CVE-2020-27213

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.0% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2023-10-10 · updated 2026-06-17
Weakness: CWE-330

Description

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.

Affected products

Ethernut Nut/Os: 5.1

References

http://lists.egnite.de/mailman/listinfo/en-nut-announce
http://www.ethernut.de/en/download/index.html
https://www.cisa.gov/news-events/ics-advisories/icsa-21-042-01
https://www.forescout.com/resources/numberjack-weak-isn-generation-in-embedded-tcpip-stacks/
http://lists.egnite.de/mailman/listinfo/en-nut-announce
http://www.ethernut.de/en/download/index.html
https://www.cisa.gov/news-events/ics-advisories/icsa-21-042-01
https://www.forescout.com/resources/numberjack-weak-isn-generation-in-embedded-tcpip-stacks/

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