It could not be determined how many of the site's 1.5 million users was affected.
The popular parenting website released a statement acknowledging the security breach saying "the worst case scenario is that the data of every Mumsnet user account was accessed".
It added:
On Friday 11 April, it became apparent that what is widely known as the 'Heartbleed bug' had been used to access data from Mumsnet users' accounts.
On Thursday 10 April we at MNHQ became aware of the bug and immediately ran tests to see if the Mumsnet servers were vulnerable. As soon as it became apparent that we were, we applied the fix to close the OpenSSL security hole (known as the Heartbleed patch). However, it seems that users' data was accessed prior to our applying this fix.
All passwords were disabled and users were asked to change them before they would be able to login.
The Canadian tax authority have also been targeted by the bug saying that 900 people's social insurance numbers had been stolen.
Here's an infographic sent to us by IPVN.net that outlines all the password changes you need to make: