A bug in Microsoft Outlook meant that sensitive emails supposed to be scrambled with S/MIME encryption before they were sent may have also been mistakenly sent in unencrypted plaintext.
The bug let plaintext-formatted encrypted mails be sent in both encrypted and unencrypted forms, according to a blog post describing the issue.
Those unencrypted messages could have exposed secret or sensitive communications for months, the researchers said.
S/MIME is an end-to-end email encryption standard that allows email clients to scramble the contents of an email before it's sent over the internet using a personal certificate. Encrypting emails doesn't just protect the contents, but ensures the authenticity of the message's contents.
The bug allowed encrypted emails sent through Outlook to be read without the private certificates of the recipient, which "results in total loss of security properties provided by S/MIME encryption," the blog post read.
Users would have been unaware of the security lapse, because the message would appear as encrypted in Outlook's "sent items" folder.
The researchers said that an attacker could intercept and read emails if they have "access to the network traffic at any point along the mails path through the network and no transport level encryption is used," or if the attacker has access to either the sender or recipient's mailboxes.
But incoming messages encrypted using S/MIME were not affected, said the researchers.
Security researcher Kevin Beaumont independently verified the bug.
Microsoft fixed the bug on Tuesday as part of its monthly release of security fixes, which rated the bug as "important."
Updates are available through the usual Windows Update and Office Update channels.