It’s a bit of bitter irony, when a security product gets used maliciously, to pull off the exact attack it was designed to prevent. Enter Proofpoint, and the EchoSpoofing attack. Proofpoint offers an email security product, filtering spam and malicious incoming emails, and also handling SPF, DKIM, and DMARC headers on outgoing email. How does an external service provide those email authentication headers?

One of the cardinal sins of running an email server is to allow open relaying. That’s when anyone can forward email though an SMTP server without authentication. What we have here is two nearly open relays, that wound up with spoofed emails getting authenticated just like the real thing. The first offender is Microsoft’s Office365, which seems to completely skip checking for email spoofing when using SMTP relaying from an allowed IP address. This means a valid Office365 account allows sending emails as any address. The other half relies on the way Proofpoint works normally, accepting SMTP traffic from certain IP addresses, and adding the authentication headers to those emails. There’s an option in Proofpoint to add the Microsoft Office 365 servers to that list, and apparently quite a few companies simply select that option.

The end result is that a clever spammer can send millions of completely legitimate looking emails every day, that look very convincing even to sophisticated users. At six months of activity, averaging three millions emails a day, this campaign managed just over half a billion malicious emails from multiple high-profile domains.

The good news here is that Proofpoint and Guardio discovered the scheme, and worked with Microsoft to develop the X-OriginatorOrg header that is now applied to every email sent from or through the Office365 servers. This header marks the account tenant the email belongs to, giving vendors like Proofpoint a simple way to determine email validity. Continue reading “This Week In Security: Echospoofing, Ransomware Records, And Github Attestations” →