WASHINGTON—The hack of the Securities and Exchange Commission’s X account this week likely complicates an expected vote Wednesday on whether to greenlight exchange-traded funds holding bitcoin. 

The decision, which people close to the agency expect to occur via a closed-door vote by the agency’s five commissioners, was already controversial. U.S. regulators have little visibility into crypto markets, which SEC Chair Gary Gensler has said are rife with fraud and manipulation. A bitcoin ETF could draw more investors into the asset class, potentially exposing them to harm, critics say.