CVE-2020-28011 in Exim
Summary
by MITRE • 05/06/2021
Exim 4 before 4.94.2 allows Heap-based Buffer Overflow in queue_run via two sender options: -R and -S. This may cause privilege escalation from exim to root.
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Analysis
by VulDB Data Team • 05/09/2021
The vulnerability CVE-2020-28011 represents a critical heap-based buffer overflow in the Exim mail transfer agent version 4.94.2 and earlier. This flaw exists within the queue_run functionality and specifically manifests when processing two sender options: -R and -S. The vulnerability stems from inadequate input validation and memory management within the command line argument parsing mechanism of the exim binary, creating a condition where maliciously crafted input can overwrite adjacent memory regions on the heap. The flaw is particularly concerning because it occurs during the execution of queue processing, which typically runs with elevated privileges. When the exim daemon processes mail queue items, it invokes the queue_run function with these specific sender options, creating an opportunity for attackers to exploit the buffer overflow condition.
The technical implementation of this vulnerability involves the manipulation of command line arguments passed to the exim binary when executing queue processing operations. The -R and -S options are processed in a manner that does not properly validate the length of input data before copying it into fixed-size heap buffers. This classic buffer overflow condition allows an attacker to overwrite adjacent memory locations, potentially including return addresses, function pointers, or other critical control data structures. The heap-based nature of the vulnerability means that the overflow occurs in dynamically allocated memory regions rather than on the stack, making exploitation more complex but still achievable through careful crafting of input parameters. According to CWE-121, this represents a classic heap-based buffer overflow vulnerability that can lead to arbitrary code execution.
The operational impact of CVE-2020-28011 extends beyond simple denial of service to potentially enable privilege escalation from the exim user to root. When the vulnerable queue_run function executes with elevated privileges, an attacker who can influence the processing of mail queue items can leverage this vulnerability to execute arbitrary code with root privileges. This privilege escalation occurs because the exim daemon typically runs with root privileges during queue processing to ensure proper file system access and mail delivery operations. The vulnerability affects systems where exim is configured to process mail queues automatically, which is standard in most email server configurations. Attackers can exploit this by crafting specially formatted email messages or queue entries that contain malicious data in the -R and -S parameters, potentially leading to complete system compromise.
Mitigation strategies for CVE-2020-28011 primarily focus on immediate patching of the exim software to version 4.94.2 or later, which contains the necessary fixes for the buffer overflow conditions. Organizations should also implement additional security controls such as restricting access to exim command line interfaces and monitoring for unusual queue processing patterns. The vulnerability aligns with ATT&CK technique T1068 which covers "Exploitation for Privilege Escalation" and CWE-787 which addresses "Out-of-bounds Write" conditions. System administrators should also consider implementing network segmentation and access controls to limit potential attack vectors, while monitoring for suspicious mail queue entries that might indicate exploitation attempts. The patching process should include thorough testing in staging environments to ensure that the updated exim version does not introduce compatibility issues with existing mail server configurations and delivery workflows.