CVE-2020-27633

criticalpriority · track · 36OT risk · elevatedstack campaign · NUMBER:JACKEPSS 0.009 · 59 %ileDEEURUUS

grimoire priorityP2 · Scheduled

Exploitable?

—

no known exploitation

Likely / severe?

0.87% (59th) EPSS

CVSS 9.1 · critical

Is it mine?

35 products

Check your exposure

Who says so?

5 sources

cnnvd · enisa_euvd · fstec_bdu · cert_bund

EPSS · scored 2026-10-27

In FNET 4.6.3, TCP ISNs are improperly random.

remediations

Vendor fix, mitigation, or workaround guidance, as published by each source.

A vendor fix is available.

FixedCapital VSTARcisa_csaf_ot
uIP-Contiki-NG has a patch in progress. See general recommendations below until a patch is made available.
FixedCycloneTCPcisa_csaf_ot
uIP-Contiki-NG has a patch in progress. See general recommendations below until a patch is made available.
FixedFNETcisa_csaf_ot
uIP-Contiki-NG has a patch in progress. See general recommendations below until a patch is made available.
FixedMPLAB Netcisa_csaf_ot
uIP-Contiki-NG has a patch in progress. See general recommendations below until a patch is made available.
FixedNDKTCPIPcisa_csaf_ot
uIP-Contiki-NG has a patch in progress. See general recommendations below until a patch is made available.
FixedNucleus NET Allcisa_csaf_ot
uIP-Contiki-NG has a patch in progress. See general recommendations below until a patch is made available.
FixedNucleus ReadyStart for ARM MIPS and PPCcisa_csaf_ot
uIP-Contiki-NG has a patch in progress. See general recommendations below until a patch is made available.
FixedNucleus Source Codecisa_csaf_ot
uIP-Contiki-NG has a patch in progress. See general recommendations below until a patch is made available.
FixedNut/Netcisa_csaf_ot
uIP-Contiki-NG has a patch in progress. See general recommendations below until a patch is made available.
FixedpicoTCP (EOL)cisa_csaf_ot
uIP-Contiki-NG has a patch in progress. See general recommendations below until a patch is made available.
FixedpicoTCP-NGcisa_csaf_ot
uIP-Contiki-NG has a patch in progress. See general recommendations below until a patch is made available.
FixeduC/TCP-IP (EOL)cisa_csaf_ot
uIP-Contiki-NG has a patch in progress. See general recommendations below until a patch is made available.
FixeduIP (EOL)cisa_csaf_ot
uIP-Contiki-NG has a patch in progress. See general recommendations below until a patch is made available.
FixeduIP-Contiki-NGcisa_csaf_ot
uIP-Contiki-NG has a patch in progress. See general recommendations below until a patch is made available.
FixeduIP-Contiki-OS (end-of-life [EOL])cisa_csaf_ot
uIP-Contiki-NG has a patch in progress. See general recommendations below until a patch is made available.
MitigationCapital VSTARcisa_csaf_ot
Nut/Net has a patch in progress. See general recommendations below until a patch is made available.
MitigationCycloneTCPcisa_csaf_ot
Nut/Net has a patch in progress. See general recommendations below until a patch is made available.
MitigationFNETcisa_csaf_ot
Nut/Net has a patch in progress. See general recommendations below until a patch is made available.
MitigationMPLAB Netcisa_csaf_ot
Nut/Net has a patch in progress. See general recommendations below until a patch is made available.
MitigationNDKTCPIPcisa_csaf_ot
Nut/Net has a patch in progress. See general recommendations below until a patch is made available.
MitigationNucleus NET Allcisa_csaf_ot
Nut/Net has a patch in progress. See general recommendations below until a patch is made available.
MitigationNucleus ReadyStart for ARM MIPS and PPCcisa_csaf_ot
Nut/Net has a patch in progress. See general recommendations below until a patch is made available.
MitigationNucleus Source Codecisa_csaf_ot
Nut/Net has a patch in progress. See general recommendations below until a patch is made available.
MitigationNut/Netcisa_csaf_ot
Nut/Net has a patch in progress. See general recommendations below until a patch is made available.
MitigationpicoTCP (EOL)cisa_csaf_ot
Nut/Net has a patch in progress. See general recommendations below until a patch is made available.
MitigationpicoTCP-NGcisa_csaf_ot
Nut/Net has a patch in progress. See general recommendations below until a patch is made available.
MitigationuC/TCP-IP (EOL)cisa_csaf_ot
Nut/Net has a patch in progress. See general recommendations below until a patch is made available.
MitigationuIP (EOL)cisa_csaf_ot
Nut/Net has a patch in progress. See general recommendations below until a patch is made available.
MitigationuIP-Contiki-NGcisa_csaf_ot
Nut/Net has a patch in progress. See general recommendations below until a patch is made available.
MitigationuIP-Contiki-OS (end-of-life [EOL])cisa_csaf_ot
Nut/Net has a patch in progress. See general recommendations below until a patch is made available.

