CVE-2021-38492Public exploitNetwork attack vector

Script execution via unblocked mk scheme handoff

Published Nov 3, 2021 · Updated Aug 4, 2024

Improper protocol handling in Mozilla Firefox, Firefox ESR, and Thunderbird allows remote attackers to run script in Internet Explorer. The Networking component delegates mk:@MSITStore: links to Windows because the mk scheme is absent from the external-protocol blocklist. A user must open a crafted link on Windows; Internet Explorer then runs attacker-controlled JavaScript at Low or AppContainer integrity, with reported paths to Medium integrity.

CVSS severity6.5
Medium
EPSS probability1.16%
Next 30 days · Sep 17, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper protocol handling in Mozilla Firefox, Firefox ESR, and Thunderbird allows remote attackers to run script in Internet Explorer. The Networking component delegates mk:@MSITStore: links to Windows because the mk scheme is absent from the external-protocol blocklist. A user must open a crafted link on Windows; Internet Explorer then runs attacker-controlled JavaScript at Low or AppContainer integrity, with reported paths to Medium integrity.

The record

CVE
CVE-2021-38492
Published
Nov 3, 2021
Updated
Aug 4, 2024
Vendor
Mozilla
Product
Firefox
Classifications
T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Nov 3, 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 <92

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <92
Mozilla · Thunderbirdversion=unspecified <78.14; version=unspecified <91.1
Mozilla · Firefox ESRversion=unspecified <78.14; version=unspecified <91.1

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 11, 2026Sep 17, 2026
Latest reporting daySep 17, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

T1203Exploitation for Client Execution

Public exploit references

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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