Windows AppX Installer Spoofing Vulnerability
CVE-2021-43890 is a spoofing vulnerability in Microsoft Windows AppX/App Installer. Microsoft reported that attackers could abuse the App Installer process, including the ms-appinstaller URI scheme, with specially crafted packages delivered via phishing or malicious links. Supporting reporting indicates the issue allowed a malicious .appinstaller/.appxbundle package to present trusted-looking publisher information in the App Installer UI despite inconsistencies with the underlying signing identity, enabling malware delivery under the guise of legitimate software. Observed campaigns used this mechanism to distribute malware families including Emotet, TrickBot, and BazarLoader/BazarBackdoor. Microsoft assigned CVE-2021-43890 to this App Installer process issue and fixed it in the January 2022 Patch Tuesday release; later, Microsoft also disabled the ms-appinstaller protocol by default to reduce abuse.
Are you exposed to this one?
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Impact, mitigation & remediation
What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.
Impact
What an attacker gets, and what they’ve been doing with it.
Mitigation
If you can’t patch tonight, do this now.
Remediation
Patch, then assume compromise.
Exploits
No public exploits tracked yet. Mallory keeps watching.
No public exploit code observed for this vulnerability.
Affected products & vendors
Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.
Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.
Recent activity
6 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
A Microsoft vulnerability that CISA KEV’s knownRansomwareCampaignUse field silently flipped to Known during 2025 (evidence of ransomware campaign use).
A Windows App Installer process vulnerability abused to deliver malware via ms-appinstaller links and malicious .appinstaller/.appxbundle packages, enabling deceptive installation of BazarBackdoor/BazarLoader payloads.
Unknown (the content only lists this CVE as a reference and does not describe the vulnerability, impact, affected products, or exploitation details).
A spoofing vulnerability in the Windows AppX Installer.
The version that knows your environment.
Query your assets running an affected version, and investigate the blast radius.
Every observed campaign linking this CVE to a named adversary.
Malware families riding this exploit, with evidence and IOCs.
YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.
Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.