CVE-2021-43890
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.
- CVSS
- 7.1Highscored by the publisher (CNA)
- Exploited in the wild
- Yes, in CISA KEV since 15 Dec 2021CISA deadline for US federal agencies: 29 Dec 2021known use in ransomware campaigns
- EPSS
- 11.3%chance of exploitation in the next 30 daystop 4% of scored CVEsFIRST score of 11 Nov 2026
- Fix
- Patch availablesee the advisories in the references
- Published
- 15 Dec 2021analysed here 12 Nov 2026
Do first. Apply updates per vendor instructions. (CISA's required action for this KEV entry)
Summary
Triage
- Exploitable
- At runtime
- Attack vector
- network
- Can a WAF block it?
- No. See why
The summary, impact and action text on this page are written by our AI analysis pipeline, or taken from the NVD description when a CVE was imported from NVD. They are not the vendor's wording: check them against the sources. A longer AI executive summary and technical analysis are generated for signed-in accounts on the Max plan.
Affected products
| Product | Affected versions | Vendor says fixed in |
|---|---|---|
| Microsoft App Installer | >= 1.0.0.0 and < publicationVendor says fixed in: not stated |
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.
Products named in our analysis
- microsoft:app_installer
Fix and mitigation
WAF and other mitigations
Can a WAF block it? No. The attack requires the user to open a specially crafted attachment, which means the malicious input is delivered through local user action rather than an HTTP request. Therefore, a web application firewall cannot filter or block this attack.
Also consider: patch
Based on an AI reading of the CVE description, and no vendor rule found yet. We check Cloudflare, AWS, Google Cloud Armor and Azure; for Akamai, Imperva, Fastly, F5, ask your vendor.
Remediation steps (AI-written)
Step-by-step remediation is generated for accounts on the Max plan, for CVEs that match a technology in their stack. Add this technology to your stack to see the fix checklist.
Exploitation evidence
- CISA KEV catalog
- Listed since 15 Dec 2021. Known use in ransomware campaigns.
- CISA due date
- 29 Dec 2021, the remediation deadline for US federal agencies
- CISA required action
- Apply updates per vendor instructions.
- Public exploit code
- None on record in Exploit-DB, Metasploit, Nuclei templates or PoC-in-GitHub.
- CISA SSVC
- exploitation active, automatable no, technical impact totalAssessed by CISA 14 Dec 2021 (SSVC 2.0.3). Exploitation is none, poc (public proof of concept) or active.
Timeline
- 14 Dec 2021CISA SSVC assessment: exploitation active
- 15 Dec 2021Added to CISA KEV (exploited in the wild)1 day later
- 15 Dec 2021Published1 day later
- 29 Dec 2021CISA remediation due date (US federal agencies)13 days later
- 6 Aug 2026CVE record last updated1,681 days (about 5 years) later
- 12 Nov 2026Analysed by vulneraibility.com98 days (about 3 months) later
CVSS vector
CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H
| Metric | Value | What it means |
|---|---|---|
| Attack vector | NetworkReachable over the internet or any routable network. | Reachable over the internet or any routable network. |
| Attack complexity | HighNeeds specific conditions, a race or a particular configuration. | Needs specific conditions, a race or a particular configuration. |
| Privileges required | LowThe attacker needs basic user privileges. | The attacker needs basic user privileges. |
| User interaction | RequiredA victim must click, open or otherwise cooperate. | A victim must click, open or otherwise cooperate. |
| Scope | UnchangedImpact stays within the vulnerable component. | Impact stays within the vulnerable component. |
| Confidentiality | HighTotal data disclosure possible. | Total data disclosure possible. |
| Integrity | HighComplete modification possible. | Complete modification possible. |
| Availability | HighFull outage possible. | Full outage possible. |
Common questions
Is CVE-2021-43890 being exploited?
Yes. CVE-2021-43890 appears in CISA's Known Exploited Vulnerabilities catalog, which means exploitation in the wild has been confirmed and reported. Treat it as active rather than theoretical, and patch it ahead of higher-scoring vulnerabilities that carry no exploitation evidence.
How severe is CVE-2021-43890?
CVE-2021-43890 is rated High with a CVSS score of 7.1. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2021-43890?
Yes. A fix has been recorded for CVE-2021-43890. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2021-43890 affect?
CVE-2021-43890 affects microsoft:app_installer. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
References
6 references in the CVE record, grouped by the tags the record gives them.
Other links (6)
- portal.msrc.microsoft.com /en-US/security-guidance/advisory/CVE-2021-43890
- bleepingcomputer.com /news/microsoft/microsoft-disables-msix-protocol-handler-abused-in-malware-attac
- thehackernews.com /2023/12/microsoft-disables-msix-app-installer.html
- microsoft.com /en-us/security/blog/2023/12/28/financially-motivated-threat-actors-misusing-app
- github.com /ChrisTitusTech/winutil/pull/26
- cisa.gov /known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-43890
Where the data on this page comes from
- CVE List (CVE.org): description, affected versions, references; CVE Terms of Use
- NVD: CVSS and CPE data
- CISA KEV catalog: exploitation, date added, due date
- CISA Vulnrichment (ADP): SSVC decision points
- FIRST EPSS: exploit probability and percentile
Summary, impact and triage tags: written by our AI analysis pipeline from the sources above. Check them against the sources.
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