Threats / Microsoft / CVE-2021-43890
CVE-2021-43890
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-08-04
Microsoft Windows vulnerability
Microsoft Windows AppX Installer spoofing vulnerability enabling unauthorized package installation with high impact to confidentiality, integrity, and availability.
Verdict
Today item, not a backlog item.
Spoofing vulnerability in Windows AppX Installer allows attackers to masquerade as legitimate application sources, leading to unauthorized code execution and system compromise. Active exploitation and ransomware deployment observed.
CISA KEV Yes · 2021-12-155Ransomware use Flagged5EPSS 0.10295 (verify live)6Exploit Weaponized · public PoC7CVSS 7.13CISA SSVC Exploitation: active · Automatable: slow · Impact: total4
01
Is it exploitable?
— the evidence, ranked above the scoreExploit available
Fully weaponized — public exploit code is cataloged for this vulnerability.We link the existence of the exploit; we do not host or redistribute payloads.
Reported exploitation
3 independent public reports of in-the-wild exploitation are cataloged.Distinct reporting sources (vendor, incident response, government); open them for the underlying claims.
Exploited in the wild
Listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-12-15), flagged for known ransomware use.
Probability (EPSS)
EPSS 0.10295 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 7.1 (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) · Affected: Microsoft, Windows. Confirm exact fixed builds in the vendor advisory.CVSS — CNA-assigned (CVE.org), version 3.1, set at disclosure.
CISA SSVC (triage disposition)
Exploitation: active · Automatable: slow · Technical Impact: totalCISA's own stakeholder-specific triage inputs, published in the CVE record (CISA Coordinator role). NOTE: this is CISA's three decision points; the final Act/Attend/Track decision also requires a Mission & Well-being input not published here, so no overall decision is asserted.
Currency of the record
CISA added this to its KEV catalog the same day the CVE record was published.KEV lag = CISA-KEV dateAdded − CVE.org datePublished; a floor on how long the known-exploited flaw sat in the public record before the federal catalog listed it. Severity/weakness here is CNA- or CISA-assigned (current at disclosure), independent of NVD's analysis backlog.
02
Who’s exploiting it?
— attribution turns risk into urgencyAttribution not established
No confirmed (advisory-backed) threat-actor attribution is established for this record. Absence of a named actor is not absence of compromise — see Coverage & confidence.
03
Why it matters
— the attack path, told twice: adversary, then board1
Front door — unauthenticated access narrative 1
Attacker
I craft a malicious AppX package that spoofs a legitimate application identity to bypass installer validation.
Business
Attackers gain ability to deploy unauthorized software, creating entry point for ransomware and data exfiltration campaigns.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I distribute the spoofed package through compromised or social engineering vectors, exploiting user trust in Windows installation mechanisms.
Business
Widespread infection potential across enterprise and consumer Windows deployments without user awareness of malicious origin.
3
Lateral reach — past segmentation narrative 3
Attacker
I execute arbitrary code with the privileges of the installation process to establish persistence and lateral movement.
Business
Complete system compromise enabling ransomware deployment, data theft, and operational disruption across affected infrastructure.
04
What to do
— defensible action- Remediate per the vendor advisory — confirm the fixed build for your version and verify exposure.1
Say it to the boardA vulnerability with this evidence profile is a defensible budget line, not a backlog ticket — fund the change against the proof above.
05