CVE-1999-0796 in FreeBSD
Summary
by MITRE
FreeBSD T/TCP Extensions for Transactions can be subjected to spoofing attacks.
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Analysis
by VulDB Data Team • 04/17/2026
The vulnerability identified as CVE-1999-0796 resides within the FreeBSD operating system's implementation of T/TCP (TCP Transactions) extensions, which were designed to improve TCP performance by reducing the number of round trips required for certain transactions. This flaw represents a significant security weakness in the network protocol stack that could enable malicious actors to exploit the transactional TCP mechanisms. The T/TCP extensions were intended to provide a more efficient way to handle small data transfers by allowing the TCP stack to optimize the three-way handshake process, but this optimization introduced a critical vulnerability that undermines the integrity of the connection establishment phase.
The technical flaw stems from inadequate validation mechanisms within the T/TCP implementation that fails to properly verify the authenticity of connection requests during the transactional handshake process. Specifically, the vulnerability allows attackers to craft spoofed packets that can trick the FreeBSD system into accepting connections from unauthorized sources. This occurs because the T/TCP extensions do not adequately authenticate the source of transactional TCP segments, creating a window where malicious actors can manipulate the connection establishment process without proper authorization. The flaw operates at the network protocol level, specifically within the TCP/IP stack implementation where transactional TCP connections are processed and validated.
The operational impact of this vulnerability extends beyond simple network disruption to encompass potential unauthorized access and data compromise within systems running FreeBSD with T/TCP extensions enabled. Attackers could exploit this weakness to perform man-in-the-middle attacks, intercept sensitive communications, or establish unauthorized connections to network services. The vulnerability particularly affects systems that rely heavily on T/TCP for performance optimization, as these systems become more susceptible to spoofing attacks that would otherwise be prevented by standard TCP security mechanisms. Organizations running FreeBSD systems with transactional TCP enabled face a heightened risk of network-based attacks that could lead to data breaches or service disruption.
Mitigation strategies for CVE-1999-0796 primarily focus on disabling the T/TCP extensions within FreeBSD systems until a proper patch is implemented, as this represents the most effective immediate solution to prevent exploitation. System administrators should also implement network-level security controls such as TCP sequence number randomization and connection tracking mechanisms to reduce the attack surface. The vulnerability aligns with CWE-294, which addresses authentication bypass issues in cryptographic protocols, and relates to ATT&CK technique T1071.004 for application layer protocol traffic shaping. Additionally, organizations should conduct thorough network monitoring to detect anomalous TCP behavior that might indicate exploitation attempts, while also ensuring that all FreeBSD systems are updated with the latest security patches that address the transactional TCP spoofing vulnerability.