The Privacy Protection Authority said in a statement that responsibility for complying with Israeli privacy law involving use of the voter registry “lies with the parties themselves.”

Explaining the ease with which the voter information could be accessed, Ran Bar-Zik, the programmer who revealed the breach, explained that visitors to the Elector app’s website could right-click to “view source,” an action that reveals the code behind a web page.

That page of code included the user names and passwords of site administrators with access to the voter registry, and using those credentials would allow anyone to view and download the information. Mr. Bar-Zik, a software developer for Verizon Media who wrote the Sunday article in Haaretz, said he chose the name and password of the Likud party administrator and logged in.

“Jackpot!” he said in an interview on Monday. “Everything was in front of me!”

“When we talk about hacking, we imagine people in hoodies doing technical stuff,” he said, but in this case, no hacking skill was necessary.

Mr. Bar-Zik said he had received a tipoff about the problem on Friday night. The message was sent in English to Cybercyber, a podcast that he hosts with two colleagues, and as evidