Criminals may have stolen personal and tax-return information from about 8,800 customers of TaxSlayer LLC, a provider of software to individuals who prepare their own tax returns, according to the company.

The data breach is the second reported by a major tax software firm this year. In January, TaxAct said information on about 450 customers had been stolen and there was suspicious activity in about 9,000 other accounts.

TaxSlayer,...