Your Social Insurance Number (SIN) is confidential.
If your SIN falls into the wrong hands, it could be used to obtain personal information and invade your privacy. When the SIN is not linked to you as its rightful owner, another person could receive your government benefits, tax refunds or bank credits. Your personal information could also be revealed to unauthorized people, which could lead to identity theft and other types of fraud.

Link: https://www.canada.ca/en/employment-social-development/programs/sin/protect.html