What is CVE-2021-1821?

CVE-2021-1821 is a medium-severity vulnerability in Apple macOS. CVSS score: 6.5/10. Published 2021-10-28.

How severe is CVE-2021-1821?

Medium severity. CVSS v3 base score is 6.5 out of 10.

Vulnerability in Apple macOS

CVE-2021-1821

A logic issue was addressed with improved state management. This issue is fixed in watchOS 7.6, macOS Big Sur 11.5. Visiting a maliciously crafted webpage may lead to a system denial of service.

Published 2021-10-28 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H.

EPSS exploit prediction

EPSS: 0.007 (49.7th percentile), scored 2026-10-02.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 50th percentile — 49.7% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.007 · newest: 0.007 · change: 0.000

Affected products

Apple macOS — versions unspecified
Apple watchOS

References

support.apple.com/en-us/HT212602 (Vendor Advisory)
support.apple.com/en-us/HT212605 (Vendor Advisory)

Frequently asked questions

What is CVE-2021-1821?: CVE-2021-1821 is a medium-severity vulnerability in Apple macOS. CVSS score: 6.5/10. Published 2021-10-28.
How severe is CVE-2021-1821?: Medium severity. CVSS v3 base score is 6.5 out of 10.