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The leading UK site for parents has had users data compromised by hackers then using 'Heartbleed' exploit.
Mumsnet sent an email to users, warning that the hackers may have passwords and personal messages before network administrators were able to fix the vulnerability.
The website has urged all users to change their passwords.
The National Security Agency (NAS) have denied that they were aware of the Heratbleed bug before the security flaw was made public.
The denial follows claims the NSA had known about the flaw for up to two years.
The US National Secutiry Agency (NSA) has knew about the Heartbleed bug for at least two years before it was revealed, according to Bloomberg.
One person 'familiar with the matter' told the news agency that the NSA preferred to keep the bug secret in order to harvest the private data the flaw exposed.
The US Government have warned that hackers are attempting to exploit the 'Heartbleed' bug by scanning networks to see if they are vulnerable, saying they could now "exploit unpatched systems".
Larry Zelvin, a Department of Homeland Security official who runs an agency centre that monitors and responds to emerging cyber threats said on his White House blog:
"While there have not been any reported attacks or malicious incidents involving this particular vulnerability at this time, it is still possible that malicious actors in cyberspace could exploit unpatched systems."
Finnish security firm Codenomicon has set up a dedicated website to give people information about the Heartbleed bug, a glitch in the OpenSSL security product that may have put internet users' personal data at risk.
It comes after the firm, along with Google Security, revealed earlier this week that the bug had gone undetected for two years and could be used by hackers to steal sensitive information such as passwords.
Several technology companies have urged the public to reset their passwords amid fears of a major security problem with a product used to protect people's personal data.
The Heartbleed bug affects OpenSSL, which many companies use to protect sensitive information, including people's password.
A small padlock icon appears on websites using OpenSSL to reassure users, but the loophole in the programme could have left it open to exploitation by hackers.
Blogging platform Tumblr posted a public notice about the bug, advising users to "take some time to change your passwords everywhere - especially your high-security services like email, file storage, and banking".
Finnish security company Codenomicon also said it would be "a good idea" to change potentially vulnerable passwords.
Latest ITV News reports
While big companies such as Facebook rush to secure their sites against a major security flaw, many smaller sites could be at risk.