CVE-2011-1506 in MailServer
Summary
by MITRE
The STARTTLS implementation in Kerio Connect 7.1.4 build 2985 and MailServer 6.x does not properly restrict I/O buffering, which allows man-in-the-middle attackers to insert commands into encrypted SMTP sessions by sending a cleartext command that is processed after TLS is in place, related to a "plaintext command injection" attack, a similar issue to CVE-2011-0411. NOTE: some of these details are obtained from third party information.
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Analysis
by VulDB Data Team • 12/01/2024
The vulnerability identified as CVE-2011-1506 represents a critical security flaw in the STARTTLS implementation of Kerio Connect 7.1.4 build 2985 and MailServer 6.x email servers. This issue stems from improper handling of input/output buffering during the secure communication establishment process, creating a window of opportunity for attackers to manipulate encrypted sessions. The flaw specifically affects the transition period between plaintext and encrypted communication channels, where the system fails to adequately validate or sanitize commands received during this critical phase. The vulnerability manifests as a plaintext command injection attack vector that exploits the temporal gap between when TLS encryption is established and when the system properly enforces encrypted communication boundaries.
The technical root cause of this vulnerability can be categorized under CWE-200, which deals with information exposure, and more specifically relates to CWE-444, which addresses insufficient input validation. The flaw occurs because the mail server implementation does not properly implement secure I/O buffering mechanisms during the STARTTLS handshake process. During the transition from plaintext to encrypted communication, the system continues to process incoming cleartext commands that should be deferred until the TLS session is fully established and secure. This creates an exploitable condition where an attacker positioned between the client and server can send malicious commands in cleartext format that will be executed once the encryption is fully negotiated, effectively bypassing the security measures intended to protect the communication channel.
From an operational perspective, this vulnerability presents a significant risk to email server security and data integrity. Attackers can leverage this weakness to inject malicious commands into encrypted SMTP sessions, potentially gaining unauthorized access to email communications, modifying message content, or even establishing persistent access to the mail server infrastructure. The impact extends beyond simple command injection as it undermines the fundamental security guarantee that TLS encryption provides. This vulnerability is particularly dangerous in environments where email servers handle sensitive corporate or personal communications, as it allows adversaries to perform man-in-the-middle attacks without detection. The attack vector is relatively straightforward, requiring only that the attacker be positioned to intercept and inject commands during the brief window when the system processes both plaintext and encrypted communications simultaneously.
The security implications of CVE-2011-1506 align with tactics described in the MITRE ATT&CK framework under the T1071.004 sub-technique for application layer protocol: email protocols. This vulnerability enables attackers to manipulate email communication flows and potentially compromise the confidentiality and integrity of email data. Organizations using affected versions of Kerio Connect or MailServer should implement immediate mitigations including updating to patched versions, implementing network-level protections such as firewall rules to restrict SMTP access, and monitoring for unusual command patterns in email server logs. The vulnerability also highlights the importance of proper secure coding practices in implementing cryptographic protocols and demonstrates the critical need for comprehensive testing of security mechanisms during protocol transitions. Additional defensive measures should include implementing strict command validation, enhancing logging capabilities to detect injection attempts, and conducting regular security assessments of email infrastructure components.