A French researcher has unleashed a free tool that exploits a weakness he recently highlighted in the SMS feature of Apple's iOS that could allow an attacker to spoof the sender of a text message.

The new tool, created by researcher pod2g, basically lets an attacker send a text message that appears to be from someone you know or trust -- such as your bank.

But the vulnerability isn't in the smartphone itself, says Errata Security CTO David Maynor; rather, it's in the network transporting the SMS messages. And there are already services available, such as SMSGang's Spoof SMS Service, that provide spoofing, Maynor says. The new tool just automates SMS-spoofing, he says.

"It's a network problem," Maynor says of the issue. Even so, phone manufacturers could add some protections for this attack that help prevent malicious activity at the User Data Header (UDH) used in SMS, he says.

Even with the new tool from pod2g and the attention it attracted, it's unlikely that this type of attack would become widespread unless it's automated, Maynor says. "Until someone figures out a way to automate and monetize this and have bots doing it, you're not going to see a lot of use of it in the wild," he says.

Apple did not respond today to a media query on the development, but last week reportedly told Engadget that one limitation of SMS is that messages can be spoofed, and noted that its iMessage service verifies text-message addresses.

"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," an Apple representative said.

Pod2g says the protocol description unit (PDU) is the format that an SMS message gets converted into when it's sent to the baseband; it then delivers it to the recipient.

"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 said in a blog post about the vulnerability. "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."

Pod2g considers the flaw -- which he says is in all versions of iOS, including iOS 6 beta 4 -- to be "severe." "Apple: please fix before the final release," he said.

Weaknesses in SMS are nothing new. Renowned Apple researcher Charlie Miller, for instance, at Black Hat USA 2009 demonstrated an SMS texting bug in the iPhone that let an attacker take over a victim's phone just by sending a text message.

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