CVE-2020-28023 in Exim
Summary
by MITRE • 05/06/2021
Exim 4 before 4.94.2 allows Out-of-bounds Read. smtp_setup_msg may disclose sensitive information from process memory to an unauthenticated SMTP client.
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Analysis
by VulDB Data Team • 05/09/2021
The vulnerability identified as CVE-2020-28023 represents a critical out-of-bounds read flaw within the Exim mail transfer agent version 4.94.2 and earlier. This issue affects the smtp_setup_msg function which handles the setup of SMTP messages during the email transmission process. The vulnerability occurs when an unauthenticated SMTP client connects to the server and sends a malformed message, allowing the system to read memory locations beyond the intended buffer boundaries. This type of vulnerability falls under the CWE-125 out-of-bounds read classification, which is categorized as a memory safety issue in the Common Weakness Enumeration framework.
The technical exploitation of this vulnerability involves an attacker sending specially crafted SMTP commands that trigger the smtp_setup_msg function to access memory regions that should not be accessible to unauthenticated users. When the function processes these malformed inputs, it reads beyond the allocated buffer space and potentially exposes sensitive data from the process memory. This sensitive information could include system credentials, encryption keys, internal memory structures, or other confidential data that remains in memory during the processing of email messages. The vulnerability specifically targets the SMTP protocol implementation within Exim, making it particularly dangerous for mail servers that are publicly accessible and receive unauthenticated connections.
The operational impact of this vulnerability is significant for organizations relying on Exim mail servers, as it creates a potential information disclosure channel that could be exploited by attackers to gain insights into the system's internal state. An attacker could potentially extract sensitive information that might aid in further exploitation attempts or provide intelligence about the server environment. This vulnerability affects the confidentiality aspect of the CIA triad and could lead to cascading security issues if the disclosed information includes authentication tokens, cryptographic keys, or system configuration details. The vulnerability exists in the SMTP handling code path, which means any email server configured to accept incoming connections without authentication could be at risk, particularly those exposed to the internet.
Organizations should immediately implement mitigations by upgrading to Exim version 4.94.2 or later, which contains the necessary patches to address the out-of-bounds read condition. Additionally, network administrators should consider implementing strict access controls and limiting SMTP service exposure to trusted networks only. The mitigation strategy aligns with ATT&CK technique T1071.004 for application layer protocol usage and T1566 for credential access through network service exploitation. Regular security monitoring and log analysis should be enhanced to detect potential exploitation attempts, particularly unusual SMTP command sequences or malformed message patterns that could indicate an active attack against this vulnerability. Security teams should also consider implementing network segmentation and firewall rules to restrict SMTP service access to authorized clients only, reducing the attack surface for this and related vulnerabilities.