Was curious about the Flexcoin hack, but the article wasn't loading, so here's an archive: https://web.archive.org/web/20240423000007/https://hackingdi... Supposedly it was this simple:

  mybalance = database.read("account-number")
  newbalance = mybalance - amount
  database.write("account-number", newbalance)
  dispense_cash(amount)   // or send bitcoins to customer

and MongoDB didn't even have a way to do this atomically? An RDBMS with read-committed would handle this fine if you did "read for update" on that row.