CVE-2021-43542Network attack vector

Installed-application disclosure via protocol-handler error oracle

Published Dec 8, 2021 · Updated Aug 4, 2024

Information disclosure in Firefox before 95 and Firefox ESR and Thunderbird before 91.4.0 allows remote attackers to identify installed applications. The XMLHttpRequest path returns distinguishable error results when a requested external protocol has a registered handler, creating an application-presence oracle. A victim must load attacker-controlled web content; the reported consequence is disclosure of installed applications, not their data or execution.

CVSS severity6.5
Medium
EPSS probability1.71%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Information disclosure in Firefox before 95 and Firefox ESR and Thunderbird before 91.4.0 allows remote attackers to identify installed applications. The XMLHttpRequest path returns distinguishable error results when a requested external protocol has a registered handler, creating an application-presence oracle. A victim must load attacker-controlled web content; the reported consequence is disclosure of installed applications, not their data or execution.

The record

CVE
CVE-2021-43542
Published
Dec 8, 2021
Updated
Aug 4, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-209, T1518
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 8, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 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 <95

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <95
Mozilla · Thunderbirdversion=unspecified <91.4.0
Mozilla · Firefox ESRversion=unspecified <91.4.0

Published CVSS scores

4.3NIST NVDAV:N/AC:M/Au:N/C:P/I:N/A:N
6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/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-209Generation of Error Message Containing Sensitive Information
T1518Software Discovery

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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