CVE-2020-28388 in Nucleus NET
Summary
by MITRE • 02/10/2021
A vulnerability has been identified in Nucleus NET (All versions < V5.2), Nucleus ReadyStart for ARM, MIPS, and PPC (All versions < V2012.12). Initial Sequence Numbers (ISNs) for TCP connections are derived from an insufficiently random source. As a result, the ISN of current and future TCP connections could be predictable. An attacker could hijack existing sessions or spoof future ones.
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Analysis
by VulDB Data Team • 02/27/2021
The vulnerability described in CVE-2020-28388 represents a critical weakness in network security protocols affecting multiple Nucleus networking products including Nucleus NET and Nucleus ReadyStart across various processor architectures. This issue stems from the improper generation of Initial Sequence Numbers within TCP connections, which fundamentally compromises the security of network communications. The vulnerability impacts all versions prior to V5.2 for Nucleus NET and V2012.12 for Nucleus ReadyStart, indicating a long-standing flaw that has persisted across multiple product iterations. The core problem lies in the insufficient randomness of the random number generator used to establish TCP connection sequence numbers, creating a predictable pattern that adversaries can exploit. This weakness directly violates fundamental security principles for TCP/IP communications where sequence numbers must be unpredictable to prevent session hijacking and man-in-the-middle attacks.
The technical flaw manifests through the use of inadequate random number generation algorithms that fail to provide sufficient entropy for TCP ISN creation. In standard TCP implementations, Initial Sequence Numbers must be generated using cryptographically secure random number generators to ensure unpredictability across different connection attempts. When systems rely on pseudo-random number generators with insufficient entropy sources or predictable seed values, attackers can compute future sequence numbers by analyzing patterns in previously observed connections. This vulnerability maps directly to CWE-330, which addresses the use of insufficiently random values in security contexts, and specifically relates to the broader category of weak random number generation that undermines cryptographic security. The predictable nature of these sequence numbers enables attackers to perform TCP sequence number prediction attacks, where they can accurately guess the next expected sequence number in an established connection.
The operational impact of this vulnerability extends beyond simple network disruption to encompass full session hijacking capabilities that could allow unauthorized access to sensitive communications. An attacker who successfully predicts TCP sequence numbers can inject malicious data into existing connections without authentication, potentially leading to data manipulation, unauthorized access to network resources, or complete system compromise. This vulnerability particularly affects embedded systems and networked devices that rely on Nucleus networking stacks, making it relevant to industrial control systems, IoT devices, and network infrastructure components. The ability to spoof future connections means that attackers could establish new sessions that appear legitimate to the target system, bypassing authentication mechanisms and gaining unauthorized access to protected resources. The implications are particularly severe in environments where these products are deployed for critical infrastructure, as the vulnerability could enable attackers to compromise operational technology systems and potentially cause physical damage.
Mitigation strategies for CVE-2020-28388 require immediate attention through software updates and patches provided by the vendor, as the vulnerability affects multiple product lines and architectures. Organizations should prioritize upgrading to the latest versions of Nucleus NET and Nucleus ReadyStart that address the insufficient randomness issue in TCP ISN generation. Additionally, network administrators should implement monitoring solutions to detect unusual TCP sequence number patterns that might indicate exploitation attempts. The vulnerability's classification under ATT&CK technique T1071.004 for application layer protocol traffic and T1566 for credential access through network sniffing highlights the importance of network segmentation and traffic monitoring. Organizations should also consider implementing additional security controls such as TCP window scaling adjustments, connection rate limiting, and network intrusion detection systems to reduce the attack surface. The fix typically involves replacing the existing random number generator with a cryptographically secure implementation that provides adequate entropy and ensures that sequence numbers cannot be predicted by attackers. Regular security assessments and vulnerability scanning should be conducted to identify any other instances of weak random number generation within the network infrastructure.