CVE-2014-3556 in nginx
Summary
by MITRE
The STARTTLS implementation in mail/ngx_mail_smtp_handler.c in the SMTP proxy in nginx 1.5.x and 1.6.x before 1.6.1 and 1.7.x before 1.7.4 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.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 02/10/2022
The vulnerability described in CVE-2014-3556 represents a critical security flaw in the nginx SMTP proxy implementation that undermines the integrity of encrypted communications. This issue affects nginx versions 1.5.x and 1.6.x before 1.6.1 and 1.7.x before 1.7.4, where the STARTTLS functionality fails to properly manage input/output buffering during the transition from plaintext to encrypted SMTP sessions. The flaw enables attackers to perform man-in-the-middle attacks by injecting cleartext commands into encrypted sessions after TLS encryption has been established, creating a window of opportunity for malicious interference with legitimate communication flows.
The technical root cause of this vulnerability lies in the improper handling of I/O buffering within the SMTP proxy module of nginx. When the STARTTLS handshake occurs, the system should maintain strict separation between plaintext and encrypted data streams to prevent command injection attacks. However, the implementation fails to properly isolate these streams, allowing an attacker to craft a cleartext command that gets processed after the TLS encryption is fully established. This creates a scenario where legitimate commands from the client can be interleaved with malicious commands from an attacker, potentially leading to unauthorized access or data manipulation within the SMTP session. The vulnerability is classified under CWE-129 as an improper validation of input boundaries, specifically related to buffer handling and data flow management in cryptographic protocols.
The operational impact of this vulnerability extends beyond simple command injection, as it fundamentally compromises the security assurances provided by TLS encryption in SMTP communications. Attackers can exploit this weakness to execute unauthorized commands against mail servers, potentially gaining access to sensitive email data, modifying message content, or redirecting email flows. The vulnerability is particularly concerning because it operates at the protocol level where encryption is expected to provide complete protection. This weakness creates a persistent threat vector that can be exploited by attackers positioned between the client and server, effectively neutralizing the cryptographic protections that should secure email communications. The attack pattern resembles previous vulnerabilities such as CVE-2011-0411, indicating a recurring pattern in how I/O buffering is managed in proxy implementations that handle encrypted protocols.
Mitigation strategies for this vulnerability require immediate patching of affected nginx installations to versions 1.6.1 or 1.7.4 and later, which contain the necessary fixes to properly manage I/O buffering during STARTTLS transitions. Organizations should also implement network-level monitoring to detect unusual command sequences that might indicate exploitation attempts, particularly around the timing of TLS handshake completion and command processing. Additional defensive measures include configuring strict TLS policies that enforce proper certificate validation and implementing network segmentation to limit exposure of SMTP services to untrusted networks. From an ATT&CK framework perspective, this vulnerability maps to technique T1071.004 for Application Layer Protocol and T1566 for Phishing, as it enables attackers to manipulate email communications through protocol-level attacks that bypass traditional network security controls. The vulnerability demonstrates the critical importance of proper buffer management in cryptographic implementations and highlights the need for comprehensive security testing of protocol-handling components in web and proxy servers.