CVE-2020-28014 in Exim
Summary
by MITRE • 05/06/2021
Exim 4 before 4.94.2 allows Execution with Unnecessary Privileges. The -oP option is available to the exim user, and allows a denial of service because root-owned files can be overwritten.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 05/09/2021
The vulnerability identified as CVE-2020-28014 affects Exim versions prior to 4.94.2 and represents a critical privilege escalation flaw that enables attackers to execute commands with unnecessary privileges. This vulnerability specifically exploits the -oP command-line option which is accessible to the exim user, creating a pathway for unauthorized file manipulation that can lead to system compromise. The flaw stems from insufficient access controls and privilege management within the mail transfer agent implementation, allowing a malicious actor to leverage the exim user account to overwrite root-owned files.
The technical nature of this vulnerability involves the improper handling of file permissions and ownership during mail processing operations. When the -oP option is utilized, it allows the exim user to specify file paths that can result in overwriting critical system files owned by root. This represents a classic privilege escalation vector where a lower-privileged user can manipulate system resources that should remain protected. The vulnerability is particularly dangerous because it bypasses normal permission checks and allows arbitrary file overwrites, potentially enabling attackers to replace system binaries or configuration files with malicious counterparts.
From an operational impact perspective, this vulnerability can lead to complete system compromise and persistent access for attackers. The ability to overwrite root-owned files means that malicious actors can modify critical system components such as binaries, configuration files, or even create backdoor access points. The denial of service aspect of this vulnerability further compounds the risk as it can be used to disrupt services while simultaneously establishing persistent access. Organizations running affected Exim versions face significant risk of unauthorized system access, data compromise, and potential lateral movement within their networks.
The vulnerability aligns with CWE-276, which describes improper file permissions, and can be mapped to ATT&CK technique T1068, which covers "Exploitation for Privilege Escalation." The flaw demonstrates poor privilege separation and inadequate input validation within the mail processing pipeline. Security professionals should note that this vulnerability can be exploited without requiring additional authentication or complex attack chains, making it particularly dangerous in environments where exim is installed with default configurations. The impact extends beyond simple privilege escalation to include potential data exfiltration, system integrity compromise, and service disruption.
Organizations should immediately upgrade to Exim version 4.94.2 or later to remediate this vulnerability, as no effective workarounds exist for the underlying privilege management flaw. System administrators should also conduct comprehensive audits of exim configurations and monitor for suspicious file modifications in critical system directories. Additional mitigations include implementing strict file permission controls, monitoring for unauthorized file access patterns, and ensuring that exim is configured with minimal required privileges. Network segmentation and access controls should be reviewed to limit potential exploitation vectors, while security monitoring systems should be configured to detect unusual file modification activities that could indicate exploitation attempts.