Federal law
FTC Safeguards Rule
If you prepare tax returns for a fee, the FTC treats you as a financial institution under the Gramm-Leach-Bliley Act. The Safeguards Rule requires you to keep a written information security program, and 16 CFR 314.4 lists what that program has to include. There's no exception for being small.
[VERIFY: pin cite for the “written” requirement itself (believed to be 16 CFR 314.3(a)); wisp-final cites Part 314 and 314.4]
Source: 16 CFR Part 314
