CVE-2020-28026 in Exim
Summary
by MITRE • 05/06/2021
Exim 4 before 4.94.2 has Improper Neutralization of Line Delimiters, relevant in non-default configurations that enable Delivery Status Notification (DSN). Certain uses of ORCPT= can place a newline into a spool header file, and indirectly allow unauthenticated remote attackers to execute arbitrary commands as root.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 05/09/2021
The vulnerability CVE-2020-28026 represents a critical command execution flaw in Exim email transfer agent versions prior to 4.94.2. This vulnerability specifically affects non-default configurations where Delivery Status Notification (DSN) functionality is enabled, creating a dangerous condition where attacker-controlled input can be injected into spool header files through improper handling of line delimiters. The flaw stems from insufficient validation of the ORCPT= parameter, which allows malicious actors to inject newline characters into critical system files. This improper neutralization of line delimiters creates a path for remote code execution with root privileges, making it particularly severe in environments where Exim runs with elevated permissions.
The technical exploitation of this vulnerability occurs through the manipulation of the ORCPT= parameter within email delivery status notifications, which are used to track delivery failures and provide status updates to senders. When DSN is enabled and the ORCPT= parameter contains unescaped newline characters, these delimiters can be written into spool header files that Exim uses to manage email delivery processes. The spool files are processed by the system with root privileges, allowing an unauthenticated attacker to inject malicious commands that will execute with the highest system privileges. This vulnerability operates at the intersection of improper input validation and privilege escalation, creating a direct path from network access to root compromise.
The operational impact of CVE-2020-28026 extends beyond simple command execution to represent a complete system compromise capability for any network-accessible Exim installation running with DSN enabled. Organizations using Exim as their primary mail transfer agent face potential complete system takeover, data exfiltration, and persistence mechanisms through this vulnerability. The fact that it requires non-default configuration makes it less likely to be immediately discovered but also means that many organizations may be unknowingly exposed. The vulnerability aligns with CWE-74 and CWE-707, which cover improper neutralization of special elements and improper control of generation of code, respectively. This flaw also maps to ATT&CK techniques including T1059 for command and scripting interpreter and T1548 for abuse of privileges, demonstrating how this vulnerability can be leveraged for both execution and privilege escalation within compromised systems.
Mitigation strategies for CVE-2020-28026 require immediate patching of Exim installations to version 4.94.2 or later, which includes proper neutralization of line delimiters in DSN handling. Organizations should also review their Exim configurations to disable DSN functionality if it is not required for their operations, as this removes the attack surface entirely. Network segmentation and access controls should be implemented to limit exposure of Exim servers to untrusted networks, while monitoring should be enhanced to detect unusual email delivery patterns that might indicate exploitation attempts. System administrators should also consider implementing additional security controls such as mandatory access controls and privilege separation to limit the impact if exploitation occurs. The vulnerability demonstrates the critical importance of proper input validation in security-critical software components and highlights how seemingly minor flaws in parameter handling can lead to complete system compromise.