# What is CVE-2021-31955? > CVE-2021-31955 is a medium-severity flaw (CVSS 5.5) in Windows 10 1809 from Microsoft, published on 8 June 2021. It is on the list of flaws known to be exploited, so criminals are already using it. The latest forecast puts its chance of use in the next 30 days at 81%. If you run an affected version, patch it first. Canonical page: https://www.precursorintelligence.com/what-is/cve-2021-31955 ## At a glance - **Verdict:** Fix this now. Attackers are using this flaw today. If you run an affected version, it goes to the top of your list. - **Precursor Intelligence risk score:** 74 out of 100 (High); action: Emergency patch. Kept up to date as the evidence changes. - **Severity:** Medium (CVSS 5.5 out of 10) - **Known to be exploited:** Yes, it is on the government list of flaws known to be exploited - **Chance of use in the next 30 days:** 81% (latest exploit-likelihood forecast) - **Published:** 8 June 2021 - **Record last updated:** 7 March 2025 - **Main product:** Windows 10 1809 from Microsoft - **Affected products:** 8 ## What is CVE-2021-31955? In plain terms An attacker has to be already on the device with an ordinary user account, and nobody has to click anything. If it works, the attacker can read data. ### The official description Windows Kernel Information Disclosure Vulnerability ### What kind of flaw is it? - CWE-497: Exposure of Sensitive System Information to an Unauthorized Control Sphere ### How an attack on it works - Where an attacker can be: Already on the device - How hard it is to pull off: Easy: no special conditions - Login needed first: An ordinary user account - Does someone have to click: No, nobody has to do anything - What an attacker gets: Can read data Severity vector: `CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N` ## Is CVE-2021-31955 being exploited? Yes Attackers are already using it. It is on the government list of flaws known to be exploited, and the latest forecast puts the chance of it being used in the next 30 days at 81%. Exploit likelihood over the last 7 days: 81%, 81%, 81%, 81%, 81%, 81%, 81%. ### Timeline - 8 June 2021: Published - 7 March 2025: Record last updated - 19 November 2025: Exploit likelihood jumped - 22 November 2025: Exploit likelihood fell ### How Precursor Intelligence ranks it CVE-2021-31955 scores **74 out of 100 (High)**, with the action "Emergency patch". Every published flaw gets a risk score from 0 to 100, kept up to date as the evidence changes. Four things go into the score, from the one that counts most to the ones that count less: - Evidence it is being exploited: biggest factor - How severe it is: strong factor - How widely exposed it is: smaller factor - Threat activity around it: smaller factor 85 and above is Critical. Anything confirmed as exploited is always marked for an emergency patch, whatever its score. When this flaw matches software you track, it lands in your list with its score and the reasons behind it. ## Who is affected by CVE-2021-31955? The published record lists 8 products. Versions are as published: "before 10.0.17763.1999" means every version up to, but not including, 10.0.17763.1999. - [Windows 10 1809](https://www.precursorintelligence.com/product/microsoft/windows_10_1809) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.17763.1999 - [Windows 10 1909](https://www.precursorintelligence.com/product/microsoft/windows_10_1909) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.18363.1621 - [Windows 10 2004](https://www.precursorintelligence.com/product/microsoft/windows_10_2004) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.19041.1052 - [Windows 10 20H2](https://www.precursorintelligence.com/product/microsoft/windows_10_20h2) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.19042.1052 - [Windows 10 21H1](https://www.precursorintelligence.com/product/microsoft/windows_10_21h1) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.19043.1052 - [Windows Server 2004](https://www.precursorintelligence.com/product/microsoft/windows_server_2004) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.19041.1052 - [Windows Server 2019](https://www.precursorintelligence.com/product/microsoft/windows_server_2019) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.17763.1999 - [Windows Server 20H2](https://www.precursorintelligence.com/product/microsoft/windows_server_20h2) by [Microsoft](https://www.precursorintelligence.com/vendors/microsoft) (Operating system): before 10.0.19042.1052 ## How do I fix CVE-2021-31955? 1. **Check whether you run it.** Compare your versions with the affected products above. Not sure what you run? Precursor Intelligence can work it out from your public website, or you can pick it from a list. 2. **Apply the vendor’s fix.** Install the update named in the vendor’s advisory. We’ve put the advisory first in the list. 3. **If you can’t patch today.** Follow any mitigation the vendor publishes, and limit who can reach the affected system from the internet until the update is in. 4. **Check it wasn’t used before you patched.** Because criminals are already using this flaw, look for signs of a break-in on affected systems using the vendor’s guidance. ### References - [portal.msrc.microsoft.com](https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31955) (Patch, Vendor advisory) ## Questions ### What is CVE-2021-31955? CVE-2021-31955 is a medium-severity flaw in Windows 10 1809 from Microsoft, published on 8 June 2021. An attacker has to be already on the device with an ordinary user account, and nobody has to click anything. If it works, the attacker can read data. ### How serious is CVE-2021-31955? It scores 5.5 out of 10 on the CVSS severity scale, which is medium. Our risk score for CVE-2021-31955 is 74 out of 100, which is High. Its action is "Emergency patch". Attackers are using this flaw today. If you run an affected version, it goes to the top of your list. ### Is CVE-2021-31955 being exploited? Attackers are already using it. It is on the government list of flaws known to be exploited, and the latest forecast puts the chance of it being used in the next 30 days at 81%. ### Which versions does CVE-2021-31955 affect? For Windows 10 1809 from Microsoft: before 10.0.17763.1999. The record lists 8 products in all, each with its versions, under "Who is affected" on this page. ### How do I fix CVE-2021-31955? Install the update the vendor names in its advisory, linked first under "How do I fix" on this page. If you can't patch today, follow the vendor's mitigation and limit who can reach the affected system from the internet. Because criminals are already using it, also check affected systems for signs of a break-in. ## Does CVE-2021-31955 affect you? Precursor Intelligence keeps checking for you: - **Tell us what you run.** Pick your software from a list, or let us work it out from your website. - **We match it to every flaw.** Over 400,000 published flaws, checked against your list. - **Ranked by real-world risk.** Every flaw gets a 0 to 100 risk score, kept up to date, led by evidence criminals are using it. - **Told as soon as we know.** An alert by email, in the app or by webhook to your own tools. Start free (no card, nothing to install): https://www.precursorintelligence.com/register Book a demo: https://www.precursorintelligence.com/contact