ConnectWise ScreenConnect: authentication bypass flaw under attack

What is affected

ConnectWise ScreenConnect 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.

ConnectWise ScreenConnect contains an improper authentication vulnerability. This vulnerability could allow a ViewState code injection attack, which could allow remote code execution if machine keys are compromised.

What an attacker can do

This is a authentication bypass flaw. It lets an attacker get in without a valid username and password and act as a legitimate user, often as an administrator.

How it is reached

Over the internet or network

Access the attacker needs

An administrator account

Does the victim have to do something?

No, works without the victim doing anything

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 ConnectWise ScreenConnect 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, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

Vendor advisories and fixes

• connectwise.comwww.connectwise.com/company/trust/security-bulletins/screenconnect-security-patch-2025.4

How urgent is it

CISA added this vulnerability to its Known Exploited Vulnerabilities catalogue on June 2, 2025, which means there is reliable evidence of attacks in the wild. US federal agencies must fix it by June 23, 2025, 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 3.5% probability that this flaw will be exploited somewhere in the next 30 days. That is higher than 89% of all scored vulnerabilities.

Its CVSS severity score is 7.2 out of 10 (high), as recorded in the US National Vulnerability Database.

Technical description

ScreenConnect versions 25.2.3 and earlier versions may be susceptible to a ViewState code injection attack. ASP.NET Web Forms use ViewState to preserve page and control state, with data encoded using Base64 protected by machine keys.  It is important to note that to obtain these machine keys, privileged system level access must be obtained. If these machine keys are compromised, attackers could create and send a malicious ViewState to the website, potentially leading to remote code execution on the server.  The risk does not originate from a vulnerability introduced by ScreenConnect, but from platform level behavior.  This had no direct impact to ScreenConnect Client. ScreenConnect 2025.4 patch disables ViewState and removes any dependency on it.