CVE-2016-9963 in Exim
Summary
by MITRE
Exim before 4.87.1 might allow remote attackers to obtain the private DKIM signing key via vectors related to log files and bounce messages.
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Analysis
by VulDB Data Team • 07/09/2019
The vulnerability identified as CVE-2016-9963 affects Exim email servers version 4.87.1 and earlier, presenting a significant security risk related to the improper handling of log files and bounce messages. This flaw enables remote attackers to potentially extract private DKIM signing keys, which are critical cryptographic components used to authenticate email messages and prevent spoofing. The vulnerability stems from insufficient sanitization of log file entries that contain bounce message data, creating an avenue for information disclosure attacks.
The technical implementation of this vulnerability involves the interaction between Exim's logging mechanism and its bounce message processing capabilities. When email messages fail to deliver, Exim generates bounce notifications that include message content and headers in its log files. In vulnerable versions, these log entries were not properly sanitized, allowing malicious actors to craft specific bounce messages that could result in the exposure of sensitive DKIM private keys stored on the server. The flaw operates through the manipulation of message headers and content that gets logged, creating a path for attackers to extract cryptographic material from server logs.
This vulnerability has substantial operational impact on email infrastructure security, particularly for organizations relying on DKIM for email authentication. The exposure of private DKIM keys compromises the entire email authentication framework, potentially enabling attackers to forge legitimate emails that appear to originate from trusted domains. This can lead to widespread phishing attacks, brand reputation damage, and successful social engineering campaigns. Organizations using Exim mail servers were at risk of having their email domains compromised, with potential for large-scale impersonation attacks that could affect thousands of users.
The vulnerability aligns with CWE-200, which addresses "Information Exposure," and represents a classic case of improper information handling in security-critical applications. From an ATT&CK framework perspective, this vulnerability maps to T1566, "Phishing," and T1552, "Unsecured Credentials," as it enables attackers to gain access to authentication credentials through information disclosure. The attack vector specifically involves remote exploitation without requiring authentication, making it particularly dangerous as it allows adversaries to leverage the vulnerability from outside the network perimeter.
Mitigation strategies for CVE-2016-9963 include immediate patching of Exim installations to version 4.87.1 or later, which contains the necessary fixes for log file sanitization. Organizations should also implement log file monitoring to detect unusual patterns that might indicate exploitation attempts, and consider implementing additional access controls to limit exposure of sensitive data in log files. Regular security audits of mail server configurations and comprehensive monitoring of authentication key usage should be established as part of the remediation process. Additionally, organizations should verify that their backup and recovery procedures do not inadvertently expose the compromised keys and ensure that all DKIM keys are properly rotated following any suspected compromise.