CVE-2021-40444

What this means for your business

• It affects Microsoft MSHTML. It matters if your company, or a supplier that handles your data, runs it.
• Attackers are already using it. CISA added it to its list of flaws used in real attacks on 3 November 2021.
• Ransomware gangs are known to have used it.
• An attacker can use it remotely, over a network, without a login, but only after tricking someone into an action such as opening a file or a link.
• FIRST's prediction model gives it a 97% chance of attack attempts being seen in the next 30 days, ranking above 99% of all known flaws.

What to do

1. 1Check whether your company or your suppliers run Microsoft MSHTML, and which version. The affected versions are listed further down this page.
2. 2If you do, apply the vendor's fix. A patch or vendor advisory has been published.
3. 3Treat it as urgent. US federal agencies must fix it by 17 November 2021. That deadline does not apply to private companies, but it shows how seriously CISA takes it.
4. 4Because attackers have used it, check the affected systems for signs of a break-in, not only for the update.