What You Should Know About the iPhone SMS Spoof Attack

SMS text messaging is certainly not exclusive to Apple or its iconic iPhone smartphone. But, apparently there is something unique about the way Apple delivers SMS messages that makes the iPhone particularly vulnerable to spoofing or smishing (SMS phishing) attacks.

An iOS security researcher wrote a blog post detailing the discovery. When an SMS text message is sent, part of the header information contains the actual number the message originated from. However, there is also an optional header called the UDH (User Data Header) which allows for a different Reply To address to be entered.

Some mobile platforms display both the actual originating number and the information from the Reply To field, hopefully raising some red flags for the recipient if the two are different. Apple’s iOS only displays--and responds to--the address specified in the Reply To field.

Why is that a problem? Well, if an attacker knows the phone number of your financial institution, or your Mom, or your boss, he (or she) could send a text message to your iPhone that appears to originate from that number. On an iPhone, the SMS text message would seem to be from a legitimate source, and you’d be much more likely to respond, or comply with requests for sensitive information you normally wouldn’t share.

Apple responded to Engadget regarding the security flaw with the following statement:

“Apple takes security very seriously. When using iMessage instead of SMS, addresses are verified which protects against these kinds of spoofing attacks. One of the limitations of SMS is that it allows messages to be sent with spoofed addresses to any phone, so we urge customers to be extremely careful if they're directed to an unknown website or address over SMS.”

The problem with that “solution” is that iMessage only works between iOS devices. So, unless everyone you might send or receive text messages from is also using an iPhone, iPad, or Mac OS X to communicate with you, iMessage isn’t actually a feasible fix.

The security researcher who revealed the flaw summed his blog post up with, “Now you are alerted. Never trust any SMS you received on your iPhone at first sight.”

That seems fair, but there are some other elements you can use to determine if the message is legitimate or not. First of all, if you receive a message from someone who is not in your iPhone contacts, it generally shows up as the originating phone number as opposed to “Mom”. As mentioned above, an attacker who knows your mom’s mobile number may be able to send a spoofed message that appears to be from your mom, but a spoofed message from another number should appear as the number itself even if the message claims to be from your mom.

Second, common sense should play a role here as well. If you and your best friend text regularly about the sports, or politics, or what the plans are for the coming weekend, and you receive a text that just says “click this link”, you should be suspicious. If your Mom barely knows what text messaging is, and never really uses SMS, it should alert you that something isn’t right if you get a message out of the blue asking for money.

SMS text messaging is a great tool, but it’s certainly not the most secure. Apple’s implementation of SMS may be more prone to spoofing than other mobile platforms, but you should think twice about clicking links or sharing sensitive information via SMS messaging on any platform.

As smartphones continue to become more mainstream, attackers will continue to seek out ways to find weaknesses and exploit them. And while iOS has so far remained relatively secure compared to other smartphone operating systems, it's by no means perfect. As attackers get more aggressive about targeting smartphones and tablets, the need for cross-device security will only continue to increase.

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