Cryptocurrency is one of those new issues that has some vagueness about it, both in terms of how it fits into understandings of shari'ah and also how it will fit into the world at large. Similar issues, in the past, were the emergence of cameras, credit cards, and democracy; it took time for there to develop both an understanding of the role they would play in the world, and also how to approach them from the angle of religious law. Only time will tell whether cyptocurrencies will eventually supplant state-regulated currencies in the way that digital bank accounts have, in many areas, supplanted cash, or state-regulated currencies supplanted things like coins with hard value (such as gold or silver).

Currently, due to the uncertainty regarding cryptocurrency, many (not all) scholars have approached the subject cautiously. Additional concerns include (a) the use of cryptocurrency for unlawful activities, (b) the illegality of cryptocurrency in some areas, and (c) the potential for harm/fraud.

Here is an article addressing fatwas from Shi'i scholars regarding Bitcoin, being the most common cyptocurrency: http://ijtihadnet.com/bitcoin-perspective-shiite-clerics-fatwas/