Used in attacks· fix by 2025-10-27Not scored yetUsed by ransomware gangs
Microsoft Windows
CVE-2021-43226
Microsoft Windows Common Log File System Driver contains a privilege escalation vulnerability that could allow a local, privileged attacker to bypass certain security mechanisms.
What this means for your business
- It affects Microsoft Windows. It matters if your company, or a supplier that handles your data, runs it.
- Attackers are already using it. CISA added it to its list of flaws used in real attacks on 6 October 2025.
- Ransomware gangs are known to have used it.
- FIRST's prediction model gives it a 3% chance of attack attempts being seen in the next 30 days, ranking above 86% of all known flaws.
What to do
- 1Check whether your company or your suppliers run Microsoft Windows.
- 2If you do, follow CISA's required action below. No patch reference has been published yet.
- 3Treat it as urgent. US federal agencies must fix it by 27 October 2025. That deadline does not apply to private companies, but it shows how seriously CISA takes it.
- 4Because attackers have used it, check the affected systems for signs of a break-in, not only for the update.
Not sure if your company is exposed?
Fastnexa’s certified penetration testers can check whether attackers could use this flaw, or others like it, against your websites, apps and network. The full test is free for our first 10 founding clients until 31 December 2026.
Required action
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
CISA’s wording, unedited.
Scoring
- CVSS
- Not yet scored
Dates
- Published
- 2025-10-06
- Added to KEV
- 2025-10-06
- Remediation due
- 2025-10-27
- Sources
- CISA KEV