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Known exploited · Apple

CVE-2021-1879: Apple iOS, iPadOS, and watchOS WebKit Cross-Site Scripting (XSS) Vulnerability

What it is

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.

Added November 3, 2021Due November 17, 2021CISA KEV catalog 2026.10.01

Vendor
Apple
Product
iOS, iPadOS, and watchOS
Added to KEV
November 3, 2021
CISA due date
November 17, 2021
Ransomware use
Unknown

What CISA requires

Apply updates per vendor instructions.

Forensic triage

No

What to do now

  1. Find every instance of iOS, iPadOS, and watchOS you run, including the forgotten ones, and check its version against the vendor advisory.
  2. Apply the vendor's fix or mitigation, before November 17, 2021 if you are bound by CISA's deadline, sooner if it faces the internet.
  3. Assume it was tried against you already: look for signs of exploitation from the date it was added, not from the day you patched.
  4. Until every instance is patched, block the attack at the edge: a WAF in blocking mode stops cross-site scripting payloads.

References

CVE-2021-1879 in the National Vulnerability Database

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The Dome's WAF runs the OWASP Core Rule Set in blocking mode and stops cross-site scripting payloads before they reach your application.

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Source: CISA Known Exploited Vulnerabilities Catalog (public domain), catalog version 2026.10.01.