The Canada Revenue Agency says the social insurance numbers of 900 taxpayers were stolen last week by someone using the Heartbleed encryption vulnerability before the taxation agency shut down public access to its online services.
It happened over a six-hour period by someone exploiting the vulnerability in many supposedly secure websites that used an open-source encryption system.
The CRA said it will send registered letters to affected taxpayers and will not be emailing them because it doesn't want fraudsters to use phishing schemes to further exploit the privacy breach.
"I want to express regret to Canadians for this service interruption," CRA commissioner Andrew Treusch said. "I share the concern and dismay of those individuals whose privacy has been impacted by this malicious act."
Other personal data and possibly businesses' information may also have been lost.
"We are currently going through the painstaking process of analyzing other fragments of data, some that may relate to businesses, that were also removed," Treusch said.
Taxpayers whose data was compromised will get bolstered CRA account protection and free access to credit protection services.
Canada's Privacy Commissioner is also investigating.
Online services, including the E-file and Netfile online income tax portals, were patched and re-launched Sunday after what the CRA called a vigourous test to ensure they are safe and secure.
The CRA cut off access to those services April 8 as word spread that the Heartbleed bug had given hackers access to passwords, credit card numbers and other information at many websites.
People whose income tax filing was delayed by last week's CRA interruption have been given until May 5 – beyond the usual April 30 filing deadline – to file returns without being penalized.
The Heartbleed vulnerability, which has existed for two years, compromised secure web browsing at some sites despite the display of a closed padlock that indicates an encrypted connection.