What is CVE-2021-30666?

CVE-2021-30666 is a high-severity vulnerability in Apple iOS, classified under Improper Restriction of Operations within the Bounds of a Memory Buffer. CVSS score: 8.8/10. Published 2021-09-08.

How severe is CVE-2021-30666?

High severity. CVSS v3 base score is 8.8 out of 10.

Is CVE-2021-30666 known to be exploited?

Yes. CVE-2021-30666 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03), indicating it is being actively exploited. 6 public proof-of-concept repositories are indexed.

Buffer overflow in Apple iOS

CVE-2021-30666

A buffer overflow issue was addressed with improved memory handling. This issue is fixed in iOS 12.5.3. Processing maliciously crafted web content may lead to arbitrary code execution. Apple is aware of a report that this issue may have been actively exploited..

Vulnerability class: Buffer Overflow

Published 2021-09-08 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 8.8 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.030 (87.1th percentile), scored 2026-10-19.

Low probability of exploitation in the next 30 days, but still tracked. 87th percentile — 87.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.030 · newest: 0.030 · change: 0.000

Affected products

Apple iOS — versions unspecified
Apple Iphone OS

Weakness classification (CWE)

CWE-119 — Improper Restriction of Operations within the Bounds of a Memory Buffer

CISA KEV (Known Exploited Vulnerabilities)

This CVE is on the CISA KEV catalog, added on 2021-11-03. CISA KEV inclusion means CISA has confirmed in-the-wild exploitation; US federal agencies are required to remediate within a published due date.

BOD 22-01 due date: 2021-11-17.

Required action: Apply updates per vendor instructions.

Public proof-of-concept exploits

References

support.apple.com/en-us/HT212341 (Release Notes, Vendor Advisory)
134c704f-9b21-4f2e-91b3-4a467353bcc0 (Third Party Advisory, US Government Resource)

Frequently asked questions

What is CVE-2021-30666?: CVE-2021-30666 is a high-severity vulnerability in Apple iOS, classified under Improper Restriction of Operations within the Bounds of a Memory Buffer. CVSS score: 8.8/10. Published 2021-09-08.
How severe is CVE-2021-30666?: High severity. CVSS v3 base score is 8.8 out of 10.
Is CVE-2021-30666 known to be exploited?: Yes. CVE-2021-30666 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03), indicating it is being actively exploited. 6 public proof-of-concept repositories are indexed.