In an email to its members, Mumsnet said: "... it became apparent that what is widely known as the Heartbleed bug had been used to access data from Mumsnet users' accounts.
"We have no way of knowing which Mumsnetters were affected by this."
"The worst case scenario is that the data of every Mumsnet user account was accessed," the email said.
People have been urged to change their details in response to the internet-wide bug, but it has emerged that changing login details may not boost security at all.
Some experts are advising users to change all their passwords across every site they have an account for, while others are being a little more selective.
Mumsnet founder and chief executive Justine Roberts said all registered Mumsnet users are required to change their passwords.
"We have no way of knowing which or how many accounts were affected but have advised users to change passwords on other sites particularly if they use the same password on Mumsnet as elsewhere."
