On May 12, 2017 Microsoft detected a new ransomware that spreads like a worm by leveraging vulnerabilities that have been previously fixed. While security updates are automatically applied in most computers, some users and enterprises may delay deployment of patches. Unfortunately, the malware, known as WannaCrypt, appears to have affected computers that have not applied the patch for these vulnerabilities. While the attack is unfolding, we remind users to install MS17-010 if they have not already done so.

Microsoft antimalware telemetry immediately picked up signs of this campaign. Our expert systems gave us visibility and context into this new attack as it happened, allowing Windows Defender Antivirus to deliver real-time defense. Through automated analysis, machine learning, and predictive modeling, we were able to rapidly protect against this malware.

In this blog, we provide an early analysis of the end-to-end ransomware attack. Please note this threat is still under investigation. The attack is still active, and there is a possibility that the attacker will attempt to achieve persistence by reacting to our detection response.

Attack vector

A ransomware threat does not normally spread so rapidly. Threats like WannaCrypt typically leverage social engineering or emails as primary attack vector, relying on users downloading and executing a malicious payload. However, in this unique case, the ransomware perpetrators incorporated publicly-available exploit code for the patched SMB EternalBlue vulnerability, CVE-2017-0145, which can be triggered by sending a specially crafted packet to a targeted SMBv1 server, was fixed in security bulletin MS17-010, released on March 14, 2017.

WannaCrypt’s spreading mechanism is borrowed from well-known public SMB exploits, which armed this regular ransomware with worm-like functionalities, creating an entry vector in machines still unpatched even after the fix had become available.

The exploit code used by WannaCrypt was designed to work only against unpatched Windows 7 and Windows Server 2008 (or earlier OS) systems, so Windows 10 PCs are not affected by this attack.

We haven’t found evidence of the exact initial entry vector used by this threat, but there are two scenarios we believe are highly possible for this ransomware family:

  • Arrival through social engineering emails designed to trick users to run the malware and activate the worm-spreading functionality with the SMB exploit
  • Infection through SMB exploit when an unpatched computer can be addressed in other infected machines

Windows Defender AV detects and removes this threat.

This ransomware can stop you from using your PC or accessing your data. It has worm-like characteristics targets PCs which do not have the latest security updates. We remind customers to install MS17-010 as soon as possible, if they have not already done so.

You can also read more on our ransomware page.