According to a new study by a security expert, a venerability with the SIM card technology may put millions of users at the risk of being spied upon or robbed by those exploiting the flaw.

Karsten Nohl, a leading security expert in the industry, has said that he has found a way to discover Sims' digital keys by sending them a special text message. He warned that the criminals might be able to snoop in on calls or even steal money from the users.

Nohl, who has been working with Berlin-based Security Research Labs, has posted the details of the vulnerability on the website of his company. Industry body representing global network operators, GSMA has said that it is looking into the matter and will investigate the flaw.

A spokeswoman for the association said, "Karsten's early disclosure to the GSMA has given us an opportunity for preliminary analysis. We have been able to consider the implications and provide guidance to those network operators and Sim vendors that may be impacted. It would appear that a minority of Sims produced against older standards could be vulnerable."

The Sim or subscriber identity module is a security token that authenticates the user's identity with their network operator.