CVE-2021-31166: Microsoft HTTP Protocol Stack Remote Code Execution Vulnerability
- Is it exploited?
Yes. CISA added it to the Known Exploited Vulnerabilities catalog on , 330 days after the CVE record was published.
- How likely is exploitation in the next 30 days?
- 99.9%
EPSS score, percentile 100 of all scored records
- How severe?
- 9.8 Critical
CVSS v3.1 base score, given by the CNA
- What should I do?
CISA deadline . Apply updates per vendor instructions.
The CVE record links no vendor advisory, patch or mitigation.
- Which versions are affected?
Affected
- from 10.0.0 to before 10.0.19041.982
- from 10.0.0 to before 10.0.19041.982
- from 10.0.0 to before 10.0.19042.982
- from 10.0.0 to before 10.0.19042.982
Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of ; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of ; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of . About the data.
What to do
Required action, set by CISA
Apply updates per vendor instructions.
CISA due date for this action: .
Advisories and patches
The CVE record links no vendor advisory, patch or mitigation.
Linked from CISA's catalog entry
Affected products and versions
As listed in the CVE record.
| Status | Versions |
|---|---|
| Affected | from 10.0.0 to before 10.0.19041.982 |
| Status | Versions |
|---|---|
| Affected | from 10.0.0 to before 10.0.19041.982 |
| Status | Versions |
|---|---|
| Affected | from 10.0.0 to before 10.0.19042.982 |
| Status | Versions |
|---|---|
| Affected | from 10.0.0 to before 10.0.19042.982 |
Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of . About the data.
About this vulnerability
Microsoft HTTP Protocol Stack contains a vulnerability in http.sys that allows for remote code execution.
Summary: CISA's KEV catalog, as of .
- CISA added CVE-2021-31166 to its Known Exploited Vulnerabilities (KEV) catalog on 6 Apr 2022.
- CISA's ransomware flag for this entry is "Unknown": no evidence of ransomware use in the catalog, which is not proof of none.
- The due date CISA sets for the required action is 27 Apr 2022.
- The CVE record was published on 11 May 2021, 330 days before it was added to KEV.
- The CVE record carries a CVSS v3.1 base score of 9.8 (Critical), given by its CNA.
- FIRST's EPSS model puts the probability of exploitation in the next 30 days at 99.9% (percentile 100), as of 28 Oct 2026.
- CISA's SSVC assessment in the CVE record: exploitation active, automatable yes, technical impact total.
Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of ; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of ; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of . About the data.
Description
HTTP Protocol Stack Remote Code Execution Vulnerability
HTTP Protocol Stack Remote Code Execution Vulnerability
Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of . About the data.
Weakness
- CWE-416: Use After Free
CWE and CAPEC are maintained by MITRE; the entries are those named in the CVE record and the KEV entry.
Timeline
| Date | Event |
|---|---|
| CVE record published | |
| Added to CISA KEV | |
| CISA due date for the required action | |
| CVE record last updated |
References
More exploited Microsoft vulnerabilities
- CVE-2026-96951: Microsoft Windows Command Injection Vulnerability
- CVE-2026-65660: Microsoft SharePoint Code Injection Vulnerability
- CVE-2026-81963: Microsoft Windows Link Following Vulnerability
- CVE-2026-85880: Microsoft Windows Heap-Based Buffer Overflow Vulnerability
- CVE-2019-1068: Microsoft SQL Server Remote Code Execution Vulnerability
- CVE-2026-33824: Microsoft Internet Key Exchange (IKE) Service Extensions Double Free Vulnerability