CVE-2021-31166 | HTTP Protocol Stack Remote Code Execution Vulnerability

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HTTP Protocol Stack Remote Code Execution Vulnerability

Published: 2021-05-11 Last update: 2026-06-16 Assigner: [email protected] Source: [email protected]
NVD Status：Analyzed，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-31166 is rated Critical Active Threat (100/100): CVSS Critical severity, with high exploitation likelihood (EPSS 99.66%, 100th percentile). CISA KEV confirms active exploitation (added 2022-04-06) affecting Microsoft / HTTP Protocol Stack. a weakness (CWE-416) Unauthenticated remote administrative access may be possible. EPSS rose +6.59% over the last day, indicating growing attacker interest. The CISA remediation deadline has passed—treat as an emergency patch priority.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

CISA KEV Record for CVE-2021-31166

Name: Microsoft HTTP Protocol Stack Remote Code Execution Vulnerability · CISA KEV detail

Exploit added: 2022-04-06

Action due: 2022-04-27

Required action: Apply updates per vendor instructions.

Exploit prediction scoring system (EPSS) score for CVE-2021-31166

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-15 93.07% 99.66% +6.59%
2 2026-04-19 92.99% 93.07% +0.08%
3 2026-03-24 — 92.99% —

Full EPSS history (28 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-31166

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
9.8 3.1 CRITICAL
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 5.9 [email protected]
9.8 3.1 CRITICAL
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 5.9 [email protected]
7.5 2.0 HIGH
AV:N/AC:L/Au:N/C:P/I:P/A:P Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:L)
Exploitation conditions are straightforward and predictable.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:P)
Partial confidentiality impact.
Integrity impact (I:P)
Partial integrity impact.
Availability impact (A:P)
Partial availability impact.
 10.0 6.4 [email protected]

Weakness enumeration for CVE-2021-31166

Affected software / configurations for CVE-2021-31166

Vendor Product Version Raw CPE
microsoft windows_10_2004 < 10.0.19041.982 cpe:2.3:o:microsoft:windows_10_2004:*:*:*:*:*:*:*:*
microsoft windows_10_20h2 < 10.0.19042.982 cpe:2.3:o:microsoft:windows_10_20h2:*:*:*:*:*:*:*:*
microsoft windows_server_2004 < 10.0.19041.982 cpe:2.3:o:microsoft:windows_server_2004:*:*:*:*:*:*:*:*
microsoft windows_server_20h2 < 10.0.19042.982 cpe:2.3:o:microsoft:windows_server_20h2:*:*:*:*:*:*:*:*

References for CVE-2021-31166

cvelogic Threat Intelligence