Facebook Lets You Harvest Account Phone Numbers

Frequent contributor Bennett Haselton writes with some strong cautions on a Facebook "feature" that lets you search for random phone numbers and find the accounts of users who have registered that number on their Facebook profile. This has privacy implications that are more serious than searching by email address. Especially in light of the expanding emphasis that Facebook is putting both on search qua search and on serving as a VoIP intermediary (not to mention the stream of robocalls that the FCC is unable to stop), this might make you think twice about where your phone number ends up. Read on for Bennett's description of the problem and some possible solutions.

A few weeks ago a friend of mine said she was getting harassing text messages from a particular phone number, which she didn't recognize and which didn't appear in any of her own records. On a whim, I suggested entering the number into the Facebook search box, whereupon we found the guy's profile (even though he had no friends in common with the account we were logged in under), realized who he was, and ratted the thirty-something out to his Mom.

Then I thought: Is it really a good idea, for this to be possible? I tried entering consecutive phone numbers (starting with a random valid number, and varying the last 2 digits from 00 to 99) into Facebook's search box, and 13 of them came up with valid matches. None of those matches had any friends in common with the account we were searching from; as far as I can tell, anybody could enter any phone number into Facebook's search box and find the account associated with it, if there is one.

I think this has non-trivial privacy implications. (I repeatedly contacted Facebook explaining why I think this is a problem, but they haven't responded.) I'm not talking about the ability to find the account associated with a particular phone number — I think relatively few people have a legitimate need to send text messages from a truly anonymous phone number, and if they do, it's their own fault if they're dumb enough to put that number on their Facebook profile. And it wouldn't be a practical way to unmask the phone number associated with a particular account, either — even if you knew the person's area code, and narrowed down the list of possible exchange numbers following the area code, you'd still have to try tens of thousands of possibilities.

Rather, the problem is that you could use this technique to build up a database of phone numbers and associated accounts without targeting any specific phone number or account. Not only would you know the names associated with each of the numbers, you could associate the phone number with anything else that was discoverable from the person's Facebook profile &mdash which usually includes their location, their interests, and the names of their other friends. (By default, all such information is visible on your Facebook profile — even to users who aren't your Facebook friends and have no friends in common with you — but your contact information is supposed to be hidden from other users unless you've confirmed them as friends.)

An attacker could do this with email addresses too, of course, if they had a long list of email addresses known to be valid, by searching to see which ones were associated with Facebook accounts. Or they could supplement it with a list of automatically generated email addresses like john001@hotmail.com, john002@hotmail.com, similar to what spammers use in a dictionary harvest attack, and hope that some of those would map to valid accounts as well. The difference is that because the space of possible email addresses is effectively infinite, and because many people use email addresses on Facebook that aren't on any publicly circulating databases, an email search would probably not hit more than a small portion of Facebook accounts that were searchable by email address. On the other hand, since the space of possible phone numbers is finite, with enough patience you could uncover every Facebook account that had an associated phone number. As my short experiment above showed (13 out of 100 random numbers mapping to accounts), you could start building up a list of valid hits pretty quickly.

Similarly, it's already trivially possible for an attacker to build up a long list of other users' Facebook accounts - start with one person's account, go through their friends list, then visit the profile of each of those users and index their friends list, etc., like a search engine recursively spidering the Web. However, you'd be left with a large list of Facebook accounts but no way to contact them — you wouldn't have their email addresses or phone numbers, and if you send a message to a non-friend on Facebook, it goes into a subfolder of their Inbox marked "Other", which most users never check. The phone number dictionary attack described above, is the only loophole I can think of that lets you harvest a large list of Facebook users and a means to contact them in a way that they will actually see.

What could somebody do with such a database? Well, even if you only had a small list of a few thousand people, you could try spamming or scamming the numbers via text message. SMS scams are nothing new, of course, but they would probably be more effective if supplemented with the details you could get from a person's Facebook profile. (For straight-up spam, you can target it based on the interests listed in a person's profile. For scams, remember that you can use names taken from a person's friends list: "Hi, this is Jessica Smith. I have to pay off a parking ticket online or my car will get towed; can I borrow your credit card number and then I'll pay you tomorrow?")

