Microsoft — Microsoft HTTP Protocol Stack Remote Code Execution Vulnerability
CISA KEV · in the CISA KEV catalog since 2022-04-06
Listed in the CISA KEV catalog, and an EPSS probability of 0.999.
Out-of-Band / Urgent Remediation
The priority band answers “how soon”, not “how bad” — see Methodology for what it weighs and where it abstains.
| Measure | Value | Source | What it means |
|---|---|---|---|
| CVSS base score | 9.8 | NVD | How severe the flaw is in itself. It does not say how likely an attack is. |
| FIRST EPSS | 0.999 (99.96th pctl) | FIRST | FIRST's estimate of the chance this is exploited in the next 30 days, and how that ranks against all CVEs. |
| Actively exploited | Confirmed | CISA KEV | Whether a source of record has confirmed attacks using it in the wild. |
| Ransomware campaign use | Unknown or none | CISA KEV | Whether CISA has seen ransomware operators use it. |
| CISA remediation deadline | 2022-04-27 — KEV since 2022 · listed over a year | CISA KEV | The date US federal agencies must have fixed or mitigated it. |
| Fixed in | windows 10 2004: update to 10.0.19041.982 or later | windows 10 20h2: update to 10.0.19042.982 or later | windows server 2004: update to 10.0.19041.982 or later | windows server 20h2: update to 10.0.19042.982 or later | NVD | The version(s) the vendor has published as fixed, where NVD records one. |
| Public exploit module | Public module exists | Metasploit | Module existing is not the same as active exploitation. |
| EUVD cross-reference | Listed | ENISA EUVD | ENISA's own vulnerability database entry for this CVE, where one exists. |
Impacted software or hardware
Fixed versions: windows 10 2004: update to 10.0.19041.982 or later | windows 10 20h2: update to 10.0.19042.982 or later | windows server 2004: update to 10.0.19041.982 or later | windows server 20h2: update to 10.0.19042.982 or later
Remediation: Apply updates per vendor instructions.
Remediation basis: CISA KEV required action
ENISA EUVD also lists this CVE at CVSS 9.8 (v3.1), assigned by microsoft. Source: euvd.enisa.europa.eu.
Metasploit lists a public normal-rank module for this CVE: Windows IIS HTTP Protocol Stack DOS (dos/windows/http/http_sys_accept_encoding_dos_cve_2021_31166) — module existing is not the same as active exploitation; see metasploit.com.
This page is an opinion about urgency. Where it and a source of record disagree, the source of record is the record.
Where this sits
CVE-2021-31166 is a Microsoft entry in the CISA KEV catalog. The priority feed ranks Microsoft's current monthly release rather than older Microsoft CVEs, so this one is not listed there; the page above scores it from the same sources.
