When I signed into my account on Monday to get a seat assignment, I noticed that almost all of my miles were gone, and a few days ago an unknown redemption to Delta Marketplace. I called and a helpful agent filled out the proper forms, and by the end of the day, my miles were returned, and I was able to reset my password.
Then yesterday, I got an email from Delta that they had "noticed a number of login attempts" and my account was locked, call to restore account.
After a discussion with a helpful supervisor, here is what I learned:
1. The "marketplace" scam has been going on for some time. If you google skymiles fraud or hacking, you'll find lots of examples.
2. The multiple login thing is more than just a onetime thing. Apparently Delta recognized this problem in several thousand accounts, and locked all of them, and sent an eail.
3. The login problem is largely influenced by two things:
a. Using an emial address to login to account
b. Using a guessable or easy/soft password.
4. Delta has recently moved to a three or four attempts then you're out for logging in.
5. If you're affected, you'll need to fill out a case on the website, and send a picture of a government id with your address. It will take up to a week to resolve.

Now the hacking of miles for purchases at the Marketplace could be solve by two-step authentication, or at least email receipts of purchases, immediately, like they do with air travel purchases. The same could be done for logging in, but that may complicate mobile interaction, especially if trying to use the app in a foreign country. For sure, logging in by your email is not safe.
Apparently, reported elsewhere, several airlines, including Delta were compromised sometime in September or October. It's not clear what was taken, logins and skymiles numbers, probably, In any case, I am recommending readers check their balances and create new robust passwords for their accounts.

In any case, the supervisor said that lots of people will be or have been impacted, so this is a heads up.

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3Cforme, Delta Forum Moderator

I'm going to let this thread stand on its own - instead of merging it. The OP has made an effort to report a meaningful conversation on the topic. Thanks.