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Mobile Networks Security SIM Cards

SIM Unlock Shims

What do Unlock SIMs sold online actually do and how do they do it?

There’s a lot of “Magic Unlock SIM” products online; IdeaLTE, U-SIM LTE 4G Pro II (sic), UltraSIM, TurboSIM etc, with no real description as to what they are or how they work,

They claim to do something to do with unlocking iPhones, but with little other info.

Being interested in SIM technology, and with no real idea what they are I ordered a few.

What are they?

They’re man-in-the-middle SIM card devices that are able to intercept requests from the UE / baseband of the device.

They sit on top of the real SIM card, between it and the SIM Slot.

One of the ones I bought had a sticker on it that helped stick it into place, the other just sat above the SIM below the phone.

This means when the UE sends the APDU to request some data from the card, the SIM-shim device analyses the request, and if it matches the rules on the SIM-Shim, intercepts it and responds with something else, ignoring the data the real SIM card would send back and injecting its own,

The use for this seems to be to do with how Apple does Carrier Locking on the iPhone. It seems in the iPhone carrier settings are ranges of ICCIDs used by the different carriers for their SIMs, and uses that to identify the carrier of the SIM.

With this information it’s able to determine if the SIM card is from the carrier the iPhone is locked to or not,

Now you’re probably seeing the value in this attack – By intercepting the request for the ICCID of the card, and instead of responding with the real ICCID, the SIM-Shim intercepts the request and sending back an ICCID of a card the iPhone is carrier locked to, the iPhone is tricked into thinking it’s talking to a card from the carrier the phone is locked to.

So let’s say we’ve got an iPhone from Carrier A, and they’ve told Apple their SIM cards have ICCIDs in the range from 0001 to 0005,
If I put a SIM card with the ICCID 0003 the iPhone knows it’s a SIM from Carrier A,
If I put in a SIM card with ICCID 9999 the iPhone knows the SIM is not from carrier A, and therefore prevents me from using the iPhone,
But if I put in one of these SIM Shims, when the iPhone ask the ICCID of the card, the SIM Shim will respond with an ICCID we set on it, so if we want to use SIM with ICCID 9999 in a phone locked to Carrier A, all we’ve got to do is setup the SIM-Shim to respond with ICCID of 0001 for example.

Phew. Ok, that’s the short run down on how it works (There’s more to activating iPhones but we’re here to talk about SIMs!).

The Hardware

So physically these are “shims” – they sit between the real SIM and the mobile phone and intercept the communications.

It blows my mind that someone’s been able to manufacture these in such a small form factor.

But there is one rather glaring flaw in having a tiny wafer that sits on top of your regular SIM, and that is if it pops up/down/ get loose and become hellish to get out.

I found their insertion and removal is a bit of a game of Russian roulette as to if it will go in, or come out, without brute force and potential damage to the device.

In the end on one iPhone I had to force the SIM tray out with a set of needle nose pliers, and my little SIM-shim was pretty beaten up and no longer useable. RIP SIM-Shim 1.

I think this may have been an early version of the same thing? Or possibly to allow dual SIM on an iPhone?

The Software

Interacting with the IdealLTE for example, is via SIM Toolkit Application for managing ICCIDs.

You can set any ICCID you want, which is cool, but limited.

Unfortunately I haven’t been able to find any way of messing with these to allow interception / replacement for other APDUs, for example if you could change the Administrative Domain to get higher access to the network.

I will at some stage put these into a SIMtrace and compare the output, and have a poke around and see if I can find anyway to change / update these, or if there’s any APDUs it responds interestingly to.

Unfortunately I’ve actually lost the new unit I had to replace the one I broke, they are very very small…

I reached out to the developer / vendor but they seem to go dark and popup under a different name, I’m not holding my breath…

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