CVE-2020-35685

CRITICALNVD 9.19.1—
EchelonGraph scoreMEDIUM confidence

This critical-severity CVE scores 9.1 under NVD CVSS v3. EPSS exploit probability: 2.1%, top 21% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, nvd
9.1
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 2%CVSS: 9.1Exploit: NoneExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

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.)

CVSS v3
9.1
EG Score
9.1(medium)
EG Risk
—
EPSS
79.2%
KEV
Not listed

Published

August 19, 2021

Last Modified

November 21, 2024

Advisory Details (4)

Auto-updated Jul 22, 2026
No patch confirmed yet. Sources: cert.
cert

VU#608209 - NicheStack embedded TCP/IP has vulnerabilities

https://www.kb.cert.org/vuls/id/608209
generic

Data Integrity Software | Embedded & Enterprise | Tuxera

https://www.hcc-embedded.com
generic

New Critical Operational Technology Vulnerabilities Found on NicheStack – Mitigation Advised - Forescout

https://www.forescout.com/blog/new-critical-operational-technology-vulnerabilities-found-on-nichestack/
generic
https://cert-portal.siemens.com/productcert/pdf/ssa-789208.pdf

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 11× in last 7d / 42× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

  1. 2026-07-22 22:39 UTCEG score recompute
  2. 2026-07-22 14:05 UTCEPSS rescore
  3. 2026-07-21 15:22 UTCEPSS rescore
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  1. 2026-07-06 16:25 UTCEPSS rescore
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  60. 2026-05-26 13:42 UTCEPSS rescore
  61. 2026-05-26 13:42 UTCEPSS rescore
  62. 2026-05-26 11:48 UTCEG score recompute
  63. 2026-05-26 11:48 UTCGHSA enrichment
  64. 2026-05-26 07:17 UTCEPSS rescore
  65. 2026-05-26 07:17 UTCEPSS rescore

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Frequently asked(5)

What is CVE-2020-35685?
CVE-2020-35685 is a critical vulnerability published on August 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…
When was CVE-2020-35685 disclosed?
CVE-2020-35685 was first published in the National Vulnerability Database on August 19, 2021, with the most recent update on November 21, 2024. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2020-35685 actively exploited?
CVE-2020-35685 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 79.2% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2020-35685?
CVE-2020-35685 has a CVSS v3 base score of 9.1 (NVD).
How do I remediate CVE-2020-35685?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2020-35685, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

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