CVE-2021-43890
Confirmed PUBLISHEDWindows AppX Installer Spoofing Vulnerability
Recommended Action
Prioritize remediation. Validate affected assets and apply vendor fixes on an accelerated timeline.
At a Glance
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.
- CVE Published
- Dec 15, 2021
- Exploitation Reported
- Dec 15, 2021
- CVSS
- 7.1 High
- EPSS
- —
Affected Versions
| Vendor | Product | Version | Status |
|---|---|---|---|
| Microsoft |
App Installer
|
1.0.0.0 to < publication |
Affected |
CVE References
- 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
Recommended Actions
- Prioritize remediation. Validate affected assets and apply vendor fixes on an accelerated timeline.
- Use the Pro API to automate enrichment, telemetry, and workflow delivery for VM, SOC, and CTI pipelines.
Known Exploited Vulnerability Sources
Catalogues that list this CVE as a known exploited vulnerability.
Per-source evidence links for KEV attestations are available through the KEVIntel Pro API.
Learn about Pro API access| Source | Added |
|---|---|
| CISA First | 2021-12-15 00:00 UTC |
No detection artifacts or sensor request patterns are available for this CVE yet.
Check back as sensor telemetry and scanner integrations are updated.
Virtual Patch
Compensating WAF rules to help reduce exposure to this CVE. Rule content and deployable vendor exports are available with KEVIntel Enterprise.
KEVIntel does not currently have a virtual patch for this CVE. When available, KEVIntel virtual patches ship as deployable ModSecurity, Cloudflare, and AWS WAF rules.
Enterprise feature. Virtual patch rule content and deployable vendor exports (ModSecurity, Cloudflare, AWS WAF) are available to KEVIntel Enterprise users.
CVSS Scores
CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C
Exploitation Status
Exploited in the wild
Recorded 2021-12-15 00:00:00 UTC · CISA
Used in malware
Recorded 2026-06-02 14:08:28 UTC · CISA
Timeline
Key exploitation, disclosure, scanner coverage, and KEV attestation events for this CVE.
-
14:08 UTC about 2 months ago14:08 UTC · about 2 months ago
First public exploitation report
Exploit observed in malware
-
14:15 UTC over 4 years ago14:15 UTC · over 4 years ago
CVE published
Vulnerability disclosed publicly
-
00:00 UTC over 4 years ago00:00 UTC · over 4 years ago
Added to CISA KEV
Listed in the CISA Known Exploited Vulnerabilities catalog
-
00:00 UTC over 4 years ago00:00 UTC · over 4 years ago
CVE ID reserved
Identifier reserved by the CNA
Automate This Intelligence with the Pro API
Confidence scoring, exploit status, sensor telemetry, PoCs, scanner integrations, mentions, and tags are available programmatically for VM, SOC, and CTI workflows.
Pro API Example
GET /api/v2/pro/kevs/CVE-2021-43890
{
"cve_id": "CVE-2021-43890",
"title": "Windows AppX Installer Spoofing Vulnerability",
"affected_vendor": "Microsoft",
"affected_product": "App Installer",
"affected_versions": [
{ "vendor": "...", "product": "...", "status": "affected", "display_label": "..." }
],
"confidence": "Confirmed",
"cvss_score": 7.1,
"epss_score": null,
"exploit_status": {
"exploited_in_the_wild": true,
"active_exploitation_observed": false
},
"sensor_telemetry": { "...": "Pro API fields" },
"proof_of_concepts": [ "..." ],
"scanner_integrations": [ "..." ]
}