Or if you spidered so many accounts that you built up a database which included a significant portion of all Facebook users with phone numbers on their profile, you could even launch your own publicly searchable website, splattered with grey-market pop-up advertisements: "Look up any Facebook user's phone number! If they've got their number on their Facebook profile, we have it here!" (While this would certainly raise awareness of the problem, I think it's more likely that the data harvester would decide they could make more money trading the data on the black market.)

I haven't seen this issue raised anywhere else, but lest you accuse me of "giving the bad guys ideas", I do think it's sufficiently obvious that some people on the dark side have probably discovered it, or would have, even if I hadn't brought it up. And even if any of these outcomes is unlikely, it would only have to be done once, to put the users' data permanently in the hands of the attackers, with Facebook unable to put the cat back into the bag. (Although they could at least rectify the problem for new users going forward.)

Balanced against this, what is the upside of being able to search for someone's profile on Facebook using their phone number? In my Facebook-using days, I never did it, since it was always easier to find someone using their email address, or by searching for their name, or by finding them in the friends list of one of our mutual friends. But even in a case where all you had was the person's phone number, is it too much to text them and ask for their first and last name, or their email address, so you can add them on Facebook?

Although Facebook did not respond to my inquiries, it's true that the existing behavior doesn't technically look like a violation of their Privacy Policy ("To make it easier for your friends to find you, we allow anyone with your contact information (such as email address or telephone number) to find you through the Facebook search bar..."). And I verified with a new test account that by default, in your privacy settings, under "How You Connect", the setting "Who can look you up using the email address or phone number you provided?" is set to "Everyone." The problem is that this setting casually lumps the two together, and users — as well as Facebook itself — might not realize that the implications of being findable by your phone number, are different from being findable by your email address.

Facebook should probably just go ahead and block searches by phone number — or, at least, make you fill out a CAPTCHA every time you do a phone number search, to make it harder to harvest them in bulk. There's no way to know if scammers are trying this already, but at least we can prevent it going forward. That would require a small edit to Facebook's privacy policy, but luckily for them, they can now do that without even calling a vote.

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Comments

SMS Spam

[krakass]

Amazingly I got spam, I'm assuming because of this, just 5 minutes ago. Saying my profile picture is cute and they want to chat on yahoo messenger. Except for that fact that my picture is the retarded kid from the Stargate movie.

funny how everyone 'wants' your phone #

[TheGratefulNet]

last time I went for a haircut, the first thing they asked me was my name. fine, they can call me when the next haircutter is open.

then they wanted my phone #. really? for a date, maybe? ;) (some of they are definitely cute).

no, they want to collect data and sell it. how absurd.

of course I declined. if you don't need it, you don't get it. and they most certainly don't need it.

reminds me of a rental app I was once asked to fill in. it had the usual ss#, date of birth, full name - but they also asked mothers maiden name. now, I realize that with some work, you can get that from public records, but you have to work for it and its still partially a password of sorts that banks use to verify your ID when you call on the phone (or lost your password for online). a housing rental that wanted pretty much all the info that the bank would ask me to verify my id. yeah, sure, I'll just give you that (not!). when I called the realtor on this, he simply said 'good luck in your search'. basically, he knew he was asking more than he had a right to and simply avoided admitting it.

watch what you give out, people. think about every bit of info and if they don't need it, don't give it to them.

Re:funny how everyone 'wants' your phone #

[bferrell]

I'd think actually the number collection is so that the next time you go in, they can put your phone number in and ID you... "Do you have a discount card? Do you have it with you?? No, can I get your phone number? There you are!"

Most small shops don't (yet) have the smarts/connections to sell customer data. But the potential IS there, yes.

Re:funny how everyone 'wants' your phone #

[geekymachoman]

They don't want only your phone number, they want everything personal about you. And step by step, they're getting it.

Facebook offered like 2 euros or something in facebook credits if you entered your phone number. Youtube is forcing a channel name change to your real name or something like that, and when you decline they ask you why.. and in "reasons" offered, you can't choose "because of privacy" or whatever. They are pretending that giving your phone number/real name is a normal thing to do. And eventually they gonna brainwash people into thinking it is a normal thing to do, so everybody will do it without thinking twice.

When you see the current trend, you can extrapolate what the future will look like. Don't need to be bloody Einstein for it.

