PatchTheCVECVE-2026-84009: Time-of-check time-of-use (toctou) race condition in Windows Projected File System allows an authorized attack
24
Routine
MEDIUM 4.7WATCHLISTUPGRADEUPGRADEUPGRADE
CVE-2026-84009 is a medium-severity flaw (CVSS 4.7) affecting windows 10 1809, windows 10 21h2, windows 10 22h2. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.
Time-of-check time-of-use (toctou) race condition in Windows Projected File System allows an authorized attacker to perform tampering locally.
Why this priority
- On your watchlist
A fixed release or distro patch exists.
Risk facts
| CVSS score | 4.7 (Medium) |
| Attack vector | Local |
| Attack complexity | High |
| Privileges needed | Low |
| User interaction | None |
| EPSS exploit chance | 0.2% (top 91%) |
| CISA exploited list | No |
| Weakness | CWE-367 |
| Published | 2026-10-13 |
| Last modified | 2026-10-19 |
Publisher (CNA): Microsoft Corporation · Vendor: microsoft
Last updated 2026-10-19 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score
Am I affected?
Timeline
- Published CVE record published
- Updated Record last modified
microsoft windows 10 1809 UPGRADE
Affected: < 10.0.17763.9376
Fixed in: 10.0.17763.9376
Safe to downgrade to: no unaffected older release
Check your OS release
cat /etc/os-release; uname -r
Debian / Ubuntu
sudo apt update && sudo apt full-upgrade
RHEL / Amazon Linux / Fedora
sudo dnf upgrade --refresh
Windows
Install the latest cumulative update from Windows Update
If no patch exists yet
OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.
microsoft windows 10 21h2 UPGRADE
Affected: < 10.0.19044.7807
Fixed in: 10.0.19044.7807
Safe to downgrade to: no unaffected older release
Check your OS release
cat /etc/os-release; uname -r
Debian / Ubuntu
sudo apt update && sudo apt full-upgrade
RHEL / Amazon Linux / Fedora
sudo dnf upgrade --refresh
Windows
Install the latest cumulative update from Windows Update
If no patch exists yet
OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.
microsoft windows 10 22h2 UPGRADE
Affected: < 10.0.19045.7807
Fixed in: 10.0.19045.7807
Safe to downgrade to: no unaffected older release
Check your OS release
cat /etc/os-release; uname -r
Debian / Ubuntu
sudo apt update && sudo apt full-upgrade
RHEL / Amazon Linux / Fedora
sudo dnf upgrade --refresh
Windows
Install the latest cumulative update from Windows Update
If no patch exists yet
OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.
microsoft windows 11 23h2 UPGRADE
Affected: < 10.0.22631.7652
Fixed in: 10.0.22631.7652
Safe to downgrade to: no unaffected older release
Check your OS release
cat /etc/os-release; uname -r
Debian / Ubuntu
sudo apt update && sudo apt full-upgrade
RHEL / Amazon Linux / Fedora
sudo dnf upgrade --refresh
Windows
Install the latest cumulative update from Windows Update
If no patch exists yet
OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.
microsoft windows 11 24h2 UPGRADE
Affected: < 10.0.26100.9711
Fixed in: 10.0.26100.9711
Safe to downgrade to: no unaffected older release
Check your OS release
cat /etc/os-release; uname -r
Debian / Ubuntu
sudo apt update && sudo apt full-upgrade
RHEL / Amazon Linux / Fedora
sudo dnf upgrade --refresh
Windows
Install the latest cumulative update from Windows Update
If no patch exists yet
OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.
microsoft windows 11 25h2 UPGRADE
Affected: < 10.0.26200.9711
Fixed in: 10.0.26200.9711
Safe to downgrade to: no unaffected older release
Check your OS release
cat /etc/os-release; uname -r
Debian / Ubuntu
sudo apt update && sudo apt full-upgrade
RHEL / Amazon Linux / Fedora
sudo dnf upgrade --refresh
Windows
Install the latest cumulative update from Windows Update
If no patch exists yet
OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.
Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
Quick answers
How do I fix CVE-2026-84009?
windows 10 1809: upgrade to 10.0.17763.9376; windows 10 21h2: upgrade to 10.0.19044.7807; windows 10 22h2: upgrade to 10.0.19045.7807. Exact commands for each OS are on this page.
Is CVE-2026-84009 being exploited?
CVE-2026-84009 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.
What does CVE-2026-84009 affect?
CVE-2026-84009 affects windows 10 1809, windows 10 21h2, windows 10 22h2. Severity is medium (CVSS 4.7). Time-of-check time-of-use (toctou) race condition in Windows Projected File System allows an authorized attacker to perform tampering locally.
How urgent is CVE-2026-84009?
Priority 24/100. On your watchlist
Software pages
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