CVE-2020-28016 in Exim
Summary
by MITRE • 05/06/2021
Exim 4 before 4.94.2 allows an off-by-two Out-of-bounds Write because "-F ''" is mishandled by parse_fix_phrase.
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Analysis
by VulDB Data Team • 05/09/2021
The vulnerability identified as CVE-2020-28016 represents a critical buffer overflow flaw in the Exim email transfer agent software affecting versions prior to 4.94.2. This issue manifests as an off-by-two out-of-bounds write condition that occurs when processing specific command-line arguments, particularly the "-F ''" parameter combination. The vulnerability resides within the parse_fix_phrase function which fails to properly validate input boundaries when handling empty string arguments, creating a scenario where memory corruption can occur beyond the intended buffer limits. Such flaws are particularly dangerous in email server environments where the software processes untrusted input from external sources.
The technical exploitation of this vulnerability stems from improper boundary checking within the Exim mail processing pipeline. When the software encounters the "-F ''" command-line option, which specifies a blank return path, the parse_fix_phrase function does not correctly calculate the buffer size required for string manipulation operations. This miscalculation results in a write operation that extends two bytes beyond the allocated memory buffer, potentially overwriting adjacent memory locations. The flaw falls under the CWE-121 category of stack-based buffer overflow, though the specific implementation details create an off-by-two error that can be particularly challenging to detect and exploit. The vulnerability demonstrates poor input validation practices and inadequate bounds checking that violates fundamental security principles for memory management.
The operational impact of CVE-2020-28016 extends beyond simple memory corruption, as it creates potential for remote code execution in environments where Exim processes untrusted email content. Email servers running vulnerable versions of Exim become susceptible to arbitrary code execution when processing specially crafted email headers or command-line parameters. Attackers could leverage this vulnerability to execute malicious code with the privileges of the Exim process, potentially leading to full system compromise. The vulnerability affects organizations that rely on Exim as their primary mail transfer agent, particularly those with complex email processing workflows that utilize command-line options for email routing or filtering. The attack surface is broad given Exim's widespread deployment across enterprise and organizational email infrastructures.
Mitigation strategies for CVE-2020-28016 require immediate patching of affected Exim installations to version 4.94.2 or later, which contains the necessary fixes for the buffer overflow condition. Organizations should conduct comprehensive inventory audits to identify all systems running vulnerable Exim versions and prioritize patch deployment across their infrastructure. Network segmentation and access controls should be implemented to limit exposure of vulnerable mail servers to untrusted networks, while monitoring systems should be configured to detect anomalous email processing patterns that might indicate exploitation attempts. Additionally, administrators should review and tighten input validation processes for all command-line parameters, particularly those related to email header processing. The remediation aligns with ATT&CK technique T1059.007 for command and scripting interpreter execution, as exploitation would likely involve crafting malicious email content to trigger the vulnerable code path, making defensive measures around email content validation essential for comprehensive protection.