Mr. Weeks lost his phone number and about a million dollars’ worth of virtual currency late last year, despite having asked his mobile phone provider for additional security after his wife and parents lost control of their phone numbers.

The attackers appear to be focusing on anyone who talks on social media about owning virtual currencies or anyone who is known to invest in virtual currency companies, such as venture capitalists. And virtual currency transactions are designed to be irreversible.

Accounts with banks and brokerage firms and the like are not as vulnerable to these attacks because these institutions can usually reverse unintended or malicious transactions if they are caught within a few days.

But the attacks are exposing a vulnerability that could be exploited against almost anyone with valuable emails or other digital files — including politicians, activists and journalists.

Last year, hackers took over the Twitter account of DeRay Mckesson, a leader of the Black Lives Matters movement, by first getting his phone number.

In a number of cases involving digital money aficionados, the attackers have held email files for ransom — threatening to release naked pictures in one case, and details of a victim’s sexual fetishes in another.

The vulnerability of even sophisticated programmers and security experts to these attacks sets an unsettling precedent for when the assailants go after less technologically savvy victims. Security experts worry that these types of attacks will become more widespread if mobile phone operators do not make significant changes to their security procedures.