What is CVE-2020-9866?

CVE-2020-9866 is a critical-severity vulnerability in Apple macOS, classified under Buffer Copy without Checking Size of Input (Classic Buffer Overflow). CVSS score: 9.8/10. Published 2020-10-27.

How severe is CVE-2020-9866?

Critical severity. CVSS v3 base score is 9.8 out of 10.

Buffer overflow in Apple macOS

CVE-2020-9866

A buffer overflow was addressed with improved bounds checking. This issue is fixed in macOS Catalina 10.15.6, Security Update 2020-004 Mojave, Security Update 2020-004 High Sierra. A buffer overflow may result in arbitrary code execution.

Vulnerability class: Buffer Overflow

Published 2020-10-27 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.019 (79.4th percentile), scored 2026-10-01.

Low probability of exploitation in the next 30 days, but still tracked. 79th percentile — 79.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.019 · newest: 0.019 · change: 0.000

Affected products

Apple macOS — versions unspecified
Apple Mac OS X

Weakness classification (CWE)

CWE-120 — Buffer Copy without Checking Size of Input (Classic Buffer Overflow)

References

support.apple.com/en-us/HT211289 (Release Notes, Vendor Advisory)

Frequently asked questions

What is CVE-2020-9866?: CVE-2020-9866 is a critical-severity vulnerability in Apple macOS, classified under Buffer Copy without Checking Size of Input (Classic Buffer Overflow). CVSS score: 9.8/10. Published 2020-10-27.
How severe is CVE-2020-9866?: Critical severity. CVSS v3 base score is 9.8 out of 10.