The company, which provides a service for companies to mass email their customers without getting blocked, said earlier this month an account of a Bitcoin-related customer was compromised and used to send phishing emails.
Further investigation by FireEye's Mandiant division had showed the attackers also compromised a SendGrid employee's account and accessed internal systems on three days in February and March, wrote David Campbell, the company's chief security officer.
The attackers would have access to usernames, email addresses, salted and hashed passwords for SendGrid customer and employee accounts, Campbell wrote. The attackers also accessed servers containing email lists for its customers and customer contact information. Payment card information was not compromised, as SendGrid does not store it.
"We have not found any forensic evidence that customer lists or customer contact information was stolen," Campbell wrote. "However, as a precautionary measure, we are implementing a system-wide password reset."
SendGrid sends 14 billion emails a month for 180,000 customers, according to its website.
Compromising providers such as SendGrid is very useful for cyberattackers since their speciality is sending email that is vetted and will not be stopped by spam filters.
Campbell recommended that those who cryptographically sign their email with DKIM (DomainKeys Identified Mail) generate new keys. DKIM wraps a cryptographic signature around an email that verifies the domain name through which the message was sent, which helps more easily filter out spoofed messages from legitimate ones.
SendGrid's customers are also advised to set up two-factor authentication, a system which involves entering a one-time passcode or other time-sensitive bit of information as an additional defense.
The company is also working on another security measure, API Keys, which will let administrators generate authentication credentials that are separate from their usernames and passwords. Those keys can be generated for different applications and servers, which makes it easier to revoke one without service disruption, according to SendGrid's website.
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