CVE-2025-21418: in the CISA KEV catalogue

Microsoft Windows Ancillary Function Driver for WinSock Heap-Based Buffer Overflow Vulnerability. CISA added CVE-2025-21418 to the Known Exploited Vulnerabilities catalogue on 2025-02-11 with a default due date of 2025-03-04. Known ransomware campaign use: Unknown. Forensic triage flag: No.

Microsoft Windows Ancillary Function Driver for WinSock contains a heap-based buffer overflow vulnerability that allows for privilege escalation, enabling a local attacker to gain SYSTEM privileges. (CISA description)

Vendor / product: Microsoft Windows
CWE: CWE-122
CISA required action: Apply mitigations per vendor instructions or discontinue use of the product if mitigations are unavailable.
EPSS: 2.1%, percentile 81 (scored 2026-11-03)

EPSS puts about 2.1 in 100 odds (2.1%) on exploitation activity being observed in the next 30 days. Its score is higher than or equal to that of 81% of scored CVEs. This is a model estimate about exploitation anywhere, not about your environment.

Vendor and reference links (from CISA)

Your own deadline under CISA BOD 26-04 depends on exposure, automatability and technical impact: work it out with the BOD 26-04 calculator.

Data dates. CISA KEV catalogue version 2026.11.03, released 2026-11-03, 1,767 entries (checked 2026-11-03). EPSS model v2026.06.15, scored 2026-11-03, 399,388 CVEs. Sources: CISA and FIRST (EPSS by FIRST; scores generated by Empirical Security). Copied to our own database and refreshed automatically; if a refresh fails, the previous copy keeps serving with its own date.

Frequently asked questions

Is CVE-2025-21418 in the CISA KEV catalogue?

Yes. CISA added CVE-2025-21418 (Microsoft Windows Ancillary Function Driver for WinSock Heap-Based Buffer Overflow Vulnerability) to the Known Exploited Vulnerabilities catalogue on 2025-02-11. CISA's default due date for it is 2025-03-04.

Is CVE-2025-21418 used in ransomware attacks?

CISA marks known ransomware campaign use as Unknown for CVE-2025-21418. That means CISA lacks confirmation, not that ransomware groups do not use it.

What should I do about CVE-2025-21418?

CISA's required action: Apply mitigations per vendor instructions or discontinue use of the product if mitigations are unavailable.

Sources

CISA, Known Exploited Vulnerabilities Catalog (accessed 2026-11-03)
FIRST, Exploit Prediction Scoring System (EPSS) (EPSS by FIRST; scores generated by Empirical Security)
NVD record for CVE-2025-21418