CVE-2020-28013 in Exim
Summary
by MITRE • 05/06/2021
Exim 4 before 4.94.2 allows Heap-based Buffer Overflow because it mishandles "-F '.('" on the command line, and thus may allow privilege escalation from any user to root. This occurs because of the interpretation of negative sizes in strncpy.
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Analysis
by VulDB Data Team • 05/09/2021
The vulnerability identified as CVE-2020-28013 represents a critical heap-based buffer overflow in Exim email transfer agent versions prior to 4.94.2. This flaw specifically manifests when the software processes command line arguments containing the string "-F '.('" which creates a dangerous condition in memory allocation and string handling operations. The vulnerability operates at the intersection of improper input validation and unsafe memory manipulation practices, creating a path for privilege escalation attacks that can elevate any user account to root privileges. The issue stems from how Exim handles negative size parameters within strncpy function calls, where the software fails to properly validate or sanitize input values before processing them in memory operations.
The technical mechanism behind this vulnerability involves the exploitation of a heap-based buffer overflow through improper handling of command line arguments. When Exim encounters the specific pattern "-F '.('" during execution, it interprets the negative size parameter incorrectly within the strncpy function implementation. This misinterpretation causes the software to allocate insufficient memory for string operations, leading to memory corruption that can be leveraged by attackers to overwrite adjacent memory locations. The vulnerability directly maps to CWE-121, which describes heap-based buffer overflow conditions where insufficient space is allocated for data structures, and also aligns with CWE-787, which covers out-of-bounds write operations in heap memory. The improper handling of negative sizes in memory copy functions creates a condition where attacker-controlled data can overwrite critical program memory regions, potentially including return addresses or function pointers.
The operational impact of this vulnerability is severe and far-reaching, as it enables arbitrary code execution with root privileges from any user account on a system running vulnerable Exim versions. This privilege escalation capability means that an attacker with minimal access to a system can potentially gain complete control over the server, including access to all user data, system files, and network resources. The vulnerability affects systems where Exim is installed and running as a service, making it particularly dangerous for mail servers, web hosting environments, and any infrastructure relying on Exim for email processing. The attack vector is relatively simple, requiring only that an attacker can influence command line arguments passed to Exim, which is often possible through various attack surfaces including web interfaces, automated scripts, or compromised user accounts. According to ATT&CK framework, this vulnerability maps to T1068, which covers privilege escalation through local exploits, and T1059, which covers execution through command and scripting interpreters.
Mitigation strategies for CVE-2020-28013 focus primarily on immediate patching and system hardening measures. Organizations must immediately upgrade to Exim version 4.94.2 or later, as this release contains the necessary fixes for the buffer overflow condition. Additionally, administrators should implement strict input validation for all command line arguments and consider implementing privilege separation mechanisms to limit the impact of potential exploitation attempts. The vulnerability highlights the importance of proper bounds checking in memory operations and demonstrates why input sanitization should be implemented at multiple layers of software architecture. System administrators should also monitor for unusual command line patterns and implement security monitoring solutions that can detect potential exploitation attempts. The fix in version 4.94.2 addresses the core issue by properly validating size parameters in memory copy operations and ensuring that negative values are handled appropriately, preventing the heap corruption that enables privilege escalation. Organizations should also consider implementing network segmentation and access controls to limit exposure to this vulnerability, particularly in environments where Exim is accessible through web interfaces or automated processes.