The ability to spoof the reply-to information on Apple iOS SMS messages is a reminder that such messages should not be considered any more secure than email when it comes to authenticating senders, experts say.
A scheme for spoofing the information was disclosed Aug. 17 on a blog dedicated to research on Apple iOS security. The issue allows attackers to disguise the sender and potentially trick whoever receives the message by modifying the SMS data header. This allows the attacker to include a different reply-to number and possibly impersonate other people or organizations.
"If you either own a smartphone, or a modem and an account in a SMS gateway, you can send texts in raw PDU format (some services also exist to send a text with an HTTP request in raw PDU format)," blogged researcher Pod2g, who disclosed the flaw last week.
"In the text payload, a section called UDH (User Data Header) is optional but defines lot(s) of advanced features not all mobiles are compatible with," the researcher continued. "One of these options enables the user to change the reply address of the text. If the destination mobile is compatible with it and if the receiver tries to answer to the text, he will not respond to the original number, but to the specified one."
"In a good implementation of this feature, the receiver would see the original phone number and the reply-to one," Pod2g blogged. "On iPhone(s), when you see the message, it seems to come from the reply-to number, and you [lose] track of the origin. Why is it an issue? Pirates could send a message that seems to come from the bank of the receiver asking for some private information, or inviting them to go to a dedicated website."
In a response to Engadget, Apple said that it takes security "very seriously," and highlighted the availability of iMessageâwhich works on the iPhone, iPad or iPod Touch running iOS 5 or later and Mac OSX 10.8 (Mountain Lion) or later.
"When using iMessage instead of SMS, addresses are verified which protects against these kinds of spoofing attacks," a spokesperson told Engadget. "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."
Michael Sutton, vice president of security research at Zscaler's ThreatLabz, said the issue was not an iOS vulnerability per se.
"The iOS application is not uniquely vulnerable due to a coding error, but rather includes and relies on a field that most SMS clients do notânamely the 'reply to' field," he said. "Just as email is easily spoofable as fields indicating the origin of a message are not verified, the same is true of SMS."
Spoofing the 'reply to' field is trivial, he added, but the damage that can be done is limited.
"While a user could be tricked into sending an SMS message to a location other than what they intended to, this issue could not be used to compromise the device in any way," he explained. "It could however be used in a social engineering attack to trick a user into believing that a message came from a particular source. This is no different than receiving a spoofed email message and users should be equally as suspicious of unexpected SMS text as they are of emailâthe true source in both cases is unverified."
Users should not be willing to disclose information over SMS that they expect to be secure, nor should they assume that it is a trustworthy channel for communications with banks or other sensitive service providers, said Derek Halliday, the Lead Security Product Manager at Lookout Mobile Security.
"SMS should not be considered a secure communication method, it is no more trustworthy than email in this regard," he said.