CVE-2020-35685 in NicheStack
Summary
by MITRE • 08/19/2021
An issue was discovered in HCC Nichestack 3.0. The code that generates Initial Sequence Numbers (ISNs) for TCP connections derives the ISN from an insufficiently random source. As a result, an attacker may be able to determine the ISN of current and future TCP connections and either hijack existing ones or spoof future ones. (Proper ISN generation should aim to follow at least the specifications outlined in RFC 6528.)
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Analysis
by VulDB Data Team • 08/21/2021
The vulnerability identified as CVE-2020-35685 affects HCC Nichestack 3.0, a TCP/IP stack implementation used in embedded systems and networked devices. This issue represents a critical weakness in the network protocol stack's security mechanisms, specifically in how it generates Initial Sequence Numbers for TCP connections. The flaw lies in the insufficient randomness of the entropy source used to create these critical sequence numbers, which forms the foundation of TCP connection security and integrity. Initial Sequence Numbers serve as the first sequence number in a TCP connection and are crucial for ensuring that data packets are correctly ordered and that connections remain secure against various attacks including sequence number prediction attacks.
The technical implementation of this vulnerability stems from the use of a weak random number generator or pseudo-random number generator that does not provide adequate entropy for generating TCP ISNs. According to RFC 6528, proper ISN generation should incorporate sufficient randomness to prevent attackers from predicting future sequence numbers. The insufficient entropy source in HCC Nichestack 3.0 means that an attacker with sufficient observation capabilities could potentially reconstruct the random number generator's state or predict future ISNs. This vulnerability falls under the CWE-330 category of using insufficiently random values, which is a well-documented weakness in cryptographic implementations. The predictable nature of ISNs enables attackers to perform TCP sequence number prediction attacks, which can lead to successful connection hijacking or spoofing operations.
The operational impact of this vulnerability is severe and far-reaching, particularly in embedded systems and IoT devices that rely on HCC Nichestack for network communications. An attacker who successfully predicts TCP sequence numbers can hijack active TCP connections, allowing them to inject malicious data, modify existing communications, or completely take control of network sessions. This capability extends to future connections as well, since the same weak entropy source generates ISNs for all new TCP connections. The vulnerability creates a persistent threat that can be exploited across multiple sessions and time periods, making it particularly dangerous for long-running network services. Network traffic interception and manipulation becomes possible, potentially leading to data breaches, service disruption, or complete system compromise depending on the networked application's security requirements.
Mitigation strategies for this vulnerability require immediate attention and implementation of multiple defensive measures. The primary solution involves updating to a patched version of HCC Nichestack that implements proper ISN generation according to RFC 6528 specifications, ensuring that the random number generator provides sufficient entropy and unpredictability. Organizations should also implement network monitoring solutions to detect unusual TCP sequence number patterns that might indicate exploitation attempts. Additional protective measures include implementing TCP timestamps, using connection tracking mechanisms, and deploying network segmentation to limit the scope of potential attacks. From an ATT&CK framework perspective, this vulnerability relates to techniques such as T1071.004 Application Layer Protocol: DNS and T1566.001 Phishing: Spearphishing Attachment, as attackers may exploit the compromised connections to establish further access or exfiltrate data. System administrators should also consider implementing intrusion detection systems that can monitor for TCP sequence number anomalies and other indicators of potential exploitation attempts.