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NVD / CVE Program· CNA microsoft

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

CriticalCVSS 9.8 · v3.1KEVWeaponized No fix recorded

Noroxi assessment

Priority: Now (action score 99/100)

  • CISA has confirmed exploitation in the wild (Apr 6, 2022). Remediation deadline for US federal agencies: Apr 27, 2022.
  • A ready-made attack module is public (Metasploit); exploiting it takes no special expertise.
  • Probability of exploitation in the next 30 days: 99.8% (FIRST EPSS); higher than 99.9% of all records.
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
  • According to CISA the attack is automatable: targets can be found by mass scanning.
  • According to the record, no fixed version is available yet.

Patch today or shut off access. Until a fix is released, restrict access to the affected component and follow the vendor's advisory.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · CISA KEV · FIRST EPSS

Published
May 11, 2021
Updated
Jun 16, 2026
EPSS
99.8% · 99th percentile

Report tools

JSON

Action score

99

Now

Patch today or shut off access.

CVSS
39 / 40 · 9.8 / 10
CISA KEV
30 / 30 · Listed
EPSS
30 / 30 · 99.8%

CISA SSVC decision

Exploitation
active
Automatable
yes
Technical impact
total

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · microsoft
9.8
NVD has not scored this yet; the score shown is the CNA’s.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Actively exploited in the wild

CISA added this to its Known Exploited Vulnerabilities catalog on Apr 6, 2022. That signals real attacks, not theory.

Remediation deadline for US federal agencies: Apr 27, 2022.

Get told when this record changes

  • In CISA KEV: exploitation in the wild confirmed
  • Weaponized exploit exists
  • No fixed version yet

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2021-31166

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
microsoftwindows 10 2004
microsoftwindows 10 20h2
microsoftwindows server 2004
microsoftwindows server 20h2

Versions reported by the vendor

Affected version ranges reported by the assigning authority (microsoft). Independent of NVD's CPE analysis and usually ahead of it.

  • Microsoft Windows 10 Version 2004

    • 10.0.0 and later · before 10.0.19041.982affected

  • Microsoft Windows 10 Version 20H2

    • 10.0.0 and later · before 10.0.19042.982affected

  • Microsoft Windows Server version 2004

    • 10.0.0 and later · before 10.0.19041.982affected

  • Microsoft Windows Server version 20H2

    • 10.0.0 and later · before 10.0.19042.982affected

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • microsoft windows server 2004before 10.0.19041.982
  • microsoft windows server 20h2before 10.0.19042.982

Same product

microsoft: all records

Other highest-scoring records for the same primary product.

  • CVE-2021-41357Win32k Elevation of Privilege VulnerabilityKEV
    61This week

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

No fix version is recorded for this entry. Check the references for vendor advisories.

Exploit status

Weaponized

A ready exploit tool or active in-the-wild use is known for this vulnerability. This should be your top priority.

Evidence

  • Metasploit module ×1
  • Proof-of-concept repository on GitHub ×12

Noroxi does not host or link to exploit code. Metasploit module names and Nuclei template ids are shown (names, not code); Exploit-DB entries and PoC repositories are given as counts only.

Public exploit evidence
SourceKindDate
Proof-of-concept repository on GitHub12 repositories2021-05-16 → 2026-10-24
Metasploit moduleauxiliary/dos/windows/http/http_sys_accept_encoding_dos_cve_2021_31166Windows IIS HTTP Protocol Stack DOSauxiliary2021-05-11

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

  1. Metasploit-1 d
  2. Published
  3. First PoC+4 d
  4. CISA KEV+329 d
  5. TodayNov 14, 2026+2013 d

Not on record for this CVE: EPSS history · patch commit or PR link · researcher credit · variant or chain candidate · public bounty program.

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
high · data can be read
Integrity
high · data or configuration can be modified
Availability
high · the service can be disrupted
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality impact
High
Integrity impact
High
Availability impact
High

Weakness class (CWE)

CWE-416 · Use After Free

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

nvd-secondary

Root cause

After a memory region is freed, another reference pointing to it is still used. By then, the same region may have been allocated for different data.

The bug in code

The whole CWE-416 class

A representative example of this vulnerability class. Not the vendor's source code; it shows the faulty pattern and its fix.

Vulnerable

c
free(dev->buf);/* ... another thread is still running ... */process(dev->buf);

Fixed

c
lock(&dev->lock);free(dev->buf);dev->buf = NULL;unlock(&dev->lock);

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

References

Show all references (1 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records