to prevent unauthorized modifications.
Harden Sendmail Configuration: Review and harden the Sendmail configuration to minimize attack surface. This includes disabling unnecessary features and services.
Regular Security Audits: Conduct regular security audits and penetration testing to identify and address vulnerabilities.