CVE-1999-0396 in NetBSD
Summary
by MITRE
A race condition between the select() and accept() calls in NetBSD TCP servers allows remote attackers to cause a denial of service.
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Analysis
by VulDB Data Team • 04/18/2026
This vulnerability exists in NetBSD TCP server implementations where a race condition occurs between the select() and accept() system calls, creating a window of opportunity for remote attackers to exploit the timing discrepancy. The flaw arises when a server uses select() to monitor multiple file descriptors for incoming connections and then calls accept() to establish new connections. During this critical window, an attacker can exploit the timing gap to cause the server to enter an inconsistent state, potentially leading to a denial of service condition. The vulnerability specifically affects TCP servers that rely on the traditional select() and accept() pattern for handling multiple concurrent connections. This type of race condition represents a classic concurrency issue where the order of operations between system calls creates an exploitable timing vulnerability that can be leveraged by remote attackers. The issue stems from the fundamental design of how these system calls interact within the network stack, where the server's ability to properly handle incoming connections becomes compromised due to the asynchronous nature of network operations. Such vulnerabilities fall under the broader category of race conditions in system programming, which are commonly categorized as CWE-362 according to the Common Weakness Enumeration framework.
The operational impact of this vulnerability extends beyond simple service disruption, as it can effectively render TCP servers unusable to legitimate clients while attackers maintain control over the service availability. When exploited, the race condition can cause the server to crash or become unresponsive, forcing administrators to restart services and potentially leading to extended downtime. The attack vector is particularly concerning because it requires minimal privileges and can be executed remotely against vulnerable NetBSD systems. Network administrators may observe unusual connection patterns or service interruptions as symptoms of this vulnerability being exploited. The timing aspect of the vulnerability makes it particularly challenging to detect through conventional monitoring approaches, as the race condition occurs rapidly and may not leave obvious traces in system logs. This type of vulnerability is especially dangerous in environments where continuous availability is critical, such as web servers, database servers, or other mission-critical network services. The exploitation can occur during normal server operation when legitimate clients attempt to establish connections, making it difficult to distinguish between normal traffic and malicious activity.
Mitigation strategies for this vulnerability involve implementing proper synchronization mechanisms and updating server code to avoid the problematic select() and accept() sequence. System administrators should ensure their NetBSD systems are updated with the latest security patches that address this specific race condition. The recommended approach includes using alternative approaches such as epoll() on Linux systems or implementing proper socket-level synchronization to prevent the race condition from occurring. Additionally, implementing connection throttling and proper error handling can help reduce the impact of potential exploitation attempts. Organizations should also consider implementing intrusion detection systems that can monitor for unusual connection patterns that might indicate exploitation attempts. The vulnerability highlights the importance of proper concurrent programming practices and adherence to security best practices when developing network applications. Updates to the underlying operating system and network libraries are essential to address this vulnerability, as the issue is deeply rooted in the system call implementation rather than application-level code. Security teams should also implement regular vulnerability assessments to identify systems running vulnerable versions of NetBSD or affected network services. According to ATT&CK framework, this vulnerability maps to techniques involving denial of service and system exploitation through timing-based attacks, emphasizing the need for proper system hardening and timely patch management procedures to maintain network resilience against such threats.