So far in this series we’ve tackled XSS, CSRF and SQLi. Next, we’ll look at the threat of unsolicited mail.

Spammers

What?

Those nasty people that send you emails suggesting you need online dating sites, prescription meds without the prescription, or offering you the once in a lifetime chance to become a millionaire just by allowing the use of your bank account to harbor vast fortunes of Nigerian princes who seem to be fleeing their country at an astonishing rate.

As a sidenote, Nigeria is a federal republic modeled after the US (once it gained independence from us Brits of course) and doesn’t have a royal family. Nor did it within the reasonable lifespan of anyone who could have been a prince. Anyway…

Real Example

On Insta-SPAM, we have a tell a friend feature, so that our users can share the love with others they know, driving traffic to our app.

It’s a simple form that lets you enter an email address and a personal message. We then send an email to the entered address with the message and a link back to Insta-SPAM. Marketing!

Spammers find this form, thank us kindly while high fiving each other, then write a script to post to this controller with thousands of email addresses and their spam payload.

Prevent this threat by:

• Consider if you need to allow customizing of the message. Is it really necessary? Instead of letting them enter a personal message, why not craft a simple, unobtrusive but catchy message for your users, thereby removing the input that could be abused.
• If you do have a message, limit the length of what can be entered, and don’t allow HTML or links.
• You can use a CAPTCHA if you want, but they are the worst UX of all time. Instead, rate limit the number of emails that can be sent in a given time period, limit the number of email addresses that can be put in the email address field of your form, use tokenization as in the CSRF step to prevent scripting.
• Have your mail sending controller check for patterns that suggest spam and flag for review before sending. And always use a message queue with a way to halt sending, so if you get hit by a spam attack, you can turn off processing of the message queue, clean it up, then turn processing back on.

Why you should care

Spam is the bane of the internet. Don’t help it propagate. You’ll also open yourself up to Phishing attacks either with your users as a target, or as a method of delivery to other victims. Don’t introduce that liability.

How to detect if someone is trying to attack you

Monitor your message queues. Multiple messages with similar or identical content (that isn’t the content you insert) are likely spam. A huge spike in the number of messages in the queue might indicate spam. And it’s a great idea to have your mailer send a copy of every sent message to an account you can look at. That way, you get the same experience as the recipient, and can quickly spot spammy emails.

Non-security benefits of protecting yourself from this threat

It’ll force you to really think about your tell a friend feature. Then you can make it not suck. (Seriously, most of them suck.)

End of Part 5

Email, an extremely old technology, is still super useful today - so anything we can do to stop giving those that seek to ruin it an easy method to do so is well worth it. Not to mention your savings in wasted sent messages! In part 6, we’ll round out our drill down in to specific threats by looking at (Distributed) Denial of Service ((D)DoS) attacks.

And, as a reminder - I will continue to repeat a disclaimer throughout: This is a barebones, do this rather than do nothing set of suggested approaches. THIS DOES NOT CONSTITUTE ROBUST, COMPLETE AND FOOLPROOF SECURITY. The goal of this effort is to provide non-security aware founders/hackers/developers/etc with a modicum of protection at a stage in the company’s growth where there are no budgets, let alone one for Information Security. The caveat is that as soon as the company experiences growth, one of their top priorities should be to mature in to a properly developed, professionally and thoroughly provisioned Information Security program, specific to their application, industry and environment.

Just as you scaffold certain items while doing rapid coding development, this is your scaffolded application security program. Think of it as the Twitter Bootstrap of web application security.

Your Feedback / Dissent

In creating this, my aim is to improve Application Security in the early stage companies that will often consider the topic "something we'll get to when we scale". As such, critiques, comments, dissenting opinions and any other type of feedback is welcomed and indeed, heartily encouraged.

Constructive feedback will be reflected in the posts themselves at the most relevant points.

If you've got feedback for me, or you have questions about how to apply this to your own startup / project, you can get in touch:

Twitter: @dstevensio

Email: [email protected]

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