How severe is CVE-2021-40523?

CVE-2021-40523 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.

Is CVE-2021-40523 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (57th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-40523?

CVE-2021-40523 affects Contiki-os Contiki. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-40523?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

When was CVE-2021-40523 published?

CVE-2021-40523 was published on 2021-09-05 and last updated on 2026-06-17.

CVE-2021-40523

CVE-2021-40523 is a high-severity vulnerability in Contiki-os Contiki with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-754.

Key facts

Severity: High (CVSS 3.x base score 7.5)
CVSS v2: 5.0
EPSS exploit prediction: 1% (57th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-754
Affected product: Contiki-os Contiki
Published: 2021-09-05
Last modified: 2026-06-17

Description

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 exhausted.

Frequently asked questions

What is CVE-2021-40523?: 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 exhausted.
How severe is CVE-2021-40523?: CVE-2021-40523 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2021-40523 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (57th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-40523?: CVE-2021-40523 affects Contiki-os Contiki. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-40523?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2021-40523 published?: CVE-2021-40523 was published on 2021-09-05 and last updated on 2026-06-17.

References

https://github.com/contiki-os/contiki/issues/2686

Affected products (1)

cpe:2.3:o:contiki-os:contiki:3.0:*:*:*:*:*:*:*

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