CVE-2014-2972 in Exim
Summary
by MITRE
expand.c in Exim before 4.83 expands mathematical comparisons twice, which allows local users to gain privileges and execute arbitrary commands via a crafted lookup value.
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Analysis
by VulDB Data Team • 02/09/2022
The vulnerability identified as CVE-2014-2972 represents a critical privilege escalation flaw within the Exim mail transfer agent software. This issue affects Exim versions prior to 4.83 and stems from a fundamental flaw in how the software processes mathematical comparisons during lookup value expansion. The vulnerability operates through a double expansion mechanism that creates opportunities for malicious input manipulation, allowing local attackers to execute arbitrary commands with elevated privileges. The flaw specifically manifests in the expand.c component of Exim's codebase, where lookup values undergo inappropriate processing that can be exploited to bypass security controls.
The technical implementation of this vulnerability involves the improper handling of mathematical expressions within Exim's lookup expansion functionality. When the software processes lookup values containing mathematical comparisons, it performs expansion operations twice, creating a scenario where attacker-controlled input can be interpreted multiple times. This double expansion creates a path for command injection where malicious operators and operands can be embedded within lookup values. The vulnerability specifically targets the lookup expansion mechanism that is commonly used for configuring access controls, routing rules, and other security-sensitive features within the mail server. Attackers can craft lookup values that contain mathematical expressions which, when expanded twice, result in unintended command execution. This behavior aligns with CWE-159, which describes improper handling of multiple expansions or transformations of data, and represents a classic example of a command injection vulnerability that can be leveraged for privilege escalation.
The operational impact of CVE-2014-2972 extends beyond simple command execution to encompass full system compromise when local users can manipulate lookup configurations. Since Exim typically runs with elevated privileges to handle mail processing tasks, successful exploitation allows attackers to execute arbitrary code with the privileges of the Exim process. This can lead to complete system takeover, data exfiltration, and persistence mechanisms being established. The vulnerability affects systems where Exim is configured with lookup tables that accept user input or where lookup values can be modified by local users. The attack vector requires local system access but does not need network connectivity, making it particularly dangerous in environments where local privilege escalation is a concern. This vulnerability maps to ATT&CK technique T1059.001 for command and scripting interpreter, specifically shell scripting, and T1068 for exploit for privilege escalation, demonstrating how this flaw can be leveraged to achieve higher system privileges.
Mitigation strategies for CVE-2014-2972 primarily focus on upgrading to Exim version 4.83 or later, which contains the necessary patches to prevent the double expansion behavior. System administrators should also implement strict access controls on Exim configuration files and lookup table files to prevent unauthorized modification of lookup values. Additional protective measures include disabling unnecessary lookup functionality, implementing proper input validation for all lookup values, and monitoring for suspicious expansion patterns in system logs. Organizations should also consider implementing principle of least privilege for Exim processes and regularly auditing lookup configurations to ensure they do not contain potentially dangerous expressions. The fix implemented in Exim 4.83 addresses the root cause by ensuring that mathematical comparisons are processed only once during lookup expansion, eliminating the opportunity for double evaluation that enabled the exploit. Security monitoring should include detection of unusual command execution patterns that might indicate exploitation attempts, particularly around lookup value processing and mathematical expression evaluation within mail server configurations.