Encryption keys that secure Xbox Live accounts have been “inadvertently disclosed”, Microsoft has said.
The keys in question are designed to be kept private so they can guarantee the authenticity of a digital certificate, invoked when users connect to xboxlive.com.
Since the keys have been leaked, connections may not be secure.
It is not clear how the disclosure happened, but Microsoft has since updated its certificates.
The leak means a hacker could intercept data transmitted between a user and Microsoft’s servers by impersonating the xboxlive.com domain name.
“The certificate could be used in attempts to perform man-in-the-middle attacks,” said the company, in an advisory note about the problem.
“This issue affects all supported releases of Microsoft Windows.
“Microsoft is not currently aware of attacks related to this issue.”
The company has recommended users install all recommended updates for Windows, which will update lists of trusted certificates on users’ systems.
“The advisory that talks about it says the key was revoked after December 1st,” Tod Beardsley, security research manager at security firm Rapid7 told the BBC.
“That wasn’t the fix, the fix is pushing down to the certificate trust authority that says don’t trust this after December 1st – but you only got that yesterday which is a week late.”
But Mr Beardsley added things could easily have been worse.
“The fact that they do it at all is great. There have been cases where keys have been disclosed for months and months and nobody noticed,” he said.
Josh Goldfarb, chief technology officer at security company FireEye, also advised users to install the latest updates.
“This type of disclosure can prove attractive to attackers looking to fool or trick users into giving over private or sensitive information,” he said.
“Although there is potential for abuse here, the risk is relatively easy to remediate by updating the list of trusted certificates.”