CVE-2021-23976

UI spoofing via inverted manifest-directory check

Published Feb 26, 2021 · Updated Aug 3, 2024

Improper manifest validation in Mozilla Firefox for Android before 86 allows local users to spoof trusted web content through a malicious app intent. FennecWebAppIntentProcessor's isUnderFennecManifestDirectory check reverses its directory-name and parent comparisons, accepting arbitrary manifest paths and origins. A malicious installed app can consequently launch an attacker-declared web app in fullscreen and target another origin, enabling UI spoofing and cross-origin access.

CVSS severity8.1
High
EPSS probability1.10%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper manifest validation in Mozilla Firefox for Android before 86 allows local users to spoof trusted web content through a malicious app intent. FennecWebAppIntentProcessor's isUnderFennecManifestDirectory check reverses its directory-name and parent comparisons, accepting arbitrary manifest paths and origins. A malicious installed app can consequently launch an attacker-declared web app in fullscreen and target another origin, enabling UI spoofing and cross-origin access.

The record

CVE
CVE-2021-23976
Published
Feb 26, 2021
Updated
Aug 3, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-1021, T1036
Attack vector
local
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=< 86

What conditions does exploitation require?

Attack vectorlocal
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=< 86

Published CVSS scores

8.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
5.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/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)
Vectorlocal
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-1021Improper Restriction of Rendered UI Layers or Frames
T1036Masquerading

Public exploit references

No public exploit references are available in this record.

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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