An ASN.1 vulnerability has been found in a security advisory of 7/18 here. It has to do with length bounds checking in the LTV triplet. A fix is provided but updating it on a large number of GSM devices is not practical. “It could be triggered remotely without any authentication in scenarios where the vulnerable code receives and processes ASN.1 encoded data from untrusted sources, these may include communications between mobile devices and telecommunication network infrastructure nodes, communications between nodes in a carrier’s network or across carrier boundaries, or communication between mutually untrusted endpoints in a data network.”
A quote from the paper – “When GSM radio stacks were implemented, attacks against end devices were not much of a concern. Hence checks on messages arriving over the air interface were lax as long as the stack passed interoperability tests and certifications. Open-source solutions such as OpenBTS (Base Transceiver Station)allow anyone to run their own GSM network at a fraction of the cost of carrier-grade equipment, using a sim- ple and cheap software-defined radio. This development has made GSM security explorations possible for a significantly larger set of security researchers. Indeed, as the reader will see in the following, insufficient verification of input parameters transmitted over the air interface can lead to remotely exploitable memory corruptions in the baseband stack.”
The cellular baseband stack of most smartphones runs on a separate processor and is significantly less hardened, if at all. GSM does not provide mutual authentication, there is no protection against fake BTSs.