iPhone Security Issue Opens Door to SMS Spoofing
The issue is related to iOS's handling of User Data Header (UDH) information, an optional section of a text payload that allows users to specify certain information such as changing the reply-to number on a message to something other than the sending number. The iPhone's handling of this optional information could leave recipients open to targeted SMS spoofing attacks.
In the text payload, a section called UDH (User Data Header) is optional but defines lot of advanced features not all mobiles are compatible with. 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.pod2g highlights several ways in which malicious parties could take advantage of this flaw, including phishing attempts linking users to sites collecting personal information or spoofing messages for the purposes of creating false evidence or gaining a recipient's trust to enable further nefarious action.
Most carriers don't check this part of the message, which means one can write whatever he wants in this section : a special number like 911, or the number of somebody else.
In a good implementation of this feature, the receiver would see the original phone number and the reply-to one. On iPhone, when you see the message, it seems to come from the reply-to number, and you [lose] track of the origin.
In many cases the malicious party would need to know the name and number of a trusted contact of the recipient in order for their efforts to be effective, but the phishing example shows how malicious parties could cast broad nets hoping to snare users by pretending to be a common bank or other institution. But with the issue resulting in recipients being shown the reply-to address, an attack could be discovered or thwarted simply by replying to the message, as the return message would go to the familiar contact rather than the malicious one.