CVE-2011-0411 in Postfix
Summary
by MITRE
The STARTTLS implementation in Postfix 2.4.x before 2.4.16, 2.5.x before 2.5.12, 2.6.x before 2.6.9, and 2.7.x before 2.7.3 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.
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Analysis
by VulDB Data Team • 12/01/2024
The vulnerability identified as CVE-2011-0411 represents a critical security flaw in the Postfix mail server's STARTTLS implementation that fundamentally undermines the integrity of encrypted email communications. This weakness affects multiple versions of Postfix across its 2.4, 2.5, 2.6, and 2.7 release lines, creating a persistent risk for organizations relying on secure SMTP communications. The flaw specifically targets the I/O buffering mechanisms that govern how plaintext and encrypted data are handled during the transition from unencrypted to encrypted SMTP sessions, creating a window of opportunity for malicious actors to exploit the protocol's transitional state.
The technical nature of this vulnerability stems from improper handling of input/output buffering during the STARTTLS handshake process, where the system fails to adequately separate cleartext commands from encrypted ones. This creates a scenario where an attacker can inject malicious commands into the encrypted session by carefully timing the transmission of plaintext commands that are processed after the TLS encryption has been established. The vulnerability operates under the CWE-200 category of Information Exposure and falls within the ATT&CK technique of T1566 for Phishing with Malicious Attachments, specifically targeting the initial stages of email delivery where command injection occurs. The flaw allows for a sophisticated man-in-the-middle attack where the attacker can manipulate the SMTP session after the encryption has been negotiated but before the system properly validates the command sequence.
The operational impact of this vulnerability extends beyond simple command injection, as it can enable attackers to execute arbitrary commands within the mail server's context, potentially leading to unauthorized email relaying, data exfiltration, or further compromise of the email infrastructure. Organizations using affected Postfix versions face significant risk of email spoofing, unauthorized access to mail queues, and potential credential theft through manipulation of SMTP commands. The attack vector specifically targets the transitional period between plaintext and encrypted communication, making it particularly dangerous as it exploits the natural flow of email protocol operations. This vulnerability particularly affects email security in environments where Postfix serves as an SMTP relay or gateway, potentially allowing attackers to bypass security controls and gain unauthorized access to email systems.
Mitigation strategies for CVE-2011-0411 require immediate patching of affected Postfix versions to the recommended secure releases, specifically upgrading to Postfix 2.4.16, 2.5.12, 2.6.9, or 2.7.3 respectively. Network administrators should implement additional monitoring for unusual command sequences in SMTP sessions and consider implementing stricter I/O buffering controls at the network level. The vulnerability highlights the importance of proper protocol implementation and the need for comprehensive testing of security features during the development lifecycle, aligning with security standards that emphasize the protection of data integrity during protocol transitions. Organizations should also consider implementing additional email security controls such as DMARC, SPF, and DKIM to provide layered protection against the potential exploitation of this vulnerability.