CVE-2020-28022 in Exim
Summary
by MITRE • 05/06/2021
Exim 4 before 4.94.2 has Improper Restriction of Write Operations within the Bounds of a Memory Buffer. This occurs when processing name=value pairs within MAIL FROM and RCPT TO commands.
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Analysis
by VulDB Data Team • 05/09/2021
The vulnerability identified as CVE-2020-28022 represents a critical buffer overflow flaw in the Exim email transfer agent software affecting versions prior to 4.94.2. This issue manifests when the software processes name=value parameter pairs within the MAIL FROM and RCPT TO SMTP commands, creating a scenario where maliciously crafted input can cause improper restriction of write operations beyond allocated memory boundaries. The vulnerability stems from insufficient validation of parameter lengths during SMTP command processing, allowing attackers to exploit memory corruption through carefully constructed email headers or command arguments.
The technical implementation of this vulnerability involves the Exim software's handling of SMTP transaction parameters where name=value pairs are parsed without adequate boundary checking. When processing these parameters, the software allocates fixed-size buffers to store the parsed values but fails to validate whether incoming data exceeds these predetermined limits. This improper restriction creates a classic buffer overflow condition that can be exploited to overwrite adjacent memory locations, potentially leading to arbitrary code execution or denial of service conditions. The flaw specifically impacts the mail delivery agent's ability to safely handle malformed input during email transaction processing.
From an operational perspective, this vulnerability poses significant risks to email infrastructure security as it allows remote attackers to potentially execute arbitrary code on affected systems with the privileges of the Exim process. The attack surface is particularly concerning given that Exim is widely deployed across enterprise email servers, web hosting providers, and mail relay systems. Successful exploitation could result in complete system compromise, data exfiltration, or disruption of email services. The vulnerability is particularly dangerous because it operates at the protocol level during normal email transaction processing, making detection more challenging and exploitation relatively straightforward for attackers with knowledge of the specific attack vectors.
Organizations should prioritize immediate patching of all Exim installations to version 4.94.2 or later, which includes the necessary memory boundary checks and input validation fixes. System administrators should also implement network monitoring to detect unusual SMTP command patterns that might indicate exploitation attempts. Additional mitigations include configuring strict SMTP command validation, implementing rate limiting for email transactions, and deploying intrusion detection systems to monitor for exploitation signatures. The vulnerability aligns with CWE-121, which addresses stack-based buffer overflow conditions, and maps to ATT&CK technique T1204.002 for exploitation through malicious input, emphasizing the need for comprehensive input validation and memory safety measures in email processing applications.