CVE-2020-28012 in Exim
Summary
by MITRE • 05/06/2021
Exim 4 before 4.94.2 allows Exposure of File Descriptor to Unintended Control Sphere because rda_interpret uses a privileged pipe that lacks a close-on-exec flag.
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Analysis
by VulDB Data Team • 05/09/2021
The vulnerability identified as CVE-2020-28012 affects Exim versions prior to 4.94.2 and represents a critical security flaw in the mail transfer agent's handling of file descriptors. This issue stems from the rda_interpret function which processes remote delivery agents and creates a privileged pipe without properly setting the close-on-exec flag. The flaw creates a dangerous exposure where file descriptors intended for privileged operations can be inherited by unintended processes, potentially allowing malicious actors to gain elevated privileges through improper file descriptor inheritance.
This vulnerability falls under the CWE-242 category of "Use of Inherently Dangerous Function" and specifically relates to improper handling of file descriptor inheritance patterns. The technical implementation issue occurs when Exim creates a pipe for communication between processes during remote delivery operations. When the close-on-exec flag is not set on the pipe file descriptor, any child process spawned by the mail transfer agent inherits this privileged file descriptor. This inheritance allows unauthorized processes to potentially access or manipulate the communication channel that should remain restricted to privileged operations.
The operational impact of this vulnerability is severe as it enables privilege escalation attacks within the mail server environment. An attacker who can influence process creation or manipulation of the mail transfer agent's execution flow could potentially exploit this weakness to gain elevated privileges or access sensitive communication channels. The vulnerability particularly affects systems where Exim handles remote delivery operations and where multiple processes may be spawned during mail processing. The exposure creates a persistent security risk that could be exploited across various attack vectors including compromised user accounts that might spawn processes with inherited file descriptors.
Mitigation strategies should focus on updating to Exim version 4.94.2 or later where the close-on-exec flag is properly implemented for privileged pipes. System administrators should also implement process monitoring to detect unauthorized process creation patterns and file descriptor inheritance. The fix involves ensuring that all privileged file descriptors created during remote delivery operations have the close-on-exec flag set to prevent unintended inheritance by child processes. Additionally, organizations should review their mail server configurations and implement proper access controls to minimize the attack surface. Network segmentation and process isolation techniques can help reduce the impact if exploitation occurs, while regular security audits should verify that no other similar file descriptor handling issues exist within the mail server's codebase. This vulnerability demonstrates the critical importance of proper file descriptor management in privileged applications and aligns with ATT&CK technique T1068 which covers local privilege escalation through improper file descriptor handling.