CVE-2020-28007 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 log directory (owned by a non-root user), a symlink or hard link attack allows overwriting critical root-owned files anywhere on the filesystem.
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Analysis
by VulDB Data Team • 05/09/2021
The vulnerability identified as CVE-2020-28007 represents a critical privilege escalation flaw in Exim email transfer agent versions prior to 4.94.2. This issue stems from improper file system permissions and directory ownership handling within the mail server's logging infrastructure. The vulnerability specifically exploits the configuration where Exim operates with root privileges in the log directory while the directory itself is owned by a non-root user account, creating an exploitable condition that allows malicious actors to manipulate critical system files through symbolic link or hard link attacks.
The technical implementation of this vulnerability involves the manipulation of file system links within the Exim log directory structure. When Exim processes log entries, it creates or updates files in the log directory without proper validation of symbolic links or hard links that might have been established by an attacker. This flaw falls under the Common Weakness Enumeration category of CWE-276, which addresses incorrect permissions for system resources, and more specifically aligns with CWE-73, which deals with external control of file name or path. The vulnerability demonstrates a classic case of insecure file handling where the system assumes that file paths refer to legitimate files without validating the underlying file system structure.
The operational impact of this vulnerability extends beyond simple privilege escalation to potentially enable complete system compromise. An attacker who can manipulate the log directory through symlink attacks can overwrite critical root-owned files anywhere on the filesystem, including system binaries, configuration files, or even the shadow password file. This capability allows for persistent backdoor establishment, privilege elevation, and potential lateral movement within the network infrastructure. The attack vector is particularly concerning because it requires minimal user interaction and can be executed through legitimate mail processing operations, making detection difficult and the attack surface broad.
From an adversarial perspective, this vulnerability aligns with techniques described in the MITRE ATT&CK framework under the privilege escalation tactics, specifically targeting the technique of "Create or Modify System Process" and "Modify System Image". The attack can be executed through legitimate mail delivery processes, making it particularly dangerous as it bypasses many traditional security monitoring mechanisms that focus on user-level activities. The vulnerability essentially allows an attacker to leverage the legitimate mail server functionality to gain unauthorized access to root privileges, which can then be used to establish persistent access, exfiltrate data, or deploy additional malicious tools.
The recommended mitigation strategy involves immediate patching of Exim installations to version 4.94.2 or later, which addresses the core permission and file handling issues. Additionally, system administrators should implement proper file system permissions and ownership controls, ensuring that log directories and their contents are properly secured. The principle of least privilege should be enforced by running Exim with minimal necessary privileges, and regular security audits should be conducted to identify and remediate similar permission-related vulnerabilities. Organizations should also consider implementing file integrity monitoring solutions to detect unauthorized modifications to critical system files, particularly those that might be targets of symlink-based attacks.