CVE-2020-27213

Name: CVE-2020-27213
Creator: NVD / NIST
Published: 2023-10-10T17:15:10.337+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

HIGHCVSS 7.5/10EPSS 0.97%

Last modified Jun 17, 2026

CVE-2020-27213 is a high-severity vulnerability rated 7.5/10 on the CVSS scale. 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. EPSS estimates a 0.97% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1

7.5/10

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

EPSS Probability

0.97%

57.5th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-330

Affected Software

Vendor	Product	Versions
Ethernut	Nut\/Os	5.1

References

http://lists.egnite.de/mailman/listinfo/en-nut-announceNot Applicable
http://www.ethernut.de/en/download/index.htmlRelease Notes
https://www.cisa.gov/news-events/ics-advisories/icsa-21-042-01Third Party Advisory, US Government Resource
https://www.forescout.com/resources/numberjack-weak-isn-generation-in-embedded-tcpip-stacks/Exploit, Third Party Advisory
http://lists.egnite.de/mailman/listinfo/en-nut-announceNot Applicable
http://www.ethernut.de/en/download/index.htmlRelease Notes
https://www.cisa.gov/news-events/ics-advisories/icsa-21-042-01Third Party Advisory, US Government Resource
https://www.forescout.com/resources/numberjack-weak-isn-generation-in-embedded-tcpip-stacks/Exploit, Third Party Advisory

Timeline

Published: Oct 10, 2023
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2020-27213?

How severe is CVE-2020-27213?

CVE-2020-27213 has a CVSS score of 7.5/10 (HIGH severity). The EPSS model estimates a 0.97% probability of exploitation in the next 30 days.

How do I fix CVE-2020-27213?

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-2020-27213?

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

Scan your code now

Source: NVD / NIST