CVE-2020-7247 in OpenSMTPD
Summary
by MITRE
smtp_mailaddr in smtp_session.c in OpenSMTPD 6.6, as used in OpenBSD 6.6 and other products, allows remote attackers to execute arbitrary commands as root via a crafted SMTP session, as demonstrated by shell metacharacters in a MAIL FROM field. This affects the "uncommented" default configuration. The issue exists because of an incorrect return value upon failure of input validation.
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Analysis
by VulDB Data Team • 02/05/2025
The vulnerability identified as CVE-2020-7247 represents a critical command injection flaw in OpenSMTPD version 6.6, specifically within the smtp_mailaddr function located in smtp_session.c. This vulnerability arises from improper input validation during SMTP session processing, allowing remote attackers to execute arbitrary commands with root privileges. The flaw is particularly dangerous because it affects the default configuration of OpenBSD 6.6 and other systems running this version of OpenSMTPD, making it exploitable in standard deployment scenarios without requiring additional configuration changes. The vulnerability stems from a fundamental error in the code's logic where the function fails to properly handle validation failures, resulting in a misinterpretation of input data that ultimately leads to command execution. This type of vulnerability falls under CWE-78, which specifically addresses OS Command Injection, and represents a direct violation of secure coding practices for input sanitization and validation.
The technical exploitation of this vulnerability occurs through manipulation of the MAIL FROM field in SMTP sessions, where attackers can inject shell metacharacters that are then processed by the vulnerable smtp_mailaddr function. When the SMTP server processes these malicious inputs, the incorrect return value handling causes the system to interpret and execute the injected commands as if they were legitimate administrative instructions. This allows an attacker to gain full root access to the system, enabling them to perform any action permitted by the root account including privilege escalation, data exfiltration, and system compromise. The vulnerability is particularly insidious because it operates at the protocol level within the mail server itself, making it difficult to detect through traditional network monitoring approaches and allowing attackers to remain persistent within the system while maintaining administrative control.
The operational impact of CVE-2020-7247 extends far beyond simple command execution, as it fundamentally compromises the integrity and confidentiality of mail servers running vulnerable versions of OpenSMTPD. Organizations using default configurations are immediately at risk, as the vulnerability does not require any specialized knowledge of the system beyond standard SMTP protocol usage. The attack surface is broad since any system accepting SMTP connections and running the vulnerable version becomes a potential target. This vulnerability directly aligns with ATT&CK technique T1059.001, which covers command and scripting interpreter execution, and represents a classic privilege escalation vector that can be leveraged for further lateral movement within networks. The implications are severe for email infrastructure security, as compromised mail servers can be used for spam distribution, data theft, or as launch points for attacks on other systems within the network perimeter.
Mitigation strategies for CVE-2020-7247 primarily focus on immediate patching of affected systems, as the vulnerability is fully remediated through software updates. Organizations should prioritize upgrading to OpenSMTPD versions that have addressed this specific issue, typically those released after the vulnerability disclosure. System administrators should also implement network-level restrictions to limit SMTP access to trusted sources only, and consider deploying intrusion detection systems that can monitor for suspicious SMTP patterns. The vulnerability highlights the importance of proper input validation and error handling in security-critical code, emphasizing the need for comprehensive testing of all input processing functions. Additionally, organizations should conduct thorough vulnerability assessments to identify any other systems running vulnerable versions of OpenSMTPD or similar mail server software that may be susceptible to similar issues. Regular security audits and adherence to secure coding practices, particularly around input sanitization and proper error handling, are essential to prevent similar vulnerabilities from emerging in the future.