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I've heard that there's a new type of security in Facebook. When logging in they automatically check if the IP Address is similar to that of the original IP during registration. That's in order to prevent hacking into someone's account via the three methods i.e keylogging, phishing or social engineering.

So, since VPNs and proxies change your IP, Facebook might just think that you're intruding in someone's account. I haven't tried it (I will) but I hope it doesn't work :P. Is it possible?

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As far as I'm aware, Facebook don't actually block the new attempts from different IPs. Instead, they send an email to the registered email address with a notification that you logged into a new device, with a button to mark this as legitimate or malicious.

The tracking appears to work not only via your IP, but also a long-term identification cookie and other browser fingerprint data (e.g. user agent header) so that switching browsers or computers within the same network will still trigger it.

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  • OK, so I am just an absolute beginner in hacking (although I know some PHP) but what if a hacker either intentionally or accidentally used the same browser and also the user have never used any cookies (except session cookies obviously which will be destroyed when browser is closed) and hacked the account, would he succeed without "triggering the alarm"? And I have used Facebook using different phones and browsers but nothing happened.
    – Slim Shady
    Apr 26 '16 at 13:34
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    @BenJunior No, because the lack of a cookie is the indicator - when you log into Facebook it sets an identifying cookie, and clearing it makes you no longer identifiable.
    – Polynomial
    Apr 26 '16 at 13:36
  • @BenJunior: it depends on what you meant by "the same browser" and "hacked the account".
    – Yuriko
    Apr 26 '16 at 13:57
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It depends.

Facebook analyzes the locations from which you use to log in from. Depending on where your new IP is located, Facebook may, or may not, give you access. Remember that it has never been your account in the first place.

In the case where Facebook finds the new location suspicious, it will use another mean to validate your identity. (E.g. an email to validate your attempt.) Once you validate that new location, you should be able to connect.

I don't know the details of the algorithm that decides whether that new connection should be considered as an abnormal behavior. Several parameters comes to mind, such as the distance between the locations, the time since your last connection, and so on and so forth. (Someone logging in in from Russia one hour after logging in from the USA 1 hour before is kinda suspicious.)

Anyway, if you have a total access to the mail box and/or the phone number associated with that account, you should be fine.

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  • Yes, that might be suspicious but do you really think that Facebook checks the previous and the current location every time the user logs in while putting the time interval in consideration, don't you think it would be very hard?
    – Slim Shady
    May 2 '16 at 5:51
  • Facebook already stores that (approximate) location, with the date. I don't think that would be hard (at least, with this two parameters. A very simple and naive algorithm could be: if (distance [km] / time_interval [hour] > 500) then alert(). (It would use less CPU usage than checking the password.) In other words, one would need to travel faster than 500 km/hr (= 310 mph) between 2 connections to raise an alert. This shouldn't annoy the casual user, unless they're traveling by plane. (And then, they'll just have to confirm their identity by email/phone the first time.)
    – Yuriko
    May 2 '16 at 6:01
  • But as I said, I don't know the algorithm they are using. (And I don't think they'd give it to me if I asked. ;-) ) It's probably not as simple as the one I wrote in my last comment. Perhaps some countries are considered as more suspicious than others, etc. The User Agent used might be one parameter too. In your case, I would say that it only depends of your location and your VPN/Proxy location. If Facebook raises an alert, just confirm your identity and you should be fine afterward.
    – Yuriko
    May 2 '16 at 6:08
  • So, what would happen if two people log in to one account at the same time (same place or not)?
    – Slim Shady
    May 2 '16 at 11:01
  • Without the algorithm, we can only guess. In my opinion, Facebook will probably raise an alert if you're suddenly logging in from a new country. However, it should not bother you afterward, even if you're logged at the same time, I tested.
    – Yuriko
    May 2 '16 at 11:14

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