Re:funny how everyone 'wants' your phone #

[Jason Levine]

its still partially a password of sorts that banks use to verify your ID when you call on the phone

Not always. My identity was stolen once. The thieves opened a credit card in my name using my address, SSN, and DOB. They got my mother's maiden name wrong. (Wasn't even close.) It didn't raise a single red flag to stop the transaction. Neither did them changing the address immediately and asking for rush delivery of the card or trying to get a $5,000 cash advance before the card even was activated.

So banks might *say* they're using Mother's Maiden Name to verify identity, but not all banks (*cough*Capital One*cough*) do.

Re:funny how everyone 'wants' your phone #

[Applekid]

Not only that, since it will be the new normal, you will be strange for not wanting to give up that data.

Re:funny how everyone 'wants' your phone #

[I Mean, What]

GameStop does this. They don't even bother asking if I have one of their rewards cards to swipe to identify me. Having my phone number on file with GameStop has two advantages for me. When I had preordered games in one state and moved to another, they were able to transfer the preorder from my old location by using my phone number as a unique identifier. They also use my phone number to text me when games I've preordered have arrived. I love video games, but not enough to mentally keep track of their release dates.

On the flip side, I would be extremely pissed off to find out they sold my number to a robocaller. When Facebook started asking for my phone number I said fuck off. They couldn't hear me because I said it to the screen and clicked the No Thanks button or w/e. But I said it again when I had my FB account perma-deleted. Fuck Facebook. Fuck them running til they limp. GameStop is a legit business (insert chuckles here), Facebook is some douchebag Harvard dropout mercenary data miner with no respect for anyone. Zuck You, Fuckerberg.

Re:funny how everyone 'wants' your phone #

[Spamalope]

I told Facebook to FO when they asked for my number too.

Facebook took my number from a friend's mobile phone's contact list and added it to my profile two weeks later. I never gave it to them. They can die in a fire.

Re:funny how everyone 'wants' your phone #

[Registered Coward v2]

last time I went for a haircut, the first thing they asked me was my name. fine, they can call me when the next haircutter is open.

then they wanted my phone #. really? for a date, maybe? ;) (some of they are definitely cute).

reminds me of a rental app I was once asked to fill in. it had the usual ss#, date of birth, full name - but they also asked mothers maiden name. now, I realize that with some work, you can get that from public records, but you have to work for it and its still partially a password of sorts that banks use to verify your ID when you call on the phone (or lost your password for online). a housing rental that wanted pretty much all the info that the bank would ask me to verify my id. yeah, sure, I'll just give you that (not!). when I called the realtor on this, he simply said 'good luck in your search'. basically, he knew he was asking more than he had a right to and simply avoided admitting it.

watch what you give out, people. think about every bit of info and if they don't need it, don't give it to them.

Why do people assume you have to give everyone real info? They have no way of knowing what your mother's maidan name and simply picking something you can remember such as some random street name you like. Unless you pick something truly bizzare, like West 52nd or Avenue of the Americas, Lindy or Ruby should be fine. Oddly enough, the only person I know who has had an issue is because her maiden name only has a few consonants (thanks to the immigration guy at Ellis Island when her grandfather emigrated) and gets questioned when she gives her name. I've done that, along with giving a long defunct corporate phone number and never had an issue; in fact 555-1212 with a random area code works fine for affinity cards.

Re:funny how everyone 'wants' your phone #

[Safety Cap]

I told Facebook to FO when they asked for my number too.

The better way to deal with such data-harvesting schemes is to fill it with plausible but junk data.

That serves two purposes:

1. You don't stand out as a "problem"
2. Finding and correcting said junk data becomes an impossible task if enough people do it.

So in the case of Facebook asking for your phone number, use the correct (or neighboring) area code and make up the other digits. Don't use 555-xxxx or Jenny's number as those are too easy to spot.

Of course, if you use two-factor authentication (which is a good idea to thwart the majority of crooks who happen to be unskilled/stupid), you'll have to provide your real number, or a working proxy.

Re:funny how everyone 'wants' your phone #

[QuietLagoon]

I'd think actually the number collection is so that the next time you go in, they can put your phone number in and ID you... "Do you have a discount card? Do you have it with you?? No, can I get your phone number? There you are!"

