CVE-2021-4221Public exploitNetwork attack vector

Address-bar spoofing via RTL domain reordering

Published Dec 22, 2022 · Updated Apr 16, 2025

Address-bar spoofing in Mozilla Firefox for Android before 92 allows remote attackers to misrepresent a visited site's domain using an RTL name. The toolbar removes the URL scheme for display without forcing left-to-right direction, so a leading RTL domain is visually reordered to the right of the path. Exploitation requires a user to open a crafted URL, after which attacker-controlled content can appear to belong to the path's named site.

CVSS severity4.3
Medium
EPSS probability0.41%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Address-bar spoofing in Mozilla Firefox for Android before 92 allows remote attackers to misrepresent a visited site's domain using an RTL name. The toolbar removes the URL scheme for display without forcing left-to-right direction, so a leading RTL domain is visually reordered to the right of the path. Exploitation requires a user to open a crafted URL, after which attacker-controlled content can appear to belong to the path's named site.

The record

CVE
CVE-2021-4221
Published
Dec 22, 2022
Updated
Apr 16, 2025
Vendor
Mozilla
Product
Firefox
Classifications
CWE-1007, T1036
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 22, 2022CVE publishedPublication date reported by the CVE source.
  2. Apr 16, 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 <92

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <92

Published CVSS scores

4.3CISA ADP VulnrichmentCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/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.

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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-1007Insufficient Visual Distinction of Homoglyphs Presented to User
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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