CVE-2020-35685: Hcc-Embedded Nichestack

Critical severity, CVSS 9.1. EPSS: 2.1% chance of exploitation in the next 30 days.

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.)

Affected products

• Hcc-Embedded Nichestack: version 3.0 only
• Siemens Sentron 3wa COM190 Firmware: before 2.0.0 (fixed in 2.0.0)
• Siemens Sentron 3wl COM35 Firmware: before 1.2.0 (fixed in 1.2.0)

Published 2021-08-19. Last modified 2026-06-17.