The Mumsnet parenting site was updated after the upgrade in the British Data Protection Supervisor. Allowed some to view details about other accounts.
A message posted on the site states that the problem occurred between 5 and 7 February.
The bills were mixed, if two users logged in exactly at the same time, said Founder Mumsneta Justine Roberts.
In total, 46 users were violated, they were reported from the website, but there was no password that would be exposed.
"You have every right to expect that your Mumsnet account will be safe and private," Roberts wrote. "We urgently want to find out how this violation happened and to learn and improve our processes."
Some of the victims have called Mumsnet already at the beginning of February 7, so they can inspect other accounts.
An affected person could see information that includes:
- e-mail address
- account details
- publication history
Mumsnet said that he now turned the software update up, which caused the problem. It also forced all users to unsubscribe by removing anyone who is still hiding in another user's account.
The ICO said it had received a report from the mummnet and that it would investigate the incident.
Analysis by Zoe Kleinman, Technology Rapporteur
Mumsnet tends to become headlines for easy reasons.
Often this is due to some of the more bizarre questions raised by parental web site members on their chat forums (the "penis glass" is a legend thing, look at it), and scratch marks such as AIBU (am I am unreasonable), DC (dear children) and LTB (let … you can guess the rest).
But for some women, it is the first platform to turn to help and advice on a number of deep personal issues: intimacy, abuse, family violence, spontaneous abortion, adultery, isolation, the special needs of their children.
They are worried that they will be able to identify them with fellow "mummies" who know them in real life, and even their partners who stumble over their jobs. The trust they have put in Mumsnet to protect their privacy – and perhaps because of their safety – are many.
The idea of writing these messages or private messages when you unintentionally log in as someone else is a real worry, and although the relief that seems not to have affected many people, disclosure will worry for some.