Following the massive Wana Decrypt0r ransomware outbreak from yesterday afternoon, Microsoft has released an out-of-bound patch for older operating systems to protect them against Wana Decrypt0r's self-spreading mechanism.
The operating systems are Windows XP, Windows 8, and Windows Server 2003. These are old operating systems that Microsoft stopped supporting years before and did not receive a fix for the SMBv1 exploit that the Wana Decrypt0r ransomware used yesterday as a self-spreading mechanism.
That mechanism is a modified version of the ETERNALBLUE exploit, an alleged NSA hacking tool leaked last month by a group known as The Shadow Brokers.
Microsoft had released a fix for that exploit a month before, in March, in security bulletin MS17-010. That security bulletin only included fixes for Windows Vista, Windows 7, Windows 8.1, Windows 10, Windows Server 2008, Windows Server 2012, and Windows Server 2016.
As the SMBv1 is a protocol that comes built-in with all Windows versions, the computers which did not receive MS17-010 remained vulnerable to exploitation via Wana Decrypt0r's self-spreading package.
"Given the potential impact to customers and their businesses, we made the decision to make the Security Update for platforms in custom support only, Windows XP, Windows 8, and Windows Server 2003, broadly available for download," Microsoft said in a statement. "This decision was made based on an assessment of this situation, with the principle of protecting our customer ecosystem overall, firmly in mind."
Researchers believe that Wana Decrypt0r — also referenced online as WCry, WannaCry, WannaCrypt, and WanaCrypt0r — infected over 141,000 computers.
While unconfirmed, many believe older Windows XP and Windows Server versions were the bulk of the infections pool, as they had no way to protect themselves.
Besides installing these out-of-band updates — available for download from here — Microsoft also advises companies and users to disable the SMBv1 protocol, as it's an old and outdated protocol, already superseded by newer versions, such as SMBv2 and SMBv3.
The current Wana Decrypt0r outbreak has been stopped last night after a security researcher found a kill switch. This is only temporary, as the attackers could release a new version of this threat. This is why patching the SMBv1 exploit is a better solution.
For those affected, you can discuss this ransomware and receive support in the dedicated WanaCrypt0r & Wana Decrypt0r Help & Support Topic. Bleeping Computer also published a technical analysis of the Wana Decrypt0r ransomware.