Parenting Mumsnet's website referred to the UK data protection watchdog after an update that allows some people to see details of other accounts.
In a message posted on the site, the problem occurred between 5 and 7 February.
The accounts got confused if two users logged in at exactly the same time, said Mumsnet founder Justine Roberts.
A total of 46 users were hacked, the site said, but the passwords are not said to have been exposed.
"You have every right to expect your Mumsnet account to be secure and private," Roberts wrote. "We are urgently working to find out exactly how this violation occurred and to learn and improve our processes".
Some of those affected launched the alarm at Mumsnet at the beginning of February 7th to be able to view other accounts.
Those interested would have been able to see the information, including:
- email address
- Account details
- publication history
- personal messages
Mumsnet said that he has now reversed the software that caused the problem. It also forced all users to disconnect, in order to remove anyone who still hovers in the account of another user.
ICO said he had received the Mumsnet report and was considering the incident.
Analysis of the technological journalist Zoe Kleinman
Mumsnet tends to make headlines for carefree reasons.
It is often the result of some of the most bizarre problems raised by members of the parenting site on its chat forums (the "penis pea" is the legendary material, it is observed) and the acronyms like AIBU (am I'm unreasonable), DC (dear children) and LTB (leave the … you can guess the rest).
However, it is also the first platform for some women to turn to for help and advice on a number of deeply personal issues: intimacy, abuse, domestic violence, abortion, adultery, loneliness, special needs of their children.
They worry about being identifiable with fellow "mumsnetters" who may know them in real life, and even their partners stumble on their posts. The trust they place in Mumsnet to protect their privacy – and perhaps as a result their security – is considerable.
The idea of u200b u200bwriting these messages or private messages while you accidentally register as someone else is genuinely troubling and while it is a relief that does not seem to have influenced many people, disclosure will be worrying for some.