Andrew Treusch, Commissioner of Canada Revenue Agency (CRA), said government security agencies notified his office "of a malicious breach of taxpayer data that occurred over a six-hour period" last week.
Treusch said approximately 900 social insurance numbers -- nine-digit codes required for working or accessing government benefits in Canada -- "were removed from CRA systems by someone exploiting the Heartbleed vulnerability."
Government officials, he added, are combing through CRA systems and "analyzing other fragments of data, some that may relate to businesses, that were also removed."
The CRA last week shuttered its website over concerns about the Heartbleed bug. It was rebooted over the weekend after a patch was installed.
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The recently-discovered flaw in online-data scrambling software OpenSSL allows hackers to eavesdrop on online communications, steal data, impersonate websites and unlock encrypted data.
OpenSSL is commonly used to protect passwords, credit card numbers and other data sent via the Internet.
More than half of websites use the software, but not all versions have the same vulnerability, according to heartbleed.Com.
Cybersecurity firm Fox-It estimates that the vulnerability has existed for about two years, since the version of OpenSSL at issue was released.