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Fencer Vulns
high 7.5

CVE-2020-27213 json

An issue was discovered in Ethernut Nut/OS 5.1.

published 2023-10-10 3 y ago modified 2026-06-17 4 mo ago also EUVD-2020-19732

cvss
7.5
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
1.0%
EPSS 1.0% chance of exploitation in 30 days, percentile 62%
public exploits
0
none known

Summary

An issue was discovered in Ethernut Nut/OS 5.1. 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. While the ISN generator seems to adhere to RFC 793 (where a global 32-bit counter is incremented roughly every 4 microseconds), proper ISN generation should aim to follow at least the specifications outlined in RFC 6528.

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

cisa ICSA-21-042-01 updated cvss 7.5

Multiple Embedded TCP/IP Stacks (Update B)

  • multiple Capital VSTAR multiple CycloneTCP <= 1.9.6 multiple FNET 4.6.3 multiple MPLAB Net <= 3.6.1 multiple NDKTCPIP <= 2.25 multiple Nucleus NET All < 5.2
    +9 more products
    multiple Nucleus ReadyStart for ARM MIPS and PPC < 2012.12 multiple Nucleus Source Code multiple Nut/Net <= 5.1 multiple picoTCP (EOL) <= 1.7.0 multiple picoTCP-NG <= 1.7.0 multiple uC/TCP-IP (EOL) <= 3.6.0 multiple uIP (EOL) <= 1.0 multiple uIP-Contiki-NG <= 4.5 multiple uIP-Contiki-OS (end-of-life [EOL]) <= 3.0
    affected: vendor fix, mitigation “uIP-Contiki-OS (end-of-life [EOL]). See general recommendations below.” github.com

From CISA ICS and IT advisories (CSAF, https://github.com/cisagov/CSAF), a U.S. Government work; changed: reduced to products, statuses and remediations. CISA notification · csaf

CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
partial
Assessed
2 y ago

Exploit prediction

0.97% chance of exploitation in the next 30 days, higher than 62% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • forescout.com https://www.forescout.com/resources/numberjack-weak-isn-generation-in-embedded-tcpip-stacks/ referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
NVD 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Classification

Weakness
  • CWE-330 Use of Insufficiently Random Values

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.