CVE-2020-28008 in Exim
Summary
by MITRE • 05/06/2021
Exim 4 before 4.94.2 allows Execution with Unnecessary Privileges. Because Exim operates as root in the spool directory (owned by a non-root user), an attacker can write to a /var/spool/exim4/input spool header file, in which a crafted recipient address can indirectly lead to command execution.
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Analysis
by VulDB Data Team • 05/09/2021
The vulnerability identified as CVE-2020-28008 represents a critical privilege escalation flaw in the Exim email transfer agent version 4.94.2 and earlier. This vulnerability stems from the improper handling of spool directory permissions and recipient address processing within the email delivery mechanism. The flaw allows attackers to exploit the way Exim manages file permissions and command execution contexts when processing incoming email messages. The spool directory structure in Exim traditionally operates with root privileges to ensure proper mail queue management, yet the system fails to adequately validate or sanitize recipient addresses that could trigger unintended command execution sequences.
The technical implementation of this vulnerability occurs through the manipulation of email header files within the /var/spool/exim4/input directory. When Exim processes incoming mail, it reads recipient addresses from spool header files and performs validation checks that can be bypassed through crafted malicious input. The vulnerability specifically targets the way recipient addresses are interpreted during message delivery, allowing an attacker to inject commands that execute with elevated privileges. This occurs because the spool directory maintains root ownership while the email processing logic does not properly isolate or sanitize user-supplied recipient information before executing any associated commands. The flaw is classified under CWE-276, which deals with improper privilege management, and represents a classic case of insecure command execution where user input directly influences system command invocation.
The operational impact of this vulnerability is severe as it provides attackers with a path to execute arbitrary commands with root privileges on systems running vulnerable versions of Exim. This privilege escalation capability enables attackers to gain full control over the affected system, potentially leading to complete compromise of email services, data exfiltration, or use as a foothold for further lateral movement within network environments. The vulnerability affects systems where Exim operates with root privileges in the spool directory context, making it particularly dangerous for email servers that handle large volumes of incoming mail. Attackers can exploit this by simply sending specially crafted emails with malicious recipient addresses that trigger the command execution path within Exim's processing pipeline.
Mitigation strategies for CVE-2020-28008 primarily involve upgrading to Exim version 4.94.2 or later, which includes patches that properly validate recipient addresses and prevent command injection through spool header files. System administrators should also implement additional security measures such as restricting write access to the spool directory, implementing proper input sanitization for email headers, and monitoring for suspicious mail processing activities. Network-level protections can include filtering incoming emails based on known malicious patterns and implementing email security appliances that can detect and block crafted recipient addresses. The vulnerability aligns with ATT&CK technique T1059.001 for command and script injection, and T1548.001 for abuse of privileges, making it a significant concern for organizations following the MITRE ATT&CK framework for threat analysis and defense planning. Organizations should also conduct regular security assessments to identify any potential exposure through misconfigured Exim installations or other email systems that might be vulnerable to similar privilege escalation techniques.