CVE-2020-35685

Name: CVE-2020-35685
Creator: NVD / NIST
Published: 2021-08-19T12:15:08.217+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

CRITICALCVSS 9.1/10EPSS 2.05%

Last modified Jun 17, 2026

CVE-2020-35685 is a critical-severity vulnerability rated 9.1/10 on the CVSS scale. 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. EPSS estimates a 2.05% chance of exploitation in the next 30 days.

Description

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.)

Metrics

CVSS 3.1

9.1/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

EPSS Probability

2.05%

78.8th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-330

Affected Software

Vendor	Product	Versions
Hcc-Embedded	Nichestack	3.0
Siemens	Sentron 3wa Com190 Firmware	< 2.0.0
Siemens	Sentron 3wl Com35 Firmware	< 1.2.0

References

https://cert-portal.siemens.com/productcert/pdf/ssa-789208.pdfMitigation, Third Party Advisory
https://www.forescout.com/blog/new-critical-operational-technology-vulnerabilities-found-on-nichestack/Mitigation, Third Party Advisory
https://www.hcc-embedded.comProduct
https://www.kb.cert.org/vuls/id/608209Third Party Advisory, US Government Resource
https://cert-portal.siemens.com/productcert/pdf/ssa-789208.pdfMitigation, Third Party Advisory
https://www.forescout.com/blog/new-critical-operational-technology-vulnerabilities-found-on-nichestack/Mitigation, Third Party Advisory
https://www.hcc-embedded.comProduct
https://www.kb.cert.org/vuls/id/608209Third Party Advisory, US Government Resource

Timeline

Published: Aug 19, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2020-35685?

How severe is CVE-2020-35685?

CVE-2020-35685 has a CVSS score of 9.1/10 (CRITICAL severity). The EPSS model estimates a 2.05% probability of exploitation in the next 30 days.

How do I fix CVE-2020-35685?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2020-35685?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST