CVE-2021-40523: In Contiki 3.0, Telnet option negotiation is mishandled

This month

HIGH 7.5REMOTEMITIGATE

CVE-2021-40523 is a high-severity flaw (CVSS 7.5) affecting contiki. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

In Contiki 3.0, Telnet option negotiation is mishandled. During negotiation between a server and a client, the server may fail to give the WILL/WONT or DO/DONT response for DO and WILL commands because of improper handling of exception condition, which leads to property violations and denial of service. Specifically, a server sometimes sends no response, because a fixed buffer space is available for all responses and that space may have been exha

Why this priority

High severity
Remotely exploitable, no login, no user action

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.0% (top 38%)
CISA exploited list	No
Weakness	CWE-754
Published	2021-09-05
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: contiki-os

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-09-05 19:15 UTCPublished CVE record published
2026-06-17 04:07 UTCUpdated Record last modified

contiki-os contiki MITIGATE

Affected: 3.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 3.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-40523?

contiki: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-40523 being exploited?

CVE-2021-40523 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-40523 affect?

CVE-2021-40523 affects contiki. Severity is high (CVSS 7.5). In Contiki 3.0, Telnet option negotiation is mishandled. During negotiation between a server and a client, the server may fail to give the WILL/WONT or DO/DONT response for DO and WILL commands because of improper handling of exception cond

How urgent is CVE-2021-40523?

Priority 47/100. High severity; Remotely exploitable, no login, no user action

Related CVEs

CVE-2021-38387 HIGH 7.5 MITIGATE In Contiki 3.0, a Telnet server that silently quits (before disconnection with clients) leads to connected cli
CVE-2021-38386 HIGH 7.5 MITIGATE In Contiki 3.0, a buffer overflow in the Telnet service allows remote attackers to cause a denial of service b
CVE-2021-28362 HIGH 7.5 UPGRADE An issue was discovered in Contiki through 3.0
CVE-2020-24336 CRITICAL 9.8 UPGRADE UPGRADE An issue was discovered in Contiki through 3.0 and Contiki-NG through 4.5
CVE-2020-13986 HIGH 7.5 UPGRADE An issue was discovered in Contiki through 3.0

References

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