CVE-2021-43890 — Microsoft Windows AppX Installer Spoofing Vulnerability
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.
Who exploits it and how urgent it is
A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.
How the score was calculated
- Confirmed exploitation (CISA KEV)+35
- Used in ransomware campaigns (CISA)+10
- EPSS 11.3% (30-day exploitation probability)+14
- Technical severity (high)+6
The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).
Triage
Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.
CISA SSVC assessment CISA
Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.
Remediation
Fixes and mitigations documented by the sources — linked to the original advisory.
- Documented patch — portal.msrc.microsoft.com (NIST National Vulnerability Database)
CISA required action CISA
Apply updates per vendor instructions.
US federal deadline: Dec 29, 2021 — passed 1751 day(s) ago
First 24 hours
Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.
- Identify exposed assets running the affected vendor/product/version combinations (Microsoft App Installer).
- Treat as emergency remediation: the CVE is in CISA KEV (confirmed exploitation).
- Apply the fixed version or the patch documented under Remediation.
- Exploitation requires user interaction: tighten email/web filtering and warn affected users.
- EPSS ahead of 96% of scored CVEs: prioritize it in the patch queue.
- Search available logs for exploit probes, errors and authentication anomalies.
Affected scope
Vendor, product and versions as the sources word them.
| Vendor | Product | Versions | Status | Source |
|---|---|---|---|---|
| Microsoft | App Installer | < 1.16< 1.11 | Vulnerable | NIST National Vulnerability Database |
| Microsoft | App Installer | ≥ 1.0.0.0 and < publication | Vulnerable | CVE Program (cve.org) e CISA ADP |
Attack characteristics
The CVSS vector, decoded. It describes the attack, not your exposure to it.
Every base score collected
Sources score independently and may disagree; each row says who scored it and under which version.
| Score | Version | Severity | Expl. | Impact | Scored by |
|---|---|---|---|---|---|
| 7.1 | CVSS 3.1 | High | 1.2 | 5.9 | secure@microsoft.com via NIST National Vulnerability Database |
| 7.1 | CVSS 3.1 | High | 1.2 | 5.9 | nvd@nist.gov via NIST National Vulnerability Database |
| 7.1 | CVSS 3.1 | High | — | — | GitHub, Inc. via GitHub Security Advisories |
| 7.1 | CVSS 3.1 | High | — | — | microsoft via CVE Program (cve.org) e CISA ADP |
| 6.0 | CVSS 2.0 | Medium | 6.8 | 6.4 | nvd@nist.gov via NIST National Vulnerability Database |
CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H (score shown at the top: nvd@nist.gov) CISA KEV details CISA
CISA lists a CVE once it has evidence of exploitation against real targets.
CISA records ransomware use only for campaigns it has confirmed, so “Unknown” means unrecorded, not ruled out. The deadline binds US federal agencies; for everyone else it is a reference date.
EPSS over time FIRST
Oct 13, 2026 → Oct 14, 2026 · 2 reading(s) · max 11.29%
Detection
Read off the CVSS vector and the weakness class. Starting points, not tested rules.
- Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
- Hunt retroactively in logs from before Dec 15, 2021 (KEV listing): exploitation may have started earlier.
Timeline
What happened to this CVE, newest first.
- 2026
- Affected products added · microsoft/app installer
- SSVC assessment updated
- Record published
- Fix documented
- Affected products added · microsoft/app installer
- CVSS changed · 7.1
- Source status changed · → Analyzed
- Added to CISA KEV (detected)
- Record created in this database
- 2021
- CVE published
- Added to CISA KEV catalog
Source activity
Repeated readings are counted rather than listed.
- 2× References added · Oct 13, 2026 – Oct 13, 2026
- 1× Official description updated · Oct 13, 2026 – Oct 13, 2026
References
10 on the record
- www.microsoft.com/en-us/security/blog/2023/12/28/financially-motivated-threat-actors-mi
- portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-43890
- www.cisa.gov/known-exploited-vulnerabilities-catalog
- www.microsoft.com/en-us/security/blog/2023/12/28/financially-motivated-threat-actors-mi
- github.com/ChrisTitusTech/winutil/pull/26
- github.com/advisories/GHSA-hvcq-2mcq-rg5f
- thehackernews.com/2023/12/microsoft-disables-msix-app-installer.html
- www.bleepingcomputer.com/news/microsoft/microsoft-disables-msix-protocol-handler-abused-in-mal
- nvd.nist.gov/vuln/detail/CVE-2021-43890
- www.bleepingcomputer.com/news/microsoft/microsoft-disables-msix-protocol-handler-abused-in-mal
Elsewhere on this site
Related CVEs (same products)
- High CVE-2026-68821 Improper Privilege Management in Microsoft App Installer Aug 11, 2026
Not in any source we poll
Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.
- No CWE classification.
- No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
- No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
- No exploit packet captures, log samples, or incident case IDs supplied.
Answered from this record
Text assembled only from the fields above — no additional claims.
What should defenders know first?
CVE-2021-43890 is "Microsoft Windows AppX Installer Spoofing Vulnerability", rated High. It affects App Installer (Microsoft); versions are in the scope table. It is in CISA KEV: there is evidence of real-world exploitation. CVSS 7.1 describes technical severity. EPSS 11.29% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.
Comments
Moderated technical discussion. Read the community guidelines.
No comments yet.
Sign in or create an account to comment and suggest corrections.