CVE-2021-23998Network attack vector

UI spoofing via inherited HTTPS lock icon

Published Jun 24, 2021 · Updated Aug 3, 2024

UI spoofing in Mozilla Firefox before 88, Firefox ESR before 78.10, and Thunderbird before 78.10 allows attackers to misrepresent HTTP as secure. Complicated cross-window navigations let an attacker-controlled HTTP page inherit the secure lock icon associated with an HTTPS page. A user must open or follow the crafted navigation; the false indicator can support phishing by making an unencrypted page appear trustworthy without compromising transport security.

CVSS severity6.5
Medium
EPSS probability0.59%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

UI spoofing in Mozilla Firefox before 88, Firefox ESR before 78.10, and Thunderbird before 78.10 allows attackers to misrepresent HTTP as secure. Complicated cross-window navigations let an attacker-controlled HTTP page inherit the secure lock icon associated with an HTTPS page. A user must open or follow the crafted navigation; the false indicator can support phishing by making an unencrypted page appear trustworthy without compromising transport security.

The record

CVE
CVE-2021-23998
Published
Jun 24, 2021
Updated
Aug 3, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-345, T1036
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 24, 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 <88

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <88
Mozilla · Thunderbirdversion=unspecified <78.10
Mozilla · Firefox ESRversion=unspecified <78.10

Published CVSS scores

6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
4.3NIST NVDAV:N/AC:M/Au:N/C:N/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)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-345Insufficient Verification of Data Authenticity
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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