CVE-2020-7247 Overview
CVE-2020-7247 is a critical command injection vulnerability in the smtp_mailaddr function within smtp_session.c in OpenSMTPD 6.6. This vulnerability allows remote attackers to execute arbitrary commands as root via a crafted SMTP session by injecting shell metacharacters in the MAIL FROM field. The vulnerability affects the default "uncommented" configuration and exists due to an incorrect return value upon failure of input validation.
This vulnerability is particularly dangerous because it allows unauthenticated remote attackers to gain root-level access to affected mail servers through a simple SMTP session. The flaw has been actively exploited in the wild and is listed in CISA's Known Exploited Vulnerabilities catalog.
Critical Impact
Remote unauthenticated attackers can execute arbitrary commands with root privileges via crafted SMTP MAIL FROM commands, leading to complete system compromise.
Affected Products
- OpenBSD OpenSMTPD 6.6
- Debian Linux 9.0 and 10.0
- Fedora 32
- Canonical Ubuntu Linux 18.04 LTS and 19.10
Discovery Timeline
- 2020-01-29 - CVE-2020-7247 published to NVD
- 2025-11-07 - Last updated in NVD database
Technical Details for CVE-2020-7247
Vulnerability Analysis
The vulnerability resides in the smtp_mailaddr function in smtp_session.c, which is responsible for validating and processing email addresses in SMTP MAIL FROM commands. The core issue stems from improper handling of the return value when input validation fails, allowing specially crafted input containing shell metacharacters to bypass security checks.
When an attacker sends a MAIL FROM command with embedded shell metacharacters, the validation function incorrectly returns a value that allows the malicious input to proceed through the mail processing pipeline. This input eventually reaches a point where it is executed in a shell context with root privileges, as OpenSMTPD typically runs with elevated permissions to bind to privileged ports and manage mail delivery.
The vulnerability is classified under CWE-78 (OS Command Injection) and CWE-755 (Improper Handling of Exceptional Conditions), reflecting both the injection nature and the underlying validation failure.
Root Cause
The root cause is an incorrect return value in the smtp_mailaddr function when input validation fails. Instead of properly rejecting malformed or malicious input containing shell metacharacters, the function returns a value that allows processing to continue. This improper error handling creates a pathway for command injection through the MAIL FROM field, which is subsequently executed with root privileges during mail processing operations.
Attack Vector
The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:
- Establishing an SMTP connection to a vulnerable OpenSMTPD server on port 25
- Sending a crafted MAIL FROM command containing shell metacharacters
- The malicious payload bypasses input validation due to the incorrect return value
- Commands embedded in the MAIL FROM field are executed with root privileges
The following patch addresses the vulnerability by correcting the input validation logic:
-/* $OpenBSD: smtp_session.c,v 1.421 2020/01/08 00:05:38 gilles Exp $ */
+/* $OpenBSD: smtp_session.c,v 1.422 2020/01/28 21:35:00 gilles Exp $ */
/*
* Copyright (c) 2008 Gilles Chehade <gilles@poolp.org>
Source: GitHub OpenBSD Commit
Detection Methods for CVE-2020-7247
Indicators of Compromise
- Unusual SMTP MAIL FROM commands containing shell metacharacters such as ;, |, $(), or backticks in mail server logs
- Unexpected processes spawned by the smtpd service running as root
- Evidence of shell command execution in mail queue or delivery logs
- Network connections originating from the SMTP daemon to unexpected destinations
Detection Strategies
- Monitor SMTP traffic for MAIL FROM commands containing shell metacharacters or command injection patterns
- Implement intrusion detection rules to flag SMTP sessions with unusual envelope sender formats
- Review OpenSMTPD logs for rejected or malformed MAIL FROM commands that may indicate reconnaissance activity
- Deploy behavioral analysis to detect anomalous process execution by the smtpd daemon
Monitoring Recommendations
- Enable verbose logging for OpenSMTPD to capture all MAIL FROM commands received
- Configure SIEM alerts for shell metacharacter patterns in SMTP envelope data
- Monitor for privilege escalation or lateral movement following SMTP-related process activity
- Audit system logs for unexpected root-level command execution correlated with SMTP activity
How to Mitigate CVE-2020-7247
Immediate Actions Required
- Upgrade OpenSMTPD to version 6.6.2p1 or later immediately
- If immediate patching is not possible, temporarily disable or restrict access to OpenSMTPD
- Review system logs for evidence of exploitation attempts or successful compromise
- Conduct a security assessment of systems running vulnerable OpenSMTPD versions
Patch Information
OpenBSD has released a security patch to address this vulnerability. The fix corrects the input validation logic in smtp_mailaddr to properly handle malformed input and return appropriate error values. Organizations should apply the patch available from the OpenBSD Security Page or upgrade to OpenSMTPD 6.6.2p1 or later.
Distribution-specific patches are available:
Workarounds
- Restrict network access to the SMTP port (25) using firewall rules to limit exposure to trusted networks only
- Implement network-level filtering to block SMTP connections containing suspicious metacharacters
- Consider temporarily switching to an alternative MTA if patching cannot be performed immediately
- Deploy a reverse proxy or mail relay in front of OpenSMTPD to sanitize incoming SMTP commands
# Restrict SMTP access to trusted networks via iptables
iptables -A INPUT -p tcp --dport 25 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 25 -j DROP
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