Most small shops don't (yet) have the smarts/connections to sell customer data. But the potential IS there, yes.

If they are big enough to have a customer card, then they have the smarts/connections to sell customer data. Indeed, the customer card service is probably run for them by a data collection company.

.

Re:funny how everyone 'wants' your phone #

[icebike]

I told Facebook to FO when they asked for my number too.

Facebook took my number from a friend's mobile phone's contact list and added it to my profile two weeks later. I never gave it to them. They can die in a fire.

Exactly.

Even those of us who have NO Facebook account at all can still be found on Facebook because so many people sync their phone contacts with Facebook, and unlike Google's address book, Facebook leaks theirs.

Re:funny how everyone 'wants' your phone #

[Charliemopps]

I refuse any request they make for data. Radioshack, for example, wont stop until you say No. I saw a guy in front of me give them his name, address, phone number, zipcode, I was astonished. Then I get up there and they ask for my name, I say no... they looked confused... then asked for my phone number... I said no... then he started to tell me he couldn't even check me out without a phone number! I told him "I guess I'm not shopping here then" at which point the manager of the store practically jumped the counter and told the checkout clerk to just use the stores number. Common sense prevails for once.

What's really funny about this whole thing is that they looked at me like I were crazy. As if I were doing something strange by not wanting to give complete strangers all of my personal info just to buy a 35 cent bulb for my flashlight. The scary part isn't that they ask for this info, or that people give it. The scary part is that they look at you like you're crazy when you refuse to give it. Society this this level of intrusion into your personal life is not only normal, but expected, and you're out of whack if you don't want to supply it.

Re:funny how everyone 'wants' your phone #

[idontgno]

The sending address? The spoofed sending address which is either (A) one of the other spam victims, or (B) a joe-job designed to slander and inconvenience someone the spammer has conceived a grudge against?

I guess hilarity will ensue when you receive a spam you sent yourself, according to the sending address you naively trust.

Re:funny how everyone 'wants' your phone #

[leenks]

You do realise that most spam is sent from legitimate email addresses, right? Ie they use an address from the list they are sending spam to as the sender.

Schemes like this just fuel spam, not reduce it.

Facebook being a bit lax with privacy....

[cant_get_a_good_nick]

The headline just writes itself sometimes.

The interesting mix, is that just a few stories down on the home page is the story about the Facebook VOIP app that only can call Facebook users that have phone numbers on their profiles. Sometimes it's obvious that Facebook is moving too fast to realize how their different systems interact.

Facebook isn't interested in your privacy ...

[gstoddart]

They want your information so they can sell it. They want as much as they can possibly get.

Do you think Facebook even try to protect your privacy? They write a feature which you might want, but which mostly benefits them.

And they've shown time and time again, they're not very good at even trying.

That fact that Zukerfucks sister got burned with privacy settings says they're deliberately obtuse.

Sure, Facebook could do all sorts of things to protect your privacy, but that's now how they get paid.

Re:Facebook isn't interested in your privacy ...

[fermion]

But who gives them that information? Is there really a reason to have a phone number anywhere on the web unless you want people to call you? I know that somethings on facebook are supposed to be private, or accessible only to select people, but we have seen in case case where that status was not protected or changes in the privacy statement made information public. This is not 2012. We don't really have the excuse to say that facebook, funded by advertisers, made my data public without my knowledge. We pretty much know that facebook is going to do this. It is like complaining that you gave a stranger $900 to buy an ipad and you never got it. Sure you were robbed, but not under threat of violence or even something that common sense should have told you not to do.

What might be interesting is why this problem of leaked phone numbers is not more widespread. To phrase it differently, why does google not recognize a phone number and perform some magic to make it link to a name. I would think because the traditionally extremely profitable and protective reverse lookup services have convinced them the ad revenue from said services would be more lucrative than any benefit the additional end user feature might provide. After all the phone book is public record, and given their agressive collection of personal information during their drive bys, they really care nothing about safety or privacy.

Re:Facebook isn't interested in your privacy ...

[gstoddart]

But who gives them that information? Is there really a reason to have a phone number anywhere on the web unless you want people to call you?

Google regularly asks me to attach a mobile number in case I lose access to my account. Facebook occasionally tells me the same lie.

