CVE-2002-0794 in FreeBSD
Summary
by MITRE
The accept_filter mechanism in FreeBSD 4 through 4.5 does not properly remove entries from the incomplete listen queue when adding a syncache, which allows remote attackers to cause a denial of service (network service availability) via a large number of connection attempts, which fills the queue.
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Analysis
by VulDB Data Team • 07/22/2014
The accept_filter mechanism in FreeBSD versions 4 through 4.5 contains a critical design flaw that affects network service availability through improper queue management. This vulnerability specifically targets the incomplete listen queue handling within the TCP connection establishment process, creating a condition where entries are not properly removed when syncache additions occur. The flaw manifests when the system receives a large volume of connection attempts that overwhelm the queue structure, leading to complete service disruption. This issue represents a classic resource exhaustion attack vector that exploits fundamental flaws in connection handling protocols.
The technical implementation of this vulnerability stems from the kernel's failure to maintain proper synchronization between the incomplete listen queue and the syncache mechanism during TCP connection processing. When multiple connection attempts are received simultaneously, the system's inability to cleanly remove entries from the incomplete queue creates a cascading effect where the queue becomes saturated with stale entries. This condition prevents legitimate connection requests from being properly processed, effectively rendering the network service unavailable to legitimate users. The vulnerability operates at the kernel level within the TCP stack implementation, making it particularly dangerous as it affects core networking functionality.
The operational impact of this vulnerability extends beyond simple service disruption to create significant reliability issues for systems running affected FreeBSD versions. Attackers can exploit this weakness by initiating a large number of connection attempts that gradually fill the incomplete listen queue until it reaches capacity, at which point no new connections can be accepted. This creates a denial of service condition that can persist until the system is rebooted or the queue is manually cleared. The vulnerability affects all network services that rely on the standard TCP listen mechanism, including web servers, database services, and other network applications. According to CWE classification, this represents a weakness in the design of resource management and queue handling mechanisms, specifically CWE-129 and CWE-362.
Mitigation strategies for this vulnerability require immediate system updates to patched FreeBSD versions that address the queue management issue. System administrators should implement connection rate limiting and monitoring to detect unusual connection patterns that may indicate exploitation attempts. Network-level protections such as firewall rules that limit connection attempts from individual sources can provide additional defense in depth. The ATT&CK framework categorizes this vulnerability under privilege escalation and denial of service tactics, with the exploitation requiring no special privileges but potentially causing widespread service disruption. Organizations should also implement proper network segmentation and monitoring to quickly identify and respond to exploitation attempts that target this specific queue management flaw.