CVE-2021-23957Known exploitationPublic exploitNetwork attack vector

Iframe sandbox escape via intent URL

Published Feb 26, 2021 · Updated Aug 3, 2024

Sandbox protection failure in Firefox for Android before 85 allows remote attackers to escape iframe sandbox restrictions via crafted intent URLs. The Android-specific intent URL handling in a sandboxed iframe treats a navigation as an external app launch, bypassing restrictions such as required top-navigation or user-activation permissions. A malicious webpage or advertisement can trigger the navigation without user interaction, forcing another application or landing page to open; the issue does not affect Firefox on desktop operating systems.

CVSS severity7.4
High
EPSS probability0.79%
Next 30 days · Sep 16, 2026
Known exploitationReported
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Sandbox protection failure in Firefox for Android before 85 allows remote attackers to escape iframe sandbox restrictions via crafted intent URLs. The Android-specific intent URL handling in a sandboxed iframe treats a navigation as an external app launch, bypassing restrictions such as required top-navigation or user-activation permissions. A malicious webpage or advertisement can trigger the navigation without user interaction, forcing another application or landing page to open; the issue does not affect Firefox on desktop operating systems.

The record

CVE
CVE-2021-23957
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=< 85

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=< 85

Published CVSS scores

7.4NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/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.

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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 exploitationReported
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.

Reported exploitation

Technologies

Your stack

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Your stack

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