Southern [4] and Meyer [5] have examined the security
impacts of interoperating GSM and UMTS networks,
specifically the weakness of the GSM encryption
algorithms [6] on GSM/UMTS heterogeneous
networks. Here we uncover a potential undocumented
vulnerability caused by interoperating GSM and
UMTS networks coupled with the weak GSM encryption
algorithms [6]. An SDR provides us the ability to
create any GSM message, package those messages into
frames, encode the frames into bursts, and modulate
the bursts in accordance with the 3rd Generation Partnership
Project (3GPP) GSM standards [7]-[13]. The
only non-configurable portion of the SDR is its hardware
that transforms the modulated digital samples,
created in software, to a transmitted analog waveform
at any desired carrier frequency.