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Comment by wkat4242 | Hacker News Reader

Smartphone modems (baseband) are super optimised for battery life. They don't send any traffic that isn't meant for the device itself on to the CPU. That would only cause unnecessary load.

They could perhaps be modified to do that but the baseband firmware is usually very closed source.

There is only one example I know, there was one particular dumbphone from the 2G era for which the baseband sourcecode was available due to a hack. You could use several (one for uplink and one for downlink) of these with modified firmware to sniff 2G traffic. I forget which model it was exactly but obviously the price ballooned on eBay :)

Haven't heard of this happening with later models. Baseband sourcecode firmware is really rare.

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Certainly Qualcomm modems can have their diagnostic mode enabled when you have access to /dev/diag - usually on rooted devices but occasionally on stock.

You can ask the processor to send higher layer information via diag, including the messages the base stations send. There’s also commands to lock on to a specific base station so you’re not constantly moving from cell to cell.

There’s plenty of commercial devices that use this functionality to provide network monitoring and management capabilities for mobile network operators checking out base station functionality in the field. TEMS comes to mind for that but they’re certainly not the only ones.

It’s a deep rabbit hole :-)

The diagnostic mode just lists the cells and their parameters afaik. It doesn't capture IMSIs or traffic to/from other devices like this does. It's like the network diagnostics menu built into Samsung and Apple phones.

It isn't even able to list some crucial parameters needed to identify neighboring cells. It's simply dumping data that's already used by the modem for its regular operation.

It does, however, more than just "listing cells" though. You can sniff all the comms, but only between your device and the base station. It won't listen to anything else, you need SDRs for that.

You may be thinking about the Motorola C123. Back in the days, you could use it to dump TMSI and IMSI using OsmoconBB : https://osmocom.org/projects/baseband/wiki/MotorolaC123

Yes, that was the one! Thanks!

FWIW, the same chipset was used as a modem in the Openmoko phones.

> Haven't heard of this happening with later models. Baseband sourcecode firmware is really rare.

You know what they say. "If you have nothing to hide, you have nothing to fear"

So I wonder what they're trying to hide from all of us. Probably all the backdoors and glaring security issues.

https://hackaday.com/2022/07/12/open-firmware-for-pinephone-...

> With the PinePhone modem.. It was quickly found that the Quectel modem ran a stripped down version of Android on its ARM core, with adb shell available over the modem’s USB interface. When a few adventurous hackers started probing it and got shell access, they found tools like ffmpeg, vim, gdb and sendmail compiled in – certainly not something you’d need on a cellular modem, but hey.

EG25 is an IoT modem and those tend to expose some extra functionality such as HTTP clients or TTS synthesis over AT commands. Some even document how to compile and run software on them - though of course it's only about the application CPU and not the actual modemy stuff that runs on separate DSPs with proprietary signed Qualcomm firmware.

Most (all?) standalone modems are basically screenless smartphones/SBCs with integrated modem these days.

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