CVE-2011-1926 in Cyrus IMAP Server
Summary
by MITRE
The STARTTLS implementation in Cyrus IMAP Server before 2.4.7 does not properly restrict I/O buffering, which allows man-in-the-middle attackers to insert commands into encrypted 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.
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Analysis
by VulDB Data Team • 01/26/2025
The vulnerability identified as CVE-2011-1926 affects the Cyrus IMAP Server version 2.4.6 and earlier, representing a critical security flaw in the implementation of the STARTTLS protocol. This issue stems from improper handling of input/output buffering mechanisms during the TLS negotiation process, creating a window of opportunity for attackers to exploit the communication channel between client and server. The flaw specifically manifests when the server transitions from plaintext to encrypted communication, leaving a vulnerable state where malicious actors can inject commands into the session.
The technical implementation flaw resides in how the Cyrus IMAP server manages I/O buffering during the STARTTLS handshake process. When a client initiates a TLS connection, the server should immediately enforce encryption for all subsequent communication. However, the vulnerable implementation allows for a race condition where cleartext commands can be processed before the TLS encryption is fully established or properly enforced. This creates a temporal gap where attackers can send malicious commands that are then executed in the context of the encrypted session, effectively bypassing the intended security protections.
This vulnerability operates under the category of plaintext command injection attacks, which are classified under CWE-444 and fall within the broader ATT&CK technique of T1071.004 for application layer protocol manipulation. The attack vector specifically targets the communication protocol layer where the transition from unencrypted to encrypted communication occurs, allowing adversaries to inject commands that are processed after the TLS encryption is supposedly active. The similarity to CVE-2011-0411 demonstrates a pattern of vulnerabilities in STARTTLS implementations where buffering mechanisms fail to properly isolate cleartext and encrypted communication streams.
The operational impact of this vulnerability is severe as it enables man-in-the-middle attackers to compromise email communications without requiring sophisticated cryptographic attacks or deep system access. An attacker positioned between the client and server can intercept the initial connection, send cleartext commands that are processed after the TLS transition, and potentially execute arbitrary commands within the user's email session. This could lead to unauthorized access to email accounts, message interception, data exfiltration, and potential privilege escalation within the email infrastructure. The vulnerability affects the confidentiality and integrity of email communications, making it particularly dangerous for organizations relying on secure email services.
The recommended mitigations for this vulnerability include immediate upgrade to Cyrus IMAP Server version 2.4.7 or later, which contains the necessary patches to address the I/O buffering issues. Organizations should also implement additional network security controls such as enforcing strict TLS requirements, implementing certificate pinning, and monitoring for anomalous command sequences in email server logs. Network administrators should consider deploying intrusion detection systems that can identify potential command injection attempts and implement proper session management policies that enforce immediate encryption upon connection establishment. The fix addresses the core buffering issue by ensuring proper isolation between cleartext and encrypted communication channels during the STARTTLS transition, preventing the injection of malicious commands after the TLS encryption is established.