IRS e-file: Secure Online Tax Filing
The IRS e-file system has never had a security breach. In fact, over 400 million returns have been electronically filed since 1986 without a security incident.
FACTS
IRS e-file transmissions are very secure because the IRS has been extremely diligent in the design, development, analysis and testing of the current infrastructure and system. IRS e-file meets or exceeds all government security standards and includes multiple firewalls.
Most e-filed online tax returns are transmitted over phone lines from the return preparer to a third-party transmitter. From there, the returns are forwarded over secured lines to the IRS. Intercepting telephone transmissions is quite difficult and requires access to phone company major transmission lines. Also, to transmit data like tax returns over telecommunications lines means that the information gets converted into digital format which could not be easily read even if it were intercepted.
Source : irs.gov