CVE-2021-23974Public exploitNetwork attack vector

Mutation XSS via unescaped noscript serialization

Published Feb 26, 2021 · Updated Aug 3, 2024

Mutation XSS in Firefox before 86 allows remote attackers to bypass HTML sanitizers through crafted noscript content parsed by DOMParser. When DOMParser creates a document with scripting disabled, the HTML serializer treats noscript text as raw markup instead of escaping angle brackets and ampersands. A user must load attacker-controlled content in a sanitizer-dependent workflow; reserialization can turn inert text into executable elements in the sanitized page.

CVSS severity6.1
Medium
EPSS probability0.75%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Mutation XSS in Firefox before 86 allows remote attackers to bypass HTML sanitizers through crafted noscript content parsed by DOMParser. When DOMParser creates a document with scripting disabled, the HTML serializer treats noscript text as raw markup instead of escaping angle brackets and ampersands. A user must load attacker-controlled content in a sanitizer-dependent workflow; reserialization can turn inert text into executable elements in the sanitized page.

The record

CVE
CVE-2021-23974
Published
Feb 26, 2021
Updated
Aug 3, 2024
Vendor
Mozilla
Product
Firefox
Classifications
T1189
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 26, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 86

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=< 86

Published CVSS scores

4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:P/A:N
6.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

T1189Drive-by Compromise

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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