# What is CVE-2021-43890? > CVE-2021-43890 is a high-severity flaw (CVSS 7.1) in App Installer from Microsoft, published on 15 December 2021. It is on the list of flaws known to be exploited, so criminals are already using it. The latest forecast puts its chance of use in the next 30 days at 11%. If you run an affected version, patch it first. Canonical page: https://www.precursorintelligence.com/what-is/cve-2021-43890 ## At a glance - **Verdict:** Fix this now. Attackers are using this flaw today. If you run an affected version, it goes to the top of your list. - **Precursor Intelligence risk score:** 74 out of 100 (High); action: Emergency patch. Kept up to date as the evidence changes. - **Severity:** High (CVSS 7.1 out of 10) - **Known to be exploited:** Yes, it is on the government list of flaws known to be exploited - **Chance of use in the next 30 days:** 11% (latest exploit-likelihood forecast) - **Published:** 15 December 2021 - **Record last updated:** 7 March 2025 - **Main product:** App Installer from Microsoft - **Affected products:** 12 ## What is CVE-2021-43890? In plain terms It can be attacked from anywhere on the internet with an ordinary user account, but a person has to take an action first. If it works, the attacker can read data, change data and take it offline. ### The official description 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. ### What kind of flaw is it? - Unknown: Unknown ### How an attack on it works - Where an attacker can be: Anywhere on the internet - How hard it is to pull off: Hard: conditions must line up - Login needed first: An ordinary user account - Does someone have to click: Yes, a person has to take an action - What an attacker gets: Can read data, change data and take it offline Severity vector: `CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H` ## Is CVE-2021-43890 being exploited? Yes Attackers are already using it. It is on the government list of flaws known to be exploited, and the latest forecast puts the chance of it being used in the next 30 days at 11%. Exploit likelihood over the last 7 days: 11%, 11%, 11%, 11%, 11%, 11%, 11%. ### Timeline - 15 December 2021: Published - 7 March 2025: Record last updated - 16 June 2026: Exploit likelihood fell ### How Precursor Intelligence ranks it CVE-2021-43890 scores **74 out of 100 (High)**, with the action "Emergency patch". Every published flaw gets a risk score from 0 to 100, kept up to date as the evidence changes. Four things go into the score, from the one that counts most to the ones that count less: - Evidence it is being exploited: biggest factor - How severe it is: strong factor - How widely exposed it is: smaller factor - Threat activity around it: smaller factor 85 and above is Critical. Anything confirmed as exploited is always marked for an emergency patch, whatever its score. When this flaw matches software you track, it lands in your list with its score and the reasons behind it. ## Who is affected by CVE-2021-43890? The published record lists 12 products. Versions are as published: "before 1.16" means every version up to, but not including, 1.16. - [App Installer](https://www.precursorintelligence.com/product/microsoft/app_installer) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Application): before 1.16, before 1.11 - [Windows 10 1507](https://www.precursorintelligence.com/product/microsoft/windows_10_1507) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): version not specified - [Windows 10 1709](https://www.precursorintelligence.com/product/microsoft/windows_10_1709) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): version not specified - [Windows 10 1803](https://www.precursorintelligence.com/product/microsoft/windows_10_1803) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): version not specified - [Windows 10 1809](https://www.precursorintelligence.com/product/microsoft/windows_10_1809) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): version not specified - [Windows 10 1903](https://www.precursorintelligence.com/product/microsoft/windows_10_1903) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): version not specified - [Windows 10 1909](https://www.precursorintelligence.com/product/microsoft/windows_10_1909) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): version not specified - [Windows 10 2004](https://www.precursorintelligence.com/product/microsoft/windows_10_2004) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): version not specified - [Windows 10 20H2](https://www.precursorintelligence.com/product/microsoft/windows_10_20h2) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): version not specified - [Windows 10 21H1](https://www.precursorintelligence.com/product/microsoft/windows_10_21h1) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): version not specified - [Windows 10 21H2](https://www.precursorintelligence.com/product/microsoft/windows_10_21h2) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): version not specified - [Windows 11 21H2](https://www.precursorintelligence.com/product/microsoft/windows_11_21h2) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): version not specified ## How do I fix CVE-2021-43890? 1. **Check whether you run it.** Compare your versions with the affected products above. Not sure what you run? Precursor Intelligence can work it out from your public website, or you can pick it from a list. 2. **Apply the vendor’s fix.** Install the update named in the vendor’s advisory. We’ve put the advisories and patch notes first in the list. 3. **If you can’t patch today.** Follow any mitigation the vendor publishes, and limit who can reach the affected system from the internet until the update is in. 4. **Check it wasn’t used before you patched.** Because criminals are already using this flaw, look for signs of a break-in on affected systems using the vendor’s guidance. ### References - [portal.msrc.microsoft.com](https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-43890) (Patch, Vendor advisory) - [microsoft.com](https://www.microsoft.com/en-us/security/blog/2023/12/28/financially-motivated-threat-actors-misusing-app-installer/) (Exploit, Vendor advisory) - [github.com](https://github.com/ChrisTitusTech/winutil/pull/26) (Issue tracking) - [thehackernews.com](https://thehackernews.com/2023/12/microsoft-disables-msix-app-installer.html) (Press/media coverage, Third-party advisory) - [bleepingcomputer.com](https://www.bleepingcomputer.com/news/microsoft/microsoft-disables-msix-protocol-handler-abused-in-malware-attacks/) (Press/media coverage, Third-party advisory) ## Questions ### What is CVE-2021-43890? CVE-2021-43890 is a high-severity flaw in App Installer from Microsoft, published on 15 December 2021. It can be attacked from anywhere on the internet with an ordinary user account, but a person has to take an action first. If it works, the attacker can read data, change data and take it offline. ### How serious is CVE-2021-43890? It scores 7.1 out of 10 on the CVSS severity scale, which is high. Our risk score for CVE-2021-43890 is 74 out of 100, which is High. Its action is "Emergency patch". Attackers are using this flaw today. If you run an affected version, it goes to the top of your list. ### Is CVE-2021-43890 being exploited? Attackers are already using it. It is on the government list of flaws known to be exploited, and the latest forecast puts the chance of it being used in the next 30 days at 11%. ### Which versions does CVE-2021-43890 affect? For App Installer from Microsoft: before 1.16, before 1.11. The record lists 12 products in all, each with its versions, under "Who is affected" on this page. ### How do I fix CVE-2021-43890? Install the update the vendor names in its advisory, linked first under "How do I fix" on this page. If you can't patch today, follow the vendor's mitigation and limit who can reach the affected system from the internet. Because criminals are already using it, also check affected systems for signs of a break-in. ## Does CVE-2021-43890 affect you? Precursor Intelligence keeps checking for you: - **Tell us what you run.** Pick your software from a list, or let us work it out from your website. - **We match it to every flaw.** Over 400,000 published flaws, checked against your list. - **Ranked by real-world risk.** Every flaw gets a 0 to 100 risk score, kept up to date, led by evidence criminals are using it. - **Told as soon as we know.** An alert by email, in the app or by webhook to your own tools. 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