CVE-2021-31166
CVE-2021-31166 is a security flaw in Microsoft HTTP Protocol Stack. Attackers can run their own code on Microsoft HTTP Protocol Stack. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 6 Apr 2022.[1]
Microsoft HTTP Protocol Stack is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. Fixed in version 10.0.19041.982 and 10.0.19042.982.
Description
CISA calls it a remote code execution in Microsoft HTTP Protocol Stack. In its words: “Microsoft HTTP Protocol Stack contains a vulnerability in http.sys that allows for remote code execution.”[1]
The underlying weakness is use after free (CWE-416), in MITRE’s list of common software weaknesses.[2]
It scores 9.8 out of 10 on the Common Vulnerability Scoring System, which rates it critical, scored by the organisation that published the CVE using CVSS 3.1.[2]
Affected versions
NVD’s record lists these fixed versions: in each line, everything before the version shown is affected.[2]
| Product | Fixed in |
|---|---|
| Windows 10 Version 2004 | 10.0.19041.982 |
| Windows Server version 2004 | 10.0.19041.982 |
| Windows 10 Version 20H2 | 10.0.19042.982 |
| Windows Server version 20H2 | 10.0.19042.982 |
Most families won’t have Microsoft HTTP Protocol Stack; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 6 Apr 2022, 330 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 27 Apr 2022 to fix it.[1]
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at over 99%. That’s in the top 1% of the flaws it scores.[4]
CISA assesses it as automatable: attackers can run it against many targets at once.
CISA doesn’t say who was attacked, or how.
Fix
Microsoft has published a fix or advice.[3] CISA’s required action: “Apply updates per vendor instructions.”[1]
- Check whether you use it.Microsoft HTTP Protocol Stack is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
- Send this page to your IT person.Ask them to confirm it’s fixed (fixed in version 10.0.19041.982 and 10.0.19042.982), or not in use. CISA gave US government agencies until 27 Apr 2022 to fix it.
- If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.
You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 9.8.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at over 99%.[4] |
See also
Other Microsoft flaws
- CVE-2026-96951, Microsoft Windows: command injection ·
- CVE-2026-65660, Microsoft SharePoint: code injection ·
- CVE-2026-85880, Microsoft Windows: heap-based buffer overflow ·
- CVE-2026-81963, Microsoft Windows: link following ·
- CVE-2019-1068, Microsoft SQL Server: remote code execution ·
Other use after free flaws
- CVE-2026-109619, PHP: stale internal request-state ·
- CVE-2026-109618, PHP: memory corruption ·
- CVE-2020-9715, Adobe Acrobat: use-after-free ·
- CVE-2026-5281, Google Dawn: use-after-free ·
- CVE-2023-43000, Apple devices: use-after-free ·
Added the same day
- CVE-2021-3156, Sudo: heap-based buffer overflow ·
- CVE-2017-0148, Microsoft SMBv1 server: remote code execution ·
Every Microsoft flaw on CISA’s list · Everything added in April 2022
References
- ^ Cybersecurity and Infrastructure Security Agency. “Microsoft HTTP Protocol Stack Remote Code Execution Vulnerability”. Known Exploited Vulnerabilities Catalog, added 6 Apr 2022.
- ^ National Institute of Standards and Technology. “CVE-2021-31166 Detail”. National Vulnerability Database, published 11 May 2021.
- ^ Microsoft. Advisories and fixes: portal.msrc.microsoft.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.999, percentile 100.0, 31 Oct 2026.
- CVE Program. “CVE-2021-31166”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.