Microsoft Exchange Server: code execution flaw under attack

What is affected

Microsoft Exchange Server 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 Exchange Server contains an unspecified vulnerability that allows for authenticated remote code execution. Dubbed "ProxyNotShell," this vulnerability is chainable with CVE-2022-41040 which allows for the remote code execution.

What an attacker can do

This is a code execution flaw. It lets an attacker run their own commands or programs on the affected system. In practice that usually means taking it over: installing malware, stealing data or using it to reach other devices on the same network.

How it is reached

From the same local network

Access the attacker needs

A basic user account

Does the victim have to do something?

No, works without the victim doing anything

CISA records this vulnerability as known to be used in ransomware campaigns: attackers use it to get in, then encrypt files and demand payment.

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 Exchange Server 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 updates per vendor instructions.

Vendor advisories and fixes

• msrc-blog.microsoft.commsrc-blog.microsoft.com/2022/09/29/customer-guidance-for-reported-zero-day-vulnerabilities

How urgent is it

CISA added this vulnerability to its Known Exploited Vulnerabilities catalogue on September 30, 2022, which means there is reliable evidence of attacks in the wild. US federal agencies must fix it by October 21, 2022, 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 100% probability that this flaw will be exploited somewhere in the next 30 days. That is higher than 100.0% of all scored vulnerabilities.

Its CVSS severity score is 8.0 out of 10 (high), as recorded in the US National Vulnerability Database.

Technical description

Microsoft Exchange Server Remote Code Execution Vulnerability