They ask for this kind of stuff all the time. I also routinely see the same stupid canned "security questions" with no option of filling in your own meaningful (and secure) questions -- and those standard questions are too easily gotten by someone else.

Re:Facebook isn't interested in your privacy ...

[I Mean, What]

When you post anything to or about your friends, anything at all, you're informing on them. The only winning move is not to play.

Apparently you can search for lots of things

[rudy_wayne]

Apparently you can search for lots of things on Facebook. For example:

http://media.gizmodo.co.uk/wp-content/uploads/2013/01/facebookgraphlols1.png

http://media.gizmodo.co.uk/wp-content/uploads/2013/01/facebooklols4.png

Re:Apparently you can search for lots of things

[Obfuscant]

Apparently you can search for lots of things on Facebook. For example:

Fearing the worst, I went to Facebook and tried searching for phone numbers. The first phone number I entered was from my own area code, made up prefix. It returned not one result, but a large number of results. People from Mexico, Indonesia, Greece, all over the world. Amazing, these people all have the same US phone number?

So I tried it with my own phone number. Again, a large number of hits, people from Mexico, Indonesia, Greece. Foreign countries like New Mexico, too. But I was not in the list. This is even more amazing. These people all have the same phone number I do, and I don't.

I think the fact that Facebook will find matches for phone numbers you enter into their search doesn't necessarily mean that those people have that phone number. Or that the number will accept SMS messages, spam or otherwise.

The phone books of the past....

[joocemann]

... screwed us over more than they helped genuine people find us... Oh wait. Nope. They were optional, like facebook, and mostly the people that called were worth answering for.

Don't use that on face book, or toss out the tin hat.

Re:The phone books of the past....

[joocemann]

Forgot to mention.

You're not as important as you think you are. Chances are good that your phone number is useless to people that don't already have it.

Re:The phone books of the past....

[Cro Magnon]

The phone books of the past gave a name, address, and phone #. FB will give name, address, phone, blood type, school, job, gross pay, and your vacation plans.

Re:The phone books of the past....

[sdnoob]

the difference between the phone companies' directories and facebook is that when you tell the phone company you want an unpublished listing... they oblige, as required by law.. facebook, on the other hand, will just fuck around with default privacy settings until your data is public.

2013: Still using Facebook

[Anonymous Coward]

Why are you people still even on Failbook in the first place? Are you really such sheep that you just have to be there "because everyone else is"? If everyone else jumped off a cliff would you follow them to your death? Don't be a Lemming.

o Facebook does NOT have your best interests at heart. You're just a "product" that it sells to advertisers.
o "I have nothing to hide" is a bullshit reason to post your whole life on the Internet. You really think the government and corporations aren't mining that data to predict -- and ultimately control -- your life? Wise up.

o "I want to stay connected to people". Here's a radical idea: How about you actually see people in person and interact and "connect" with them that way? This is what you people don't seem to get: The Internet does NOT "connect" anything except computers; your "friends" on Facebook are not your "friends" unless you actually SEE them and TALK TO THEM in person on at least a semi-regular basis. Failbook "friends" may as well be machine intelligence pretending to be people for all you know. Words on a page do not constitute a relationship!

You and everyone you know who says it is wrong: Your privacy is worth something, and it is real. Don't give it away to some fucking corporation, don't give it away to ANY government for ANY reason. The Internet is not your "friends"; it is just HARDWARE. Meet with real, live people; spend time with them, TALK to them, KNOW them, not just words on a page.

Jenny?

[magarity]

Search FB in all area codes for 867-5309 and ask to speak to Jenny. Lolz

putting on my tin foil hat for a moment...

[logicassasin]

There's more to it than meets the eye. I don't have a FB account, so I can't fathom why they would ask for you to include your phone number on it for any reason. I do know that Google now REQUIRES it just to open a Gmail account.

Some part of me simply doesn't trust this. We all know about correlation engines and how they work, and we know that the NSA collects and reads your emails (http://www.guardian.co.uk/technology/2012/sep/15/data-whistleblower-constitutional-rights). Now we add into the mix your phone number, which, as we already know is subject to warrant-less tapping (http://www.businessinsider.com/senate-renews-controversial-law-which-allows-warrantless-wiretapping-of-us-citizens-2012-12) and if the number you provides happens to belong to your cellphone, we know that it can act as a covert "roving bug" (http://yro.slashdot.org/story/06/12/02/0415209/fbi-taps-cell-phone-microphones-in-mafia-case). All of this provides more data to track you, what you do, who you interact with, who you're near at any given moment and those individuals interactions... All in the name of "keeping you/this country safe".

