CVE-2011-1407 in Exim
Summary
by MITRE
The DKIM implementation in Exim 4.7x before 4.76 permits matching for DKIM identities to apply to lookup items, instead of only strings, which allows remote attackers to execute arbitrary code or access a filesystem via a crafted identity.
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Analysis
by VulDB Data Team • 11/07/2021
The vulnerability identified as CVE-2011-1407 represents a critical security flaw in the DomainKeys Identified Mail implementation within Exim email servers version 4.7x before 4.76. This weakness stems from improper handling of DKIM identity matching mechanisms that should have strictly validated string-based identifiers but instead allowed for arbitrary lookup item processing. The flaw exists in the mail transfer agent's cryptographic signature verification system, which is designed to authenticate email messages through domain-based authentication. When an attacker crafts a malicious DKIM identity, the vulnerable implementation processes this input as a lookup item rather than a simple string, creating a path for code execution or unauthorized file system access. The vulnerability specifically affects the DKIM verification process where identity strings are not properly sanitized before being used in lookup operations, potentially allowing attackers to manipulate the verification flow.
This technical weakness creates a significant attack surface that can be exploited through carefully crafted email messages containing malicious DKIM signatures. The flaw allows remote attackers to leverage the DKIM identity matching functionality to execute arbitrary commands or access filesystem resources, effectively bypassing the intended security controls of the email authentication system. The vulnerability operates by manipulating how the DKIM implementation processes identity fields during signature verification, where lookup items are interpreted in a way that enables code injection or file access rather than the expected string comparison operations. This misinterpretation of identity handling creates a path for privilege escalation or unauthorized data access within the mail server environment, potentially compromising the entire email infrastructure. The attack vector requires sending specially crafted emails to the vulnerable Exim server, where the DKIM verification process triggers the malicious code execution or file system access.
The operational impact of CVE-2011-1407 extends beyond simple code execution to encompass potential data breaches and system compromise within email infrastructure. Organizations running vulnerable Exim versions face risks of unauthorized access to email communications, potential message interception, and compromise of the underlying mail server operating system. The vulnerability affects the integrity of email authentication mechanisms that are critical for preventing spam and phishing attacks, essentially undermining the security controls designed to protect email systems. Attackers can exploit this weakness to gain unauthorized access to email archives, potentially access sensitive communications, or even establish persistent access to the mail server infrastructure. The widespread adoption of Exim as a mail transfer agent makes this vulnerability particularly dangerous, as numerous organizations could be affected by a single exploit. This flaw demonstrates the critical importance of proper input validation in cryptographic systems and the potential consequences when identity handling mechanisms are not properly secured.
Mitigation strategies for CVE-2011-1407 require immediate patching of affected Exim installations to version 4.76 or later, which contains the necessary fixes for the DKIM identity handling vulnerability. Organizations should also implement network-level controls such as email filtering rules that can detect and block suspicious DKIM signatures or malformed email headers. The implementation of proper input validation procedures for all DKIM identity processing should be enforced, ensuring that lookup items are properly sanitized before being processed. Security monitoring should be enhanced to detect unusual DKIM verification patterns or unauthorized filesystem access attempts. Additionally, organizations should consider implementing email authentication best practices that include DKIM key rotation, proper signature validation, and regular security audits of mail server configurations. The vulnerability aligns with CWE-20, which describes improper input validation, and can be mapped to ATT&CK technique T1190 for exploit public-facing application, highlighting the need for proper application security controls and network segmentation to limit potential attack impact. Organizations should also review their email security policies to ensure that DKIM implementations follow industry standards and that proper security controls are in place to prevent similar vulnerabilities in other cryptographic authentication systems.