A post on Steam eases fears about a report of a data breach affecting a third-party Steam vendor. There were concerns that details on as many as 89 million accounts were compromised, but Valve reassures users that there is no reason to worry:

You may have seen reports of leaks of older text messages that had previously been sent to Steam customers. We have examined the leak sample and have this was NOT a breach of Steam systems.

We’re still digging into the source of the leak, which is compounded by the fact that any SMS messages are unencrypted in transit, and routed through multiple providers on the way to your phone.

The leak consisted of older text messages that included one-time codes that were only valid for 15-minute time frames and the phone numbers they were sent to. The leaked data did not associate the phone numbers with a Steam account, password information, payment information or other personal data. Old text messages cannot be used to breach the security of your Steam account, and whenever a code is used to change your Steam email or password using SMS, you will receive a confirmation via email and/or Steam secure messages.

You do not need to change your passwords or phone numbers as a result of this event. It is a good reminder to treat any account security messages that you have not explicitly requested as suspicious. We recommend regularly checking your Steam account security at any time at https://store.steampowered.com/account/authorizeddevices

We also recommend setting up the Steam Mobile Authenticator if you haven’t already, as it gives us the best way to send secure messages about your account and your account’s safety.