Basic Facebook Guide: How Can I Get Phone Number From Facebook

How I got your phone number through Facebook
Don’t have the time to read the entire article? Go to the FAQ section below for everything you should know.

Update: someone pointed out that PayPal actually reveals the last four digits of the phone numbers, so this technique may work for large countries as well if the target has its phone linked to its PayPal account.

Verifying one of the phone numbers I discovered

Last month, I discovered it is relatively simple to reveal private phone numbers on Facebook, uncovering some phone numbers of Belgian celebs and politicians. Even though this trick only seems to work in small countries such as Belgium (+/- 11.2 million people), a significant number of people is affected by this simple, yet effective privacy leak.

When I notified the fine folks of the Facebook Security team with my concerns, I got an answer I didn’t quite expect:

Not an issue, according to Facebook
When the “who can look me up by phone” setting is set to public, your phone number is public.

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There are a few issues with this:
The setting is set to public by default
It’s confusing: even though your phone number on your profile is set to ‘only me’, the ‘who can look me up’-setting overrules this. While people think their phone number is private, it’s not:

This setting only indicates whether the phone number is visible on your profile. It does not indicate whether your phone number is public.
If this setting is set to ‘Everyone’, which is the default value, your phone number is considered public.
‘Who can look me up’ also implies the person ‘looking you up’ already has your phone number. It implies that someone if looking for your specific Facebook profile based on your phone number, and not the other way around.
There is simply no only me setting

Facebook really wants your phone number, nagging you for one as soon as you join. This isn't all bad since it can help secure your account with two-factor authentication. On the flipside, this makes it easy to reveal the private phone numbers of virtually anyone on Facebook, including celebrities and politicians. We're going to look at how a hacker would do this and how to protect yourself.

Many Facebook users may not even realize that their private phone number is connected to their Facebook account, having forgotten that they did so. Facebook isn't allowed to simply extract your number from your phone, but they can do what I refer to as the "app equivalent of cyberbullying" by repeatedly asking you to confirm and save your number each time you launch Facebook.

The default privacy setting on Facebook allows anyone to search for you by your phone number once you add it. This is not a new issue. It has been around as long as the Facebook Graph search, but Facebook chooses to see this issue as a feature, as a letter received by Belgian researcher Inti De Ceukelaire shows.

Image by Inti De Ceukelaire/Twitter

Certainly, some people, such as celebrities and politicians, should be more concerned than others about revealing their private number online. However, anyone could potentially have a cyberstalker or hacker target them. Once a hacker has a phone number and your name, they can quickly use open-source intelligence (OSINT) tools that we've covered on Null Byte to grab further public data like occupation, employer, spouse, relationship, any other public info.

A hacker could use the information to further social-engineering attacks by calling you directly. Think of the classic " Microsoft tech support" scam, only the caller trying to trick you knows your name and intimate details of your personal life. Armed with these, it's easy to make the target think the caller is legitimate.
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How would a hacker actually go about finding your number? In theory, if they had a lot of time, they could just search all 9,999,999,999 potential numbers until they stumbled upon yours. Clearly, this isn't very efficient, so let's see the right way of doing it. For a practice subject, I'll be using DC Mayor Muriel Bowser (2017) as a random city official. In the examples below, her number was changed to protect her real number.

Step 1Use the Area Code
If you think of a target's phone number as one of all the possible 10-digit US phone numbers, you can quickly see that 10 billion North American phone numbers it far too large a list to effectively search through. Luckily for the hacker, he can cut this down thanks to the North American Numbering Plan (NANP) which lays out the guidelines for phone numbers in the US.

Let's take an example: 234-235-5678. Looking at the NANP, we can see that the first three numbers (234) are the area code, and the plan allows for 2–9 as the first digit and 0-9 for the second and third digits. That information right there eliminates one billion possible numbers from the hacker's list.

The hacker can also quickly take advantage of this if they know or can take an educated guess at where you live, as it's as easy a Google search. By doing this, the hacker can remove a further 9 billion 990 million numbers from the list of potential guesses.

The next three numbers after the area code in our example (235) are the central office prefix. Again, the plan calls for 2–9 for the first digit and 0–9 for both the second and third digits, but with a caveat.

In area codes where the second digit is 1, the third can't also be 1. This yet again removes a large number of phone numbers from the hacker's list. The last four digits of the phone number is the line number, in this case, 5678.

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