CVE-2018-6789 in Exim
Summary
by MITRE
An issue was discovered in the base64d function in the SMTP listener in Exim before 4.90.1. By sending a handcrafted message, a buffer overflow may happen. This can be used to execute code remotely.
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Analysis
by VulDB Data Team • 11/07/2025
The vulnerability identified as CVE-2018-6789 represents a critical buffer overflow condition within the Exim email server software's SMTP listener component. This flaw exists in the base64d function which processes base64 encoded data during email transmission. The vulnerability affects Exim versions prior to 4.90.1, making it a significant concern for organizations running older installations. The buffer overflow occurs when the SMTP listener processes malformed base64 encoded data, creating an opportunity for remote code execution. This type of vulnerability falls under CWE-121, which describes stack-based buffer overflow conditions that can lead to arbitrary code execution.
The technical exploitation of this vulnerability requires an attacker to craft a specific email message containing maliciously formatted base64 data that exceeds the allocated buffer space. When the Exim server processes this message through its SMTP listener, the base64 decoding function fails to properly validate input length, resulting in memory corruption. This memory corruption can be leveraged to overwrite adjacent memory locations, potentially allowing attackers to inject and execute arbitrary code with the privileges of the Exim process. The vulnerability demonstrates characteristics consistent with CWE-787, which covers out-of-bounds writes that can be exploited for code execution.
From an operational perspective, this vulnerability poses severe risks to email infrastructure security as it enables remote attackers to gain unauthorized access to systems running vulnerable Exim versions. Organizations relying on Exim for email services face potential data breaches, system compromise, and lateral movement opportunities for attackers. The vulnerability can be exploited without authentication, making it particularly dangerous as it allows attackers to target systems from outside the network perimeter. This aligns with ATT&CK technique T1190, which covers exploits for execution through SMTP protocols, and T1059, which involves command and scripting interpreters for remote code execution.
The impact extends beyond immediate system compromise as successful exploitation can lead to persistent access, data exfiltration, and use of compromised systems for further attacks. Organizations should prioritize patching their Exim installations to version 4.90.1 or later, as this update includes proper bounds checking and input validation for the base64 decoding function. Additional mitigations include implementing network segmentation, monitoring SMTP traffic for anomalous base64 patterns, and deploying intrusion detection systems to identify exploitation attempts. The vulnerability also highlights the importance of regular security updates and proper input validation in email server software, as similar flaws could exist in other components of the email infrastructure. Organizations should conduct comprehensive vulnerability assessments to identify all systems running affected Exim versions and ensure proper patch management procedures are in place to prevent similar issues in the future.