CVE-2020-28010 in Exim
Summary
by MITRE • 05/06/2021
Exim 4 before 4.94.2 allows Out-of-bounds Write because the main function, while setuid root, copies the current working directory pathname into a buffer that is too small (on some common platforms).
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Analysis
by VulDB Data Team • 05/09/2021
The vulnerability identified as CVE-2020-28010 represents a critical out-of-bounds write flaw in Exim versions prior to 4.94.2 that operates with elevated privileges through setuid root execution. This vulnerability occurs within the main function of the mail transfer agent when it attempts to copy the current working directory pathname into a buffer that is insufficiently sized for the operation. The flaw specifically manifests when Exim executes with root privileges, creating a dangerous condition where malicious input could trigger memory corruption. The issue stems from the inadequate buffer size allocation when handling directory pathnames, particularly on common platforms where the buffer capacity cannot accommodate the full path length that may be encountered during normal operation.
The technical exploitation of this vulnerability involves an attacker manipulating the current working directory to exceed the allocated buffer space, resulting in a memory overwrite that can potentially be leveraged for privilege escalation. When Exim runs with setuid root permissions, any buffer overflow in this context can provide attackers with elevated system access. The vulnerability is classified under CWE-121 as a stack-based buffer overflow, which directly maps to the attack pattern described in the ATT&CK framework under T1068 for exploit for privilege escalation. The specific nature of this flaw makes it particularly dangerous because it operates within a privileged execution context, allowing for potential system compromise when exploited by unauthorized users.
The operational impact of CVE-2020-28010 extends beyond simple memory corruption, as it provides a pathway for attackers to gain root privileges on affected systems. Systems running vulnerable Exim versions are at risk of complete compromise when the attack vector is successfully executed, as the buffer overflow can be manipulated to overwrite critical memory locations including return addresses or function pointers. This vulnerability particularly affects mail servers and systems that rely on Exim for email processing, where attackers could exploit the flaw to execute arbitrary code with root privileges. The implications are severe for organizations that depend on Exim for their email infrastructure, as this vulnerability could allow attackers to establish persistent access, install backdoors, or exfiltrate sensitive data from the compromised system.
Mitigation strategies for CVE-2020-28010 primarily involve immediate patching of Exim installations to version 4.94.2 or later, which contains the necessary buffer size corrections and privilege management improvements. System administrators should also implement additional security controls including regular vulnerability scanning, monitoring for unusual directory access patterns, and ensuring proper privilege separation in mail processing environments. The fix addresses the root cause by increasing the buffer size to accommodate the maximum possible pathname length on target platforms, preventing the overflow condition that previously allowed for memory corruption. Organizations should also consider implementing network segmentation, access controls, and intrusion detection systems to minimize the attack surface and reduce the likelihood of exploitation in environments where patching may not be immediately possible.