cvss

Technical severity, 0–10. One row per scoring source.

9.1

010

base score 0–10 · higher = more severe · hover a vector to decode⚠ sources disagree · medium–critical · 6.5–9.1

v3.19.1criticalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Nprimary · NVD / NIST USexploitability 3.9 · impact 5.2

v3.07.5highCVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:Nsecondary · CISA US

v3.16.5mediumAV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:Nsecondary · FSTEC (BDU) RU

v3.18.3highCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/E:Usecondary · CVSS-BT (t0sche)

ssvc

A stakeholder decision — act, attend, or track — from exploitation and impact.

Exploitation: none · Automatable: yes · Technical Impact: totalcoordinatorcisa_ssvcUS

weaknesses

The CWE weakness classes this flaw belongs to.

CWE-330USprimaryCWE-330USCWE-330US

affected products (35)

Products and version ranges known to be affected.

Open Source TCP/IP Stackall versionscert_bundDEcpe:/a:open_source:tcpip_stack:-
Siemens Nucleus RTOSall versionscert_bundDEcpe:/o:siemens:nucleus_rtos:-
Capital VSTAR: All Versionsall versionscisa_csaf_otUS
CycloneTCP: Version 1.9.6 and priorall versionscisa_csaf_otUS
FNET: Version 4.6.3all versionscisa_csaf_otUS
MPLAB Net: Version 3.6.1 and priorall versionscisa_csaf_otUS
NDKTCPIP: Version 2.25 and priorall versionscisa_csaf_otUS
Nucleus NET All: versions prior to Version 5.2all versionscisa_csaf_otUS
Nucleus ReadyStart for ARM MIPS and PPC: All versions prior to Version 2012.12all versionscisa_csaf_otUS
Nucleus Source Code: All Versionsall versionscisa_csaf_otUS
Nut/Net: Version 5.1 and priorall versionscisa_csaf_otUS
picoTCP (EOL): Version 1.7.0 and priorall versionscisa_csaf_otUS
picoTCP-NG: Version 1.7.0 and priorall versionscisa_csaf_otUS
uC/TCP-IP (EOL): Version 3.6.0 and priorall versionscisa_csaf_otUS
uIP (EOL): Version 1.0 and priorall versionscisa_csaf_otUS
uIP-Contiki-NG: Version 4.5 and priorall versionscisa_csaf_otUS
uIP-Contiki-OS (end-of-life [EOL]): Version 3.0 and priorall versionscisa_csaf_otUS
multiple Capital VSTARall versionscisa_csaf_otUS
multiple CycloneTCP≤ 1.9.6cisa_csaf_otUS
multiple FNETall versionscisa_csaf_otUS

showing 20 of 35

advisories (2)

Vendor and ecosystem advisories describing this.

WID-SEC-W-2023-1987TCP/IP Stack: Mehrere SchwachstellenDeutsch2021-02-11csaf · cert_bundDE
Ein entfernter, anonymer Angreifer kann mehrere Schwachstellen in TCP/IP Stack ausnutzen, um beliebigen Programmcode auszuführen, einen Denial-of-Service-Zustand herbeiführen oder Sicherheitsvorkehrungen umgehen.
ICSA-21-042-01Multiple Embedded TCP/IP Stacks (Update B)2021-02-11icsa · cisa_csaf_otUS
Successful exploitation of weak initial sequence numbers (ISN) can be used to hijack or spoof TCP connections, cause denial-of-service conditions, inject malicious data, or bypass authentication.

also known as (4)

Other names and identifiers for this entity.

sources

Which sources assert this record, and when.

cnnvd enisa_euvd fstec_bdu cert_bund cisa_csaf_ot NVD CVE.org MITRE

published 2023-10-10modified 2026-06-17

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+568 more

references (11)

External citations — patches, advisories, analysis.

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