CVE-2021-1879: in the CISA KEV catalogue

Apple iOS, iPadOS, and watchOS WebKit Cross-Site Scripting (XSS) Vulnerability. CISA added CVE-2021-1879 to the Known Exploited Vulnerabilities catalogue on 2021-11-03 with a default due date of 2021-11-17. Known ransomware campaign use: Unknown. Forensic triage flag: No.

Apple iOS, iPadOS, and watchOS WebKit contain an unspecified vulnerability that allows for universal cross-site scripting (XSS) when processing maliciously crafted web content. This vulnerability could impact HTML parsers that use WebKit, including but not limited to Apple Safari and non-Apple products which rely on WebKit for HTML processing. (CISA description)

Vendor / product: Apple iOS, iPadOS, and watchOS
CWE: CWE-79
CISA required action: Apply updates per vendor instructions.
EPSS: 7.1%, percentile 94 (scored 2026-11-03)

EPSS puts about 7.1 in 100 odds (7.1%) on exploitation activity being observed in the next 30 days. Its score is higher than or equal to that of 94% of scored CVEs. This is a model estimate about exploitation anywhere, not about your environment.

Vendor and reference links (from CISA)

https://nvd.nist.gov/vuln/detail/CVE-2021-1879

Your own deadline under CISA BOD 26-04 depends on exposure, automatability and technical impact: work it out with the BOD 26-04 calculator.

Data dates. CISA KEV catalogue version 2026.11.03, released 2026-11-03, 1,767 entries (checked 2026-11-03). EPSS model v2026.06.15, scored 2026-11-03, 399,388 CVEs. Sources: CISA and FIRST (EPSS by FIRST; scores generated by Empirical Security). Copied to our own database and refreshed automatically; if a refresh fails, the previous copy keeps serving with its own date.

Frequently asked questions

Is CVE-2021-1879 in the CISA KEV catalogue?

Yes. CISA added CVE-2021-1879 (Apple iOS, iPadOS, and watchOS WebKit Cross-Site Scripting (XSS) Vulnerability) to the Known Exploited Vulnerabilities catalogue on 2021-11-03. CISA's default due date for it is 2021-11-17.

Is CVE-2021-1879 used in ransomware attacks?

CISA marks known ransomware campaign use as Unknown for CVE-2021-1879. That means CISA lacks confirmation, not that ransomware groups do not use it.

What should I do about CVE-2021-1879?

CISA's required action: Apply updates per vendor instructions.

Sources

CISA, Known Exploited Vulnerabilities Catalog (accessed 2026-11-03)
FIRST, Exploit Prediction Scoring System (EPSS) (EPSS by FIRST; scores generated by Empirical Security)
NVD record for CVE-2021-1879