Social Referral Hacking


Every pre-product launch or beta invitation these days has some sort of social referral program. I'm talking about those cliched waiting lists that give your online friends and followers another reason to hate you. But it turns out most of these referral programs are really easy to game, and I've been first in line for the ones I care about.

I got a free five year membership to Jet, scored a really early invite to Robinhood by jumping from position 300k to the top 100, and climbed the ranks to number eight on Final. Do I really have that many friends?

Image of your spot in line in a waiting list

I definitely didn't lose any friends from some sort of shameless referral begging that most people probably resort to. I'd say I've even gained one. After jumping my way to number eight on Final, I got an email from Aaron, the CEO. But it wasn't an email scolding me for hacking their system, but rather one thanking me for my enthusiasm. “Hacking” is probably the wrong term to use though. Aaron calls them (me) spammers — and a decent minority of their submissions consist of fake referrals. I learned this at Final headquarters in Mountain View, as Aaron had invited my team over to practice for a YC interview.

They used to try and block these spammers by requiring tokens, or email verification, but ultimately decided fake referrals weren't really hurting them anyways. Your numbers look a little higher and you have the emails of people who really want to use your product. These are lighthouse customers that are die-hard enough to spend time testing shitty beta releases all because they like being on the “cutting edge.” Yes, I'm one of these people and will occasionally flip shit over new product releases if the use cases are appealing enough.

Let's get to hacking… err — spamming?

Final's signup system is the most permissive I've encountered so far. You can sign up randomly generated emails with your original referral code, and no email confirmation is required. That's great and all, but I've got better things to do than come up with fake email addresses. Robinhood required email verification (clicking links after signup) and also required me to send a session token. According to RFC 2822, the “local-part” of an email address (the portion before the @ sign) is interpreted arbitrarily by the host. Many sites over-sanitize email addresses, disallowing many ascii characters that are actually perfectly valid.

What's important is the concept of subaddressing:

Subaddressing is the practice of augmenting the local-part of an RFC2822 address with some 'detail' information in order to give some extra meaning to that address. One common way of encoding 'detail' information into the local-part is to add a 'separator character sequence', such as “+”, to form a boundary between the 'user' (original local-part) and 'detail' sub-parts of the address, much like the “@” character forms the boundary between the local-part and domain.

This flexibility was created to help users file away emails from specific sites and mailing lists. That way, if you make an account on with, when you start getting loads of unwanted messages at that address you know who sold your information to spammers.

The other nice thing about subaddressing is that you can use it to refer yourself, to yourself.

Most sites I've encountered so far allow subaddressing, since they are valid email addresses and all. More importantly, they treat subaddressed emails as unique addresses. For example, if the target email you want to move up to first place in line is, then you can sign up through's referral code, and both addresses will be considered unique and valid. This means that when referring myself, I don't have to have hundreds of fake email accounts set up if email verification is required. With that in mind, all that's left to do is find the endpoint for the signup page and write a script that makes a bunch of POST requests with uniquely subaddressed emails, being sure to pass the correct headers so that your “target” email will actually get referral credits.

Open up chrome dev tools, choose the “Network” tab, and click the “Preserve Log” checkbox. Then fill in your target email and sign up.

Copy as cURL

You'll have to find the POST request that hits the signup endpoint. Chrome Dev Tools has a nice feature that allows you to right click the request and “Copy as cURL.” This will come in handy for generating all the referrals.

I've removed a lot of headers from the curl request below for readability's sake.

In this case, the site required x-www-form-urlencoded encoded data of the new email address to sign up, which is included after the -d flag.

From here on out, you can just generate a script to hit the signup endpoint with a bunch of subaddressed emails. You'll have to check on a case by case basis for each site, because they all have their unique way of applying referrals. In this example, the important part is the data we're sending with the -d flag, and the referral URL passed in the Referer header. Notice the $i variable after the -d flag.

for i in `seq 1 100`;
    CMD="curl -X POST -H \"Origin:\" -H \"Referer:\" -d 'signup%5Bemail%5D=foo%2B$'"   
    eval $CMD 

I know this isn't the prettiest way of automating curl requests, and it doesn't answer a lot of questions. How would you automate for sites that require email confirmation? What if unique session tokens are required? Most of these problems are solvable, but the effort required to overcome them only eats into the time you saved from not spamming your friends.

Now that everyone can be in first place, no one will be. Hopefully we will find more effective ways of building hype for new products without forcing people to share things with their friends before they even know if they like the product or not.