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. Microsoft...
Vulnerability report
Exploited in the wild
Confirmed confidence
In CISA KEV
CVE-2021-43890
Windows AppX Installer Spoofing Vulnerability
Microsoft / App Installer · affected before publication
- Severity
- CVSS 7.1 · High
- Confidence
- Confirmed
- Exploit status
- Exploited in the wild
- EPSS
- 10.3%
- First observed
- —
- Last observed
- —
Decision summary
What security teams need to know first
Direct answers before the deeper technical record.
Is it exploited?
Yes. KEV Intelligence tracks this CVE as a known exploited vulnerability. 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.
Vendor
Product
Affected
Status
Microsoft
App Installer
Before publication
Affected
- Published
- 15 Dec 2021
- Exploitation Reported
- 15 Dec 2021
- Attack vector
- Remote
- Complexity
- High
- Privileges
- Low
- User interaction
- Required
Tags
cisa
windows
microsoft
malware
CVE References
- CVE Record CVE.org · CVE Record https://www.cve.org/CVERecord?id=CVE-2021-43890
- portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-43890 portal.msrc.microsoft.com · CVE Record https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CV...
- bleepingcomputer.com/news/microsoft/microsoft-disables-msix-proto... bleepingcomputer.com · CVE Record https://www.bleepingcomputer.com/news/microsoft/microsoft-disables-ms...
- thehackernews.com/2023/12/microsoft-disables-msix-app-installe... thehackernews.com · CVE Record https://thehackernews.com/2023/12/microsoft-disables-msix-app-install...
- microsoft.com/en-us/security/blog/2023/12/28/financially-m... microsoft.com · CVE Record https://www.microsoft.com/en-us/security/blog/2023/12/28/financially-...
- GitHub — ChrisTitusTech/winutil github.com · CVE Record https://github.com/ChrisTitusTech/winutil/pull/26
Exploitation evidence
Why KEV Intelligence marks this CVE as exploited
Third-party attestation and first-party sensor observation are shown separately so teams can judge the evidence chain.
Exploited in the wild
CISA
Independent exploitation attestation added to the KEV Intelligence record.
Used in malware
CISA
Malware families have been linked to exploitation of this CVE.
Known exploited vulnerability sources
Per-source evidence links for KEV attestations are available through the KEV Intelligence 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.
Observed signals
- Request targets
- 0
- User-Agents
- 0
Raw values available in Pro and Enterprise.
Scanner coverage
No scanner integrations recorded yet.
Virtual patch status
No KEV Intelligence 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
CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H
EPSS
10.3%
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
-
KEV confirmed by CVE
Exploitation attested by an external source
-
First public exploitation report
Exploit observed in malware
-
CVE published
Vulnerability disclosed publicly
-
Added to CISA KEV
Listed in the CISA Known Exploited Vulnerabilities catalog
-
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.
- 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
Free JSON includes basic KEV fields{
"cve_id": "CVE-2021-43890",
"confidence": "Confirmed",
"cvss_score": 7.1,
"cvss_estimated": false,
"epss_score": 0.10295,
"exploit_status": {
"exploited_in_the_wild": true,
"active_exploitation_observed": false
},
"sensor_telemetry": { "attempts": 0, "sensors": 0 }
}Early warning alerts
Get alerts on high-impact exploitation
Receive curator-selected alerts when exploitation activity warrants attention. Each alert includes the evidence and context needed to decide what requires attention now.