CVE-2002-1563 in stunnel
Summary
by MITRE
stunnel 4.0.3 and earlier allows attackers to cause a denial of service (crash) via SIGCHLD signal handler race conditions that cause an inconsistency in the child counter.
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Analysis
by VulDB Data Team • 06/28/2021
The vulnerability identified as CVE-2002-1563 affects stunnel versions 4.0.3 and earlier, representing a critical denial of service weakness that exploits race conditions within the signal handling mechanism of the application. This flaw specifically targets the SIGCHLD signal handler implementation, which is responsible for managing child process termination notifications in the stunnel daemon. The vulnerability arises from improper synchronization between the parent process and its child processes when handling process lifecycle events, creating a scenario where concurrent access to shared resources can lead to inconsistent state management.
The technical implementation of this vulnerability stems from a race condition in the signal handler that manages child process cleanup operations. When multiple child processes terminate simultaneously, the SIGCHLD signal handler may not properly account for all process terminations due to insufficient locking mechanisms or atomic operations. This race condition causes the child counter variable to become inconsistent, leading to potential integer overflow conditions or invalid memory access patterns. The flaw is particularly dangerous because it can be triggered through legitimate network connections, making it difficult to distinguish between malicious attacks and accidental triggering.
From an operational perspective, this vulnerability enables attackers to remotely crash the stunnel service by carefully crafting network traffic that induces multiple child processes to terminate simultaneously. The resulting inconsistency in the child counter causes the parent process to enter an undefined state, ultimately leading to a complete service crash. This denial of service condition can be particularly devastating in environments where stunnel serves as a critical SSL/TLS termination point for network services, as it can disrupt legitimate client connections and potentially provide a vector for more sophisticated attacks targeting the underlying infrastructure.
The vulnerability maps to CWE-362, which describes a race condition in signal handling where multiple threads or processes access shared resources without proper synchronization mechanisms. This weakness falls under the broader category of concurrency issues that are particularly challenging to detect and remediate in daemon applications that manage multiple child processes. From an ATT&CK framework perspective, this vulnerability represents a denial of service technique that can be classified under T1499.004, specifically targeting network infrastructure services to disrupt availability. The attack surface is particularly concerning as it requires minimal privileges and can be executed through standard network communication patterns, making it an attractive target for automated exploitation tools.
Mitigation strategies for this vulnerability involve immediate upgrading to stunnel versions 4.0.4 and later, which contain proper synchronization mechanisms to address the race condition in signal handling. System administrators should also implement process monitoring to detect unusual patterns of service crashes that might indicate exploitation attempts. Additionally, network-level protections such as rate limiting and connection tracking can help reduce the effectiveness of exploitation attempts by limiting the number of concurrent connections that can trigger the race condition. The fix typically involves implementing proper atomic operations or mutex locking around the child counter updates within the signal handler, ensuring that all process termination events are properly accounted for regardless of timing variations between concurrent operations.