CVE-1999-1150 in Portmaster
Summary
by MITRE
Livingston Portmaster routers running ComOS use the same initial sequence number (ISN) for TCP connections, which allows remote attackers to conduct spoofing and hijack TCP sessions.
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Analysis
by VulDB Data Team • 04/17/2026
The vulnerability described in CVE-1999-1150 affects Livingston Portmaster routers operating on ComOS firmware, representing a critical weakness in TCP session management that fundamentally undermines network security protocols. This issue stems from the predictable nature of TCP sequence numbers, which are essential for maintaining connection integrity and preventing unauthorized access to network sessions. The use of identical initial sequence numbers across all TCP connections creates a deterministic pattern that attackers can exploit to predict subsequent sequence numbers in ongoing communications.
The technical flaw manifests through the lack of proper randomization in TCP sequence number generation within the ComOS operating system. When a TCP connection is established, each side generates an initial sequence number that should be unpredictable to prevent spoofing attacks. However, the Livingston Portmaster routers consistently use the same ISN value, making it trivial for remote attackers to calculate future sequence numbers and insert malicious data into existing connections. This vulnerability directly violates the fundamental security principle of randomness in cryptographic protocols and represents a classic example of poor entropy implementation in network security systems.
The operational impact of this vulnerability extends beyond simple session hijacking to encompass complete network compromise potential. Attackers can leverage this weakness to perform man-in-the-middle attacks, redirect traffic, inject malicious content, or gain unauthorized access to sensitive network resources. The predictability of sequence numbers allows attackers to maintain persistent access to established connections without detection, as their spoofed packets would appear legitimate to the network infrastructure. This vulnerability particularly affects environments where the Portmaster routers serve as critical network gateways, potentially enabling attackers to compromise entire network segments through simple sequence number prediction techniques.
Security professionals should recognize this vulnerability as a classic example of CWE-330, which addresses the use of insufficent entropy in security functions, and aligns with ATT&CK technique T1071.004 for application layer protocol, specifically targeting TCP/IP protocol weaknesses. The remediation approach requires immediate firmware updates from Livingston to implement proper randomization of TCP sequence numbers, along with network segmentation and monitoring to detect potential exploitation attempts. Organizations should also implement additional security controls such as TCP sequence number validation, connection tracking mechanisms, and network intrusion detection systems to mitigate the risk until complete patching is achieved. The vulnerability underscores the critical importance of proper entropy implementation in network security protocols and serves as a historical example of how predictable cryptographic elements can lead to complete system compromise.