What is CVE-2021-43399?

CVE-2021-43399 is a high-severity vulnerability in Yubico Yubihsm 2 Software Development Kit, classified under Out-of-bounds Write. CVSS score: 7.5/10. Published 2021-12-08.

How severe is CVE-2021-43399?

High severity. CVSS v3 base score is 7.5 out of 10.

Buffer overflow in Yubico Yubihsm 2 Software Development Kit

CVE-2021-43399

The Yubico YubiHSM YubiHSM2 library 2021.08, included in the yubihsm-shell project, does not properly validate the length of some operations including SSH signing requests, and some data operations received from a YubiHSM 2 device.

Vulnerability class: Buffer Overflow

Published 2021-12-08 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

EPSS exploit prediction

EPSS: 0.014 (71.8th percentile), scored 2026-10-07.

Low probability of exploitation in the next 30 days, but still tracked. 72nd percentile — 71.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.014 · newest: 0.014 · change: +0.000

Affected products

Yubico Yubihsm 2 Software Development Kit

Weakness classification (CWE)

CWE-787 — Out-of-bounds Write

References

www.yubico.com/support/security-advisories/ysa-2021-04/ (Vendor Advisory)
blog.inhq.net/posts/yubico-yubihsm-shell-vuln3/ (Exploit, Third Party Advisory)

Frequently asked questions

What is CVE-2021-43399?: CVE-2021-43399 is a high-severity vulnerability in Yubico Yubihsm 2 Software Development Kit, classified under Out-of-bounds Write. CVSS score: 7.5/10. Published 2021-12-08.
How severe is CVE-2021-43399?: High severity. CVSS v3 base score is 7.5 out of 10.