CVE-2013-2125 in OpenSMTPD
Summary
by MITRE
OpenSMTPD before 5.3.2 does not properly handle SSL sessions, which allows remote attackers to cause a denial of service (connection blocking) by keeping a connection open.
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Analysis
by VulDB Data Team • 03/22/2022
OpenSMTPD version 5.3.2 and earlier contains a critical vulnerability in its SSL session handling mechanism that enables remote attackers to execute denial of service attacks through persistent connection maintenance. This flaw resides in the server's inability to properly manage SSL session states, creating a condition where malicious actors can maintain open connections indefinitely. The vulnerability specifically affects the SSL/TLS handshake and session resumption processes within the OpenSMTPD implementation, allowing attackers to exploit the connection management logic to prevent legitimate users from establishing new connections. The flaw stems from inadequate session cleanup procedures and improper handling of connection timeouts during SSL negotiations, creating a resource exhaustion scenario that effectively blocks new mail transmission attempts.
The technical implementation of this vulnerability manifests when an attacker establishes an SSL connection to the OpenSMTPD server and maintains it in a half-open state without properly completing the SSL handshake or sending mail commands. The server fails to detect and terminate these stale connections, leading to progressive resource consumption that eventually prevents the system from accepting new legitimate connections. This behavior aligns with CWE-400, which categorizes resource exhaustion vulnerabilities, and demonstrates how improper connection state management can create denial of service conditions. The flaw operates at the protocol level within the SSL/TLS implementation, specifically affecting the session resumption mechanism that should normally clean up inactive connections after a timeout period.
The operational impact of CVE-2013-2125 extends beyond simple service disruption to potentially compromise the entire mail server availability and reliability. Organizations relying on OpenSMTPD for email services face significant risk of email delivery failures, connection timeouts, and complete service unavailability during attack windows. The vulnerability is particularly dangerous because it can be exploited with minimal resources, requiring only a single connection to maintain the denial of service condition. Attackers can leverage this weakness to target critical email infrastructure, potentially disrupting business communications and email-based applications that depend on reliable SMTP services. The attack vector requires no authentication and can be executed from any network location, making it an attractive target for automated scanning and exploitation.
Mitigation strategies for this vulnerability should prioritize immediate patching to OpenSMTPD version 5.3.2 or later, which contains the necessary fixes for SSL session handling. Organizations should also implement connection rate limiting and monitoring to detect unusual connection patterns that may indicate exploitation attempts. Network-level protections such as connection tracking rules and firewall policies can help limit the number of concurrent connections to the SMTP service. The implementation of proper SSL session timeout configurations and connection cleanup routines provides additional defense layers. Security teams should monitor system logs for connection establishment patterns and implement automated alerting for unusual connection behavior. This vulnerability demonstrates the importance of proper session management in cryptographic protocols and aligns with ATT&CK technique T1499.004, which covers network denial of service attacks through connection exhaustion. Organizations should also consider implementing intrusion detection systems to monitor for exploitation attempts and establish incident response procedures for handling such denial of service conditions.