It could be a long wait before consumers can obtain their FICO 08 scores. Quinn says Fair Isaac is still revising the model with the three major credit reporting agencies, after which the company will focus on getting lenders to test and use FICO 08. He says it's too early to speculate on when consumers will get access to their new scores.

Pros and cons of authorized users

If piggybackers were gaming the system, could people still do so by getting a family member with great credit to add them as an authorized user?

Asked about legitimate authorized users inflating their FICO scores in this way, Quinn responded in an e-mail: "The innovation we are adding to the FICO 08 model will help control for the kind of score tampering you describe. The details of how it works must remain confidential."

It remains to be seen how people with otherwise low scores will benefit from their authorized-user accounts after FICO 08 is in place.

Authorized-user accounts aren't a cure-all, though, cautions Curtis Arnold, founder of CardRatings.com and author of "How You Can Profit from Credit Cards." He says credit scores look at many factors and there are bigger components to worry about.

Authorized user accounts do impact your FICO scores, but they aren't a major factor in and of themselves. Here's a look at what influences your FICO scores:

Elements of your credit score

Source: myFICO.com

Being either an authorized user or a primary accountholder carries some risk. Authorized users could see their credit score plummet if the primary cardholder defaults on that account. Primary cardholders are liable for any debt incurred by the authorized user, and high balances could damage their credit scores.

Issuers must report authorized-user information when the authorized user is the accountholder's spouse, thanks to the Equal Credit Opportunity Act of 1974. Issuers decide whether or not to report payment history data for other authorized users.

Arnold suggests the primary accountholder call and ask the issuer if authorized-user data gets reported.

If the issuer doesn't report authorized-user information when that person is not a spouse, it could make sense to add the other person as a joint accountholder. Doing that, however, makes both parties responsible for debt acquired by either party.

Last week Fair Isaac announced that its latest credit-scoring model, dubbed "FICO 08," will include authorized user accounts when calculating someone's FICO credit score.

The company estimates that 50 million consumers are "legitimate" authorized users on someone else's credit card. Legitimate authorized users have a relationship with the primary accountholder and a reason to share access to the account, such as spouses, and parents and children.

FICO 08 was originally designed to ignore authorized user accounts, in an effort to stem "piggybacking" on a stranger's credit card and artificially inflating one's score. (Editor's note: All previous stories describing the design of FICO 08 to exclude authorized user accounts were based on information that is now outdated. At the time of this writing, FICO 08 will include authorized user accounts in scoring.)

Previous versions of the FICO scoring model included authorized user accounts in scoring. Some credit repair firms exploited that loophole, and offered a way for folks with bad credit to raise their FICO scores. The consumer would pay thousands of dollars to get added as an authorized user to the credit card of someone with a stellar credit history. When that account showed up on the authorized user's credit report, the person's credit score would rise. Typically, the authorized user would not receive a credit card. Rather, the consumer paid to get a credit score boost.

Why the change

Fair Isaac said lenders complained that using FICO 08 would inhibit compliance with Federal Reserve Regulation B, which requires lenders assessing a married person's credit risk to consider the credit history of accounts shared by the spouses.

Fair Isaac is keeping the specifics of their new analytic approach secret but says they've found a way to restore authorized-user accounts to the formula but also reduce the impact of piggybacking.

"The FICO 08 scores of legitimate authorized users will now reflect the information on their credit reports about the account(s) on which they are authorized users," says Tom Quinn, vice president of global scoring solutions for Fair Isaac.

A competing scoring model called VantageScore, which was created by the three major credit reporting agencies, has never considered authorized-user accounts in its scoring. "VantageScore excludes authorized-user trade lines, whether with good or bad payment histories, to ensure the risk assessment of a credit applicant represents the true credit risk of the prospect and not the originating borrower with whom the authorized trade line is associated," says Barrett Burns, president and CEO of VantageScore Solutions.

Burns says VantageScore complies with Regulation B since "lenders do not have to consider the credit history of spousal authorized users unless the information is available." Credit reports don't indicate the existence of a spousal relationship between authorized user and accountholder.

How the change impacts you