He studied those accounts for answers to common security questions, like favorite colors or a parent's middle name, and then used the information to hack into the women's e-mail accounts.
He convinced one woman to send him naked pictures by threatening to share sensitive information with her e-mail contacts.
Bronk, who lives in the Sacramento suburb of Citrus Heights, pleaded guilty Thursday to seven felonies in Sacramento County Superior Court, including computer intrusion, false impersonation and possession of child pornography.
Prosecutors are seeking a six-year prison term when Bronk returns for a sentencing evaluation March 10.
The Associated Press contributed to this report.