CVE-2015-3644 in Stunnel
Summary
by MITRE
Stunnel 5.00 through 5.13, when using the redirect option, does not redirect client connections to the expected server after the initial connection, which allows remote attackers to bypass authentication.
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Analysis
by VulDB Data Team • 11/25/2024
The vulnerability identified as CVE-2015-3644 affects stunnel versions 5.00 through 5.13 and represents a critical authentication bypass flaw that undermines the security of SSL/TLS proxy connections. This vulnerability specifically manifests when the redirect option is enabled within the stunnel configuration, creating a fundamental weakness in the connection handling mechanism that allows attackers to manipulate the intended destination of client connections. The issue stems from improper handling of connection redirection logic where the system fails to properly enforce the intended server redirection after the initial connection establishment, creating an opportunity for malicious actors to intercept and manipulate traffic.
The technical flaw resides in the stunnel application's connection management and redirection implementation, where the redirect option does not properly validate or enforce the intended server destination after the initial connection phase. This creates a scenario where client connections can be diverted from their intended target without proper authentication verification. The vulnerability operates at the application layer and specifically impacts the SSL/TLS proxy functionality, where stunnel acts as an intermediary between clients and servers. According to CWE classification, this vulnerability maps to CWE-284 Access Control Bypass, as it allows unauthorized access to services that should require proper authentication. The flaw represents a breakdown in the principle of least privilege and proper access control enforcement within the proxy infrastructure.
The operational impact of this vulnerability is severe as it enables remote attackers to bypass authentication mechanisms that are fundamental to secure communications. Attackers can exploit this weakness to redirect client connections to unintended servers, potentially gaining access to sensitive services or data that should be protected by authentication. This vulnerability undermines the trust model that stunnel is designed to provide, allowing man-in-the-middle attacks where malicious actors can intercept and manipulate encrypted communications. The attack surface expands significantly when considering that stunnel is commonly used in enterprise environments for securing communications, making this vulnerability particularly dangerous for organizations that rely on proper authentication and authorization controls.
The implications extend beyond simple connection redirection as this vulnerability can be leveraged for various malicious activities including data interception, service disruption, and unauthorized access to protected resources. Security professionals should consider this vulnerability in the context of the ATT&CK framework, particularly under the T1071.004 technique for application layer protocol and T1566 for credential harvesting, as the bypass mechanism can facilitate both reconnaissance and exploitation phases of an attack. Organizations using affected stunnel versions face significant risk of unauthorized access to their secured communications infrastructure, potentially leading to data breaches, service availability issues, and compliance violations. The vulnerability demonstrates the critical importance of proper input validation and connection handling in security-critical applications.
Mitigation strategies should prioritize immediate patching of affected stunnel versions to the latest releases that contain the necessary fixes for the redirection logic. Organizations should also implement network monitoring to detect unusual connection patterns that might indicate exploitation attempts. Configuration reviews should ensure that the redirect option is not unnecessarily enabled, and when it is required, proper access controls and authentication mechanisms are implemented to compensate for the reduced security posture. Security teams should consider implementing additional layers of protection such as network segmentation, intrusion detection systems, and comprehensive logging to monitor for potential exploitation attempts. The vulnerability underscores the necessity of thorough testing of security features, particularly those involving connection handling and redirection, before deployment in production environments.