Learning GSM: USSD fuzzing and attacking the network

Prerequisites of attacks

Some points why USSD is a good choice:
- USSD and USSD replies are free compared to SMS (except special, VAS, etc. numbers)

- USSD and USSD replies interact with 3rd party USSD Gateways software which most probably can be attacked more easy compared to SMSC

- USSD Gateways (if not crashed by a border-case/not-tested/unusual/malformed USSD message or USSD reply), forward the messages to Applications. Most probably “Third party content and application providers” suffer from buffer overflow, script injection, SQL injection, etc.

According to http://www.truteq.com/tips/ussd/:
“The menus are served by applications. This may not be at the GSM network operator, but at a content provider connected to the USSD infrastructure. Applications or content can therefore be served from :
1. Standard supplementary services
2. GSM Network Operators value-added services
3. Third party content and application providers
“

- USSD sessions implementation mechanisms can be exploited in USSD Gateways (grow huge sessions, open huge number of sessions, etc.)

Means to practically implement attacks

Fuzzing requires a lot of messages/replies back and forth through TELCO’s equipment. Many may say that such activity may not go unnoticed, and this is true.

Read more! »

Learning GSM: Mobile/Cell Phone Power-Off vs Mobile Not-reachable/Battery-discharged

Power-Off vs Not-reachable/battery-discharged

It was interesting for me to find out and read an old paper called “Forensics and the GSM mobile telephone system” (original article file 03_spring_art1.pdf).

The point I want to discuss here is also somehow related to trust or mis-trust whether a given called subscriber really went out of GSM network reach/had the battery discharged during idle OR the subscriber actually shut-off his phone and pretends he is out of network reach/battery discharched.

This trust/mis-trust often comes as a facade dialogue template:
John: “I tried to called you regarding XYZ”
Bob: “Umm, I am really sorry - I really wanted to talk to you, but I lost network/I had phone battery discharged” (when actually Bob did switch off his phone on purpose not to be reachable specifically by John and/or other calling parties)

Now there is really a way, without having any technical device or very specific knowledge to find out whether a subscriber has shut down his phone or went out of network-reach or had his battery discharched.

Read more! »