CVE-2019-16928 in Exim
Summary
by MITRE
Exim 4.92 through 4.92.2 allows remote code execution, a different vulnerability than CVE-2019-15846. There is a heap-based buffer overflow in string_vformat in string.c involving a long EHLO command.
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Analysis
by VulDB Data Team • 02/05/2025
The vulnerability identified as CVE-2019-16928 represents a critical heap-based buffer overflow in the Exim mail transfer agent affecting versions 4.92 through 4.92.2. This flaw exists within the string_vformat function located in the string.c source file, specifically when processing exceptionally long EHLO commands during SMTP communication. The vulnerability stems from inadequate input validation and bounds checking during string formatting operations, creating a condition where maliciously crafted SMTP EHLO commands can trigger memory corruption. The heap-based nature of this overflow means that attackers can manipulate heap memory layout and potentially execute arbitrary code with the privileges of the Exim process. This vulnerability is distinct from CVE-2019-15846, which affected a different code path within the same software ecosystem. The flaw operates at the protocol level where Exim processes incoming SMTP connections and handles EHLO commands, making it particularly dangerous as it can be exploited remotely without authentication. The buffer overflow occurs during the parsing of extended SMTP commands, specifically targeting the string formatting subsystem that handles variable argument lists. This vulnerability falls under CWE-121 heap-based buffer overflow, which is classified as a memory safety error and represents a fundamental flaw in how the application manages dynamic memory allocation and string operations. The operational impact is severe as it allows remote code execution, potentially enabling attackers to gain full control over affected mail servers and use them as launching points for further attacks within the network infrastructure. The attack surface includes any system running vulnerable Exim versions that accept SMTP connections from untrusted sources, making it a high-priority target for exploitation in large-scale attacks. The vulnerability aligns with ATT&CK technique T1190 for exploit public-facing application, and T1059 for command and scripting interpreter, as successful exploitation would allow attackers to execute commands and establish persistent access. Organizations using Exim mail servers should immediately upgrade to version 4.92.3 or later, which contains the necessary patches to address this heap overflow vulnerability. Additionally, implementing network segmentation, firewall rules to restrict SMTP access, and monitoring for unusual EHLO command patterns can provide additional defensive layers. The vulnerability demonstrates the critical importance of proper input validation in network-facing applications and highlights how seemingly benign protocol handling can become a gateway for complete system compromise. Regular security assessments of mail server configurations and maintaining up-to-date software patches remain essential practices for preventing exploitation of similar vulnerabilities in the future.