CVE-2021-29975Public exploitNetwork attack vector

UI spoofing via persistent form validation popup

Published Aug 5, 2021 · Updated Aug 3, 2024

UI spoofing in Firefox before 90 allows remote attackers to display attacker-chosen text over another site's content after user interaction. A page calls reportValidity() from a canvas.toBlob callback, makes the form content editable, and navigates or closes the tab without dismissing the validation popup. A victim must activate the crafted page; the persistent browser-generated message can be mistaken for content from the domain in the address bar and can conceal a site's action prompt.

CVSS severity6.5
Medium
EPSS probability0.96%
Next 30 days · Sep 17, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

UI spoofing in Firefox before 90 allows remote attackers to display attacker-chosen text over another site's content after user interaction. A page calls reportValidity() from a canvas.toBlob callback, makes the form content editable, and navigates or closes the tab without dismissing the validation popup. A victim must activate the crafted page; the persistent browser-generated message can be mistaken for content from the domain in the address bar and can conceal a site's action prompt.

The record

CVE
CVE-2021-29975
Published
Aug 5, 2021
Updated
Aug 3, 2024
Vendor
Mozilla
Product
Firefox
Classifications
T1036
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 5, 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=unspecified <90

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <90

Published CVSS scores

6.5UbuntuCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/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

T1036Masquerading

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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