This simply doesn't sit well with me.

Re:putting on my tin foil hat for a moment...

[blueg3]

I do know that Google now REQUIRES it just to open a Gmail account.

Nonsense. It requires name, birthdate (without any verification), gender (including "other"), and solving a CAPTCHA. There is a mobile phone number field, but it doesn't complain if you leave it blank.

Just don't give FB your phone number

[DaveAtFraud]

I gave FB 555-1212 as my phone number. If someone wants to contact me, FB provides lots of ways for people I know to get in touch or request I "friend" them so they can.

Cheers,
Dave

Re:Just don't give FB your phone number

[Overzeetop]

It wasn't clear FTFA whether the phone numbers were marked as private or public on the FB accounts. If the information is marked to be shared only with "friends" then I don't believe it's searchable. I can't test it on mine...everything on my account is generally shared globally or simply not in facebook.

I do think it's funny that the dummy account I set up on FB for use with websites which want to use FB as their login criteria (and in which profile I put NSA as my employer) asks me from time to time if I happen to know "these other people who are work for the NSA."

Re:Just don't give FB your phone number

[erice]

Still, this is a pretty serious permissions flaw. Users that are not privileged to see information should not be able to search for it either.

As far as I can tell, if they have your phone number but it's set to not be visible to anyone else, it can't be searched for.

The only tests the author seems to have performed would not give any indication of what privacy setting was assigned to the phone number. So, all of his results could have been from people who had public phone numbers on Facebook.

I tested it with a friend's email address. Her "real" email address is not visible but by searching for it, I can find her page.

Re:similar too.....

[Applekid]

Except with bathroom stall numbers, the odds are greater that the call will be for a good time instead of how I can refinance my left nut for fast ca$$$$h

Re:oh no!

[SilverJets]

Where I live phone books only list landlines and the name and address tied to that land line.

A lot of people are going cell phone only. One of the benefits being you have a private number without having to pay the phone company extortion money to keep your name, number and address out of their phone book.

Re:This must have been changed quite recently

[blueg3]

Do you have something like NoScript that inhibits the action of reCaptcha? Gmail requires a phone confirmation if you don't fill out the reCaptcha.

I've had to create throwaway Gmail accounts for a variety of things and have never seen the forced-phone-number thing.

Facebook allows users to set who sees the phone

[origamy]

You can change that in Facebook > Privacy Settings > Who can look me up? > Who can look me up by e-mail or phone
Simply change from "Everyone" to either "Friends" or "Friends of Friends".

Alternatively, do not give Facebook your phone number.

Re:I see stupid people...

[waddgodd]

Well, I was going to wait for your karma to catch up with you, your reply really should be -1; flamebait, but clearly that isn't happening, so you'll get the response you deserve. Congratulations on selling your privacy cheap, I'm sure that the rest of the world appreciates you for it. What's the old saying, "if you didn't pay for it, YOU are the product", and you have clearly given them a lot of product to work with, and for that I thank you, as it makes the system work. People like me, who sell their privacy dear, don't get very far without the suck^Wusers that make the "you are the product" a net positive transaction for the Zuckermans of the world, as in they can provide whatever sops of value they allow us all because they're making substantially more from all the free information you're giving them. As for your armchair analysis of my past lives, good on you for giving more of a fuck about my past lives than I do, I hope it serves you well.

ha

[sootman]

> And it wouldn't be a practical way to unmask the phone number
> associated with a particular account, either -- even if you knew
> the person's area code, and narrowed down the list of possible
> exchange numbers following the area code, you'd still have to
> try tens of thousands of possibilities.

That's why God made computers. Even if FB blocks cURL and the like, there are many ways to automate a browser.

A service for mankind

[gmuslera]

Facebook has been creating a culture of security awareness for all mankind since 2005. Most people, don't know, is new to computers/tablets or technology in general, but slowly the entire world is getting a clue on what should not to be done in the network, not because boring teaching or manuals, but feeling in real life the consequences, because facebook.