Equifax has mailed out W-2 forms to its employees with their Social Security numbers visible through a window on the envelope.

"Equifax became aware of the problem on January 19 and informed employees in a letter dated January 27," writes CNET News' Elinor Mills.

"Specifically, some of the tax forms mailed by Equifax's payroll vendor through the U.S. Postal Service had the Social Security number in a Control Number field, which was partially or fully viewable through the return address window," Mills writes.

Click here to read the CNET News article.