Microsoft SmartScreen Prompt: security bypass flaw under attack

What is affected

Microsoft SmartScreen Prompt is mostly run by companies, schools and public bodies, not on home computers. You are unlikely to have it yourself, but organisations that hold your data may. Breaches that start with flaws like this one are how personal data ends up leaked.

Microsoft SmartScreen Prompt contains a security feature bypass vulnerability that allows an attacker to bypass the Mark of the Web (MotW) feature. This vulnerability can be chained with CVE-2023-38831 and CVE-2024-21412 to execute a malicious file.

What an attacker can do

This is a security bypass flaw. It lets an attacker slip past a built-in protection, so warnings and checks that normally stop malicious content never appear.

How it is reached

Over the internet or network

Access the attacker needs

No account needed

Does the victim have to do something?

Yes, the victim must open or click something

What to do

1. At home: nothing to install. Keep your own devices updated and use a different password for every service, so that a breach at one organisation does not open your other accounts.
2. If you administer Microsoft SmartScreen Prompt at work: apply the vendor's fix or mitigation now and review logs for signs of compromise, because the flaw was being exploited before it was listed.
3. If the product is past its support date and no fix exists, take it off the internet or retire it.

Apply mitigations per vendor instructions or discontinue use of the product if mitigations are unavailable.

Vendor advisories and fixes

• msrc.microsoft.commsrc.microsoft.com/update-guide/vulnerability/CVE-2024-29988

How urgent is it

CISA added this vulnerability to its Known Exploited Vulnerabilities catalogue on April 30, 2024, which means there is reliable evidence of attacks in the wild. US federal agencies must fix it by May 21, 2024, a deadline that has already passed. That deadline does not bind anyone else, but it shows how seriously the agency rates it.

The EPSS model estimates a 45% probability that this flaw will be exploited somewhere in the next 30 days. That is higher than 99% of all scored vulnerabilities.

Its CVSS severity score is 8.8 out of 10 (high), as recorded in the US National Vulnerability Database.

Technical description

SmartScreen Prompt Security Feature Bypass Vulnerability