CVE-2003-0740 in Stunnel
Summary
by MITRE
Stunnel 4.00, and 3.24 and earlier, leaks a privileged file descriptor returned by listen(), which allows local users to hijack the Stunnel server.
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Analysis
by VulDB Data Team • 07/25/2025
The vulnerability identified as CVE-2003-0740 represents a critical security flaw in stunnel software versions 4.00 and earlier, as well as 3.24 and prior releases. This issue stems from improper handling of file descriptors within the stunnel process, creating a persistent security risk that can be exploited by local attackers. The vulnerability specifically affects the server-side implementation of stunnel, which is commonly used to provide SSL/TLS encryption for various network services including web servers, mail servers, and database connections. When stunnel initializes and establishes listening sockets for incoming connections, it creates file descriptors that should be properly managed and closed after use. However, in the affected versions, the software fails to correctly close these privileged file descriptors, leaving them accessible to unauthorized processes running with the same privileges as the stunnel service.
The technical exploitation of this vulnerability occurs through a file descriptor leakage mechanism that allows local users to gain access to the network listening sockets that stunnel maintains for incoming connections. This leakage enables attackers to intercept and manipulate network traffic that should be encrypted and protected by the stunnel service. The flaw is particularly dangerous because it operates at the kernel level where file descriptors are managed, and it specifically targets the listen() system call which is fundamental to network service operation. When stunnel creates a listening socket, it returns a file descriptor that represents the socket connection point. The vulnerability occurs when this file descriptor is not properly closed or managed after the initial socket setup, allowing subsequent processes to inherit or access these descriptors. This type of vulnerability falls under the CWE-119 weakness category, which deals with improper access to critical resources, and specifically relates to CWE-362 which addresses concurrent execution using shared resource with improper synchronization.
The operational impact of CVE-2003-0740 extends beyond simple privilege escalation to encompass complete network traffic interception and potential man-in-the-middle attacks. Local users who can access the leaked file descriptors can essentially hijack the stunnel server's listening functionality, allowing them to capture all incoming encrypted traffic destined for services protected by stunnel. This compromises the confidentiality and integrity of network communications, as attackers can potentially decrypt traffic, modify data in transit, or redirect connections to malicious endpoints. The vulnerability is particularly concerning because it affects the core functionality of stunnel, which is designed specifically to protect network communications through encryption. Attackers could leverage this flaw to gain unauthorized access to sensitive data, including authentication credentials, personal information, and business-critical data that should remain protected by SSL/TLS encryption. The attack vector is relatively simple since it requires only local access to the system where stunnel is running, making it accessible to users who may have legitimate access but could potentially abuse their privileges.
The mitigation strategies for this vulnerability primarily focus on immediate software updates and proper system administration practices. Organizations should immediately upgrade to stunnel versions that have patched this vulnerability, typically stunnel 4.01 or later, which properly closes the file descriptors after use. System administrators should also implement proper access controls and privilege management to limit local user access to systems running stunnel services. The vulnerability demonstrates the importance of proper resource management in security-critical applications and aligns with ATT&CK technique T1068 which covers local privilege escalation through improper resource handling. Additional mitigations include implementing monitoring for unusual file descriptor access patterns and conducting regular security audits of network services. The vulnerability also highlights the need for proper input validation and resource cleanup in system-level programming, as it represents a classic example of a resource leak that can be exploited to gain unauthorized access to system resources. Organizations should also consider implementing network segmentation and monitoring to detect potential exploitation attempts, as the attack can occur without external network access once local privileges are obtained. This vulnerability serves as a reminder of the critical importance of proper resource management in security software and the potential consequences of failing to properly handle system-level resources such as file descriptors and network sockets.