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CVE-2021-4221: If a domain name contained a RTL character, it would cause the domain to be rendered to the right of the path.

If a domain name contained a RTL character, it would cause the domain to be rendered to the right of the path. This could lead to user confusion and spoofing attacks. <br>*This bug only affects Firefox for Android. Other operating systems are unaffected.*<br>*Note*: Due to a clerical error this advisory was not included in the original announcement, and was added in Feburary 2022. This vulnerability affects Firefox < 92.

MediumCVSS 4.3Not KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

CVE-2021-4221 is a Firefox for Android URL display flaw. A domain containing a right-to-left character could appear to the right of the path, making a malicious or misleading link look more trustworthy. The main business risk is phishing and spoofing, not system takeover.

Executive priority

Treat this as a moderate-priority mobile phishing risk. It should be remediated through browser updates and device inventory, but it does not indicate remote code execution or confirmed active exploitation in the provided sources.

Technical view

Mozilla describes a rendering issue in Firefox for Android where right-to-left characters in a domain could reorder how the domain and path were displayed. It affects Firefox versions before 92. CVSS 3.1 is 4.3, with network attack vector and required user interaction.

Likely exposure

Exposure is limited to users running Firefox for Android versions earlier than 92. Mozilla states other operating systems are unaffected. Organizations without mobile browser inventory may have uncertain exposure.

Exploitation context

The provided sources do not show active exploitation, and the CVE is not marked KEV. Abuse would rely on user interaction and visual confusion in the browser address display, supporting spoofing or phishing scenarios.

Researcher notes

The advisory was added later because of a clerical error, so historical reporting may be incomplete. Focus validation on Firefox for Android URL rendering exposure and version state, not desktop Firefox or other operating systems.

Mitigation direction

  • Update Firefox for Android to version 92 or later.
  • Confirm managed Android devices no longer run affected Firefox versions.
  • Check Mozilla advisory MFSA2021-38 for vendor guidance.
  • Prioritize mobile users exposed to untrusted links or phishing risk.

Validation and detection

  • Inventory Android devices for installed Firefox versions.
  • Confirm affected versions are below Firefox 92 only.
  • Verify other operating systems are excluded from scope.
  • Review mobile security telemetry for spoofing or phishing reports.
Prepared
Confidence
high
Sources
4

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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cve · low confidence lookup

CVE-2021-4221 mapping review

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Vulnerability profileCVE Program record
Severity
Medium
CVSS
4.3 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

1CVSS vectors
0Timeline events
0ADP providers
3Source links

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
4.3CVSS 3.1MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N2.81.4Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

4.3Medium
CVSS 3.1 vector shape for CVE-2021-4221Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
MozillaFirefoxunspecifiedListed
Weakness

CWE details

CWE-1007

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CWE-1007 · source CWE mapping

Insufficient Visual Distinction of Homoglyphs Presented to User

Insufficient Visual Distinction of Homoglyphs Presented to User represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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