Is CVE-2021-40523 actively exploited?

No active exploitation of CVE-2021-40523 has been confirmed so far. Continue monitoring, as exploit status can change.

Can CVE-2021-40523 be exploited remotely?

Yes. CVE-2021-40523 is exploitable remotely over the network (CVSS attack vector: Network).

What is the CVSS score of CVE-2021-40523?

CVE-2021-40523 has a CVSS score of 7.5 (HIGH).

HOME VULNERABILITIESCVE-2021-40523

HIGH

CVE-2021-40523

CWE-754Published: September 5, 2021· Updated: Jun 17, 2026

7.5

CVSS v3.1

Official 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.

NVD Source

Technical Analysis

CVE-2021-40523 can be exploited remotely over the network without requiring physical or adjacent access, significantly expanding the attack surface for threat actors.

The vulnerability requires no privileges and no user interaction, making it a prime target for automated exploitation campaigns and worm-like propagation.

A successful exploit results in availability disruption (denial of service), with a CVSS base score of 7.5.