Vulnerability report

Trusted Third Party Report Confirmed Confidence In CISA KEV

CVE-2021-43890

Windows AppX Installer Spoofing Vulnerability

Microsoft / App Installer · affected before publication

Watch CVE-2021-43890 Browse KEV feed
Severity
CVSS 7.1 · High
Confidence
Confirmed
Exploit status
Trusted Third Party Report
EPSS
11.3%
First observed
—
Last observed
—

Decision summary

What security teams need to know first

Direct answers before the deeper technical record.

What it is

CVE-2021-43890 is Windows AppX Installer Spoofing Vulnerability affecting Microsoft App Installer. We have investigated reports of a spoofing vulnerability in AppX installer that affects Microsoft Windows.

Have exploitation attempts been observed?

Yes. Exploitation attempts have been reported for this vulnerability. Previdian lists it as a known exploited vulnerability. Listed in CISA KEV. Also reported by third-party sources. Confidence is confirmed.

Who is affected?

Microsoft / App Installer affected before publication.

What should we do?

Prioritize remediation. Validate affected assets and apply vendor fixes on an accelerated timeline.

Overview

We have investigated reports of a spoofing vulnerability in AppX installer that affects Microsoft Windows

We have investigated reports of a spoofing vulnerability in AppX installer that affects Microsoft Windows. Microsoft is aware of attacks that attempt to exploit this vulnerability by using specially crafted packages that include the malware family known as Emotet/Trickbot/Bazaloader. An attacker could craft a malicious attachment to be used in phishing campaigns. The attacker would then have to convince the user to open the specially crafted attachment. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights.

Please see the Security Updates table for the link to the updated app. Alternatively you can download and install the Installer using the links provided in the FAQ section. Please see the Mitigations and Workaround sections for important information about steps you can take to protect your system from this vulnerability. December 27 2023 Update: In recent months, Microsoft Threat Intelligence has seen an increase in activity from threat actors leveraging social engineering and phishing techniques to target Windows OS users and utilizing the ms-appinstaller URI scheme. To address this increase in activity, we have updated the App Installer to disable the ms-appinstaller protocol by default and recommend other potential mitigations.

Microsoft Affected
App Installer
Before publication
Published
15 Dec 2021
Exploitation Attempts Reported
15 Dec 2021
Attack vector
Remote
Complexity
High
Privileges
Low
User interaction
Required

Tags

cisa windows microsoft malware

CVE References

Exploitation evidence

Evidence of exploitation attempts

Third-party attestation and first-party sensor observation are shown separately so teams can judge the evidence chain.

Exploitation Attempts Observed in the Wild

CISA

Recorded 15 Dec 2021

A trusted third party reported exploitation attempts.

Targeted by Malware-Related Exploitation Attempts

CISA

Recorded 02 Jun 2026

Exploitation attempts against this vulnerability are associated with malware-related payloads.

Known exploited vulnerability sources

Per-source evidence links for KEV attestations are available through the Previdian Pro API.

Learn about Pro API access
Source Added
CISA First 2021-12-15 00:00 UTC
CVE 2026-08-04 04:41 UTC

Detection

Operational artifacts and observed signals

Make the evidence actionable in scanner, SOC, and edge-control workflows.

Scanner coverage

No scanner integrations recorded yet.

Virtual patch status

No Previdian virtual patch is currently available. Future rules ship for ModSecurity, Cloudflare, and AWS WAF.

Learn about virtual patches →

No detection artifacts or sensor request patterns are available for this CVE yet.

Check back as sensor telemetry and scanner integrations are updated.

Risk and context

Severity, weaknesses, and research context

CVSS v3.1

7.1 High

Potential damage if exploited. Separate from whether attackers are using it.

CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H

EPSS

11.3%

Estimated chance of exploitation in the next 30 days. Previdian's warning comes from evidence, not this score.

All CVSS Scores

CVSS v3.1 7.1 High

CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H

CVSS v2.0 6.0 Medium

AV:N/AC:M/Au:S/C:P/I:P/A:P

Timeline

From disclosure to observed exploitation attempts

  1. 04:41 UTC

    KEV confirmed by CVE

    Exploitation attempts reported by an external source

  2. 14:08 UTC

    Targeted by malware-related exploitation attempts

    Malware-related exploitation attempts or payloads reported

  3. 14:15 UTC

    CVE published

    Vulnerability disclosed publicly

  4. 00:00 UTC

    Added to CISA KEV

    Listed in the CISA Known Exploited Vulnerabilities catalog

  5. 00:00 UTC

    CVE ID reserved

    Identifier reserved by the CNA

Pro API

Automate this intelligence

Confidence, exploit status, sensor telemetry, PoCs, scanner integrations, mentions, and tags are available programmatically for VM, SOC, and CTI workflows.

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  • Evidence confidence and provenance
  • First-party sensor telemetry
  • PoC and scanner references
  • Affected versions and enrichment
  • Automation-ready JSON delivery

GET /api/v2/pro/kevs/CVE-2021-43890

Sample API response 
{
  "cve_id": "CVE-2021-43890",
  "confidence": "Confirmed",
  "cvss_score": 7.1,
  "cvss_estimated": false,
  "epss_score": 0.11294,
  "exploit_status": {
    "exploited_in_the_wild": true,
    "active_exploitation_observed": false
  },
  "sensor_telemetry": { "attempts": 0, "sensors": 0 }
}