A free new service from the U.S. Postal Service that provides scanned images of incoming mail before it is slated to arrive at its destination address is raising eyebrows among security experts who worry about the service’s potential for misuse by private investigators, identity thieves, stalkers or abusive ex-partners. The USPS says it hopes to have changes in place by early next year that could help blunt some of those concerns.

The service, dubbed “Informed Delivery,” has been available to select addresses in several states since 2014 under a targeted USPS pilot program, but it has since expanded to include many ZIP codes nationwide, according to the Postal Service. U.S. residents can tell if their address is eligible by visiting informeddelivery.usps.com.

According to the USPS, some 6.3 million accounts have been created via the service so far. The Postal Service says consumer feedback has been overwhelmingly positive, particularly among residents who travel regularly and wish to keep close tabs on any mail being delivered while they’re on the road.

But a review of the methods used by the USPS to validate new account signups suggests the service is wide open to abuse by a range of parties, mainly because of weak authentication and because it is not easy to opt out of the service.

Signing up requires an eligible resident to create a free user account at USPS.com, which asks for the resident’s name, address and an email address. The final step in validating residents involves answering four so-called “knowledge-based authentication” or KBA questions. KrebsOnSecurity has relentlessly assailed KBA as an unreliable authentication method because so many answers to the multiple-guess questions are available on sites like Spokeo and Zillow, or via social networking profiles.

Once signed up, a resident can view scanned images of the front of each piece of incoming mail in advance of its arrival. Unfortunately, because of the weak KBA questions (provided by recently-breached big-three credit bureau Equifax, no less) stalkers, jilted ex-partners, and private investigators also can see who you’re communicating with via the Postal mail.

Perhaps this wouldn’t be such a big deal if the USPS notified residents by snail mail when someone signs up for the service at their address, but it doesn’t.

Peter Swire, a privacy and security expert at Georgia Tech and a senior counsel at the law firm of Alston & Bird, said strong authentication relies on information collected from multiple channels — such as something you know (a password) and something you have (a mobile phone). In this case, however, the USPS has opted not to leverage a channel that it uniquely controls, namely the U.S. Mail system.

“The whole service is based on a channel they control, and they should use that channel to verify people,” Swire said. “That increases user trust that it’s a good service. Multi-channel authentication is becoming the industry norm, and the U.S. Postal Service should catch up to that.”

I also wanted to know whether there was any way for households to opt out of having scanned images of their mail sent as part of this offering. The USPS replied that consumers may contact the Informed Delivery help desk to request that the service not be presented to anyone in their household. “Each request is individually reviewed and assessed by members of the Postal Service Informed Delivery, Privacy and Legal teams,” the Postal Service replied.

There does not appear to be any limit on the number of people who can sign up for the service at any one address, except that one needs to know the names and KBA question answers for a valid resident of that address.

“Informed Delivery may be accessed by any adult member of a household,” the USPS wrote in response to questions. “Each member of the household must be able to complete the identity proofing process implemented by the Postal Service.”

The Postal Service said it is not possible for an address occupant to receive emailed, scanned images of incoming mail at more than one email address. In other words, if you wish to prevent others from signing up in your name or in the name of any other adults at the address, the surest way to do that may be to register your own account and then urge all other adult residents at the address to create their own accounts.

A highly positive story about Informed Delivery published by NBC in April 2017 suggests another use for the service: Reducing mail theft. However, without stronger authentication, this service could let local ID thieves determine with pinpoint accuracy exactly when mail worth stealing is set to arrive.

The USPS says businesses are not currently eligible to sign up as recipients of Informed Delivery. However, people running businesses out of their home could also be the target of competitors hoping to steal away customers, or to pose as partner firms in demanding payment for outstanding invoices.

Informed Delivery seems like a useful service for those residents who wish to take advantage of it. But lacking stronger consumer validation the service seems ripe for abuse. The USPS should use its own unique communications channel (snail mail) to alert Americans when their physical address has been signed up for this service.

Bob Dixon, the executive program director for Informed Delivery, said the Postal Service is working on an approach that it hopes to make available to the public in January 2018 which would allow USPS to send written notification to addresses when someone at that residence signs up for Informed Delivery.

Dixon said that capability will build on technology already in place to notify Americans via mail when a change of address is requested. Currently, the USPS allows address changes via the USPS Web site or in-person at any one of more than 3,000 post offices nationwide. When a request is processed, the USPS sends a confirmation letter to both the old address and the new address.

If someone already signed up for Informed Delivery later posts a change of address request, the USPS does not automatically transfer the Informed Delivery service to the new address: Rather, it sends a mailer with a special code tied to the new address and to the username that requested the change. To resume Informed Delivery at the new address, that code needs to be entered online using the account that requested the address change.

“Part of coming up with a mail-based verification system will also let us do some additional notification that, candidly, we just haven’t built yet,” Dixon said. “It is our intent to have this ready by January 2018, and it is one of our higher priorities to get it done by then.”

There is a final precaution that should block anyone from signing up as you: Readers who have taken my advice to freeze their credit files with the four major consumer credit reporting bureaus (Equifax, Experian, Innovis and Trans Union) will find they are not able to sign up for Informed Delivery online. That’s because having a freeze in place should block Equifax from being able to ask you the four KBA questions.

By the way, this same dynamic works with other services that you may not wish to use but which require you otherwise to plant your flag of identity to prevent others from doing so on your behalf, such as managing your relationship to the Internal Revenue Service online and the Social Security Administration. For more information on why you should get a freeze and how to do that, see this piece.

Update, 3:48 p.m. ET: Added bit about how a freeze can block someone from signing up in your name.

Update, Oct. 4, 11:01 a.m.: Several readers have written in to say that although the Postal Service says citizens can opt out of Informed Delivery at a specific address by contacting the Informed Delivery Help Desk, none of those readers have successfully been able to achieve this result. One reader forwarded a response from the Help Desk folks that stated emphatically, “I do understand your concern about fraud and theft but there is no way to make your home address ineligible for Informed Delivery.” No way, that is, kexcept to register as every adult at your address, as stated above.

Tags: Alston & Bird, Equifax, georgia tech, Informed Delivery, KBA, knowledge-based authentication, Peter Swire, U.S. Postal Service, USPS