CVE-2020-28015 in Exim
Summary
by MITRE • 05/06/2021
Exim 4 before 4.94.2 has Improper Neutralization of Line Delimiters. Local users can alter the behavior of root processes because a recipient address can have a newline character.
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Analysis
by VulDB Data Team • 05/09/2021
The vulnerability identified as CVE-2020-28015 affects Exim versions prior to 4.94.2 and represents a critical security flaw in the email transfer agent's handling of recipient addresses. This issue stems from improper neutralization of line delimiters within the email processing pipeline, creating a potential privilege escalation vector that could allow local attackers to manipulate the behavior of root processes executing email operations. The vulnerability specifically occurs when Exim processes recipient addresses that contain newline characters, which should be properly sanitized but are instead interpreted as command terminators.
The technical flaw manifests in Exim's email address parsing mechanism where the system fails to adequately validate and sanitize recipient addresses before processing them in system commands. When a local user crafts an email message with a recipient address containing newline characters, these characters can be interpreted by the underlying system commands as delimiters between separate commands, enabling command injection attacks. This improper neutralization of line delimiters directly maps to CWE-174, which describes the weakness of insufficient sanitization of control characters in input data. The vulnerability operates at the intersection of input validation and command execution, where untrusted data flows directly into system command invocations without proper sanitization.
The operational impact of this vulnerability is severe as it allows local users to potentially escalate privileges and execute arbitrary commands with the privileges of the Exim process, which typically runs with elevated permissions. When Exim processes emails with maliciously crafted recipient addresses containing newline characters, it can cause the system to interpret these characters as command separators, potentially enabling attackers to inject additional commands that get executed with root privileges. This creates a significant risk for systems where Exim is used to process emails with untrusted content, particularly in multi-user environments where local attackers might attempt to exploit this vulnerability to gain unauthorized access to system resources. The vulnerability is particularly concerning in environments where Exim handles emails from external sources or where email processing occurs with elevated privileges.
Mitigation strategies for CVE-2020-28015 involve immediate patching of Exim installations to version 4.94.2 or later, which contains the necessary fixes for proper line delimiter neutralization. Organizations should also implement additional security measures including input validation for all email addresses processed by Exim, particularly focusing on sanitizing control characters and whitespace sequences. Network segmentation and access controls should be enforced to limit local user access to Exim processes, while monitoring systems should be configured to detect unusual email processing patterns that might indicate exploitation attempts. The fix implemented in Exim 4.94.2 addresses this vulnerability by ensuring that newline characters and other control characters are properly escaped or removed from recipient addresses before they are processed in system commands, aligning with the principle of least privilege and input sanitization. This vulnerability also relates to ATT&CK technique T1059.001 for Command and Scripting Interpreter, specifically focusing on the execution of system commands through improper input handling.