CVE-2021-23980Public exploitNetwork attack vector

Script execution via namespace-changing HTML reparsing

Published Feb 16, 2023 · Updated Mar 19, 2025

Mutation XSS in Mozilla Bleach before 3.3.0 allows remote attackers to execute script when sanitized HTML is rendered in a browser. The bleach.clean parser leaves HTML comment contents unescaped; browser reparsing then uses p or br to eject a raw-text tag from an SVG or MathML namespace and expose executable markup. Exploitation requires a nondefault allowlist containing svg or math, p or br, and a raw-text tag, strip_comments=False, and a user who renders the result.

CVSS severity6.1
Medium
EPSS probability0.48%
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 Mozilla Bleach before 3.3.0 allows remote attackers to execute script when sanitized HTML is rendered in a browser. The bleach.clean parser leaves HTML comment contents unescaped; browser reparsing then uses p or br to eject a raw-text tag from an SVG or MathML namespace and expose executable markup. Exploitation requires a nondefault allowlist containing svg or math, p or br, and a raw-text tag, strip_comments=False, and a user who renders the result.

The record

CVE
CVE-2021-23980
Published
Feb 16, 2023
Updated
Mar 19, 2025
Vendor
Mozilla
Product
Bleach
Classifications
CWE-79, T1059.007
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 16, 2023CVE publishedPublication date reported by the CVE source.
  2. Mar 19, 2025Record 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=unspecified <3.3.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Bleachversion=unspecified <3.3.0

Published CVSS scores

6.1CISA-ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
6.1NISTCVSS: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

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1059.007Command and Scripting Interpreter: JavaScript

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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