2-factor authentication is vulnerable to code interception and exploits of account recovery systems.
It is difficult for experts to assess the quality of the large number of 2-factor implementations.
Bitcoin holders have been a focus of hacks, but the practices are widespread. Account recovery systems at wireless service providers are the weakest link - often requiring only a billing address to hack, and they provide a host of social engineering opportunities.
SMS 2-factor code delivery should not be used.
The emphasis is beginning to shift from hack prevention to detection (classifying usage patterns and ambient signals).
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