OTTAWA, Ontario (AP) — Canada's tax agency says the ID numbers of roughly 900 people were stolen from its systems, which were left vulnerable by a computer bug called "Heartbleed."

The Canada Revenue Agency blocked public access to its online services for several days last week until it addressed the security risk, but said Monday there was nonetheless a data breach over a six-hour period.

The agency said Monday everyone affected will receive a registered letter and free access to credit protection services.

The Heartbleed bug has caused major security concerns across the Internet. It is caused by a flaw in OpenSSL software, which is used on the Internet to provide security.

The bug is affecting many global IT systems in both private and public sector organizations and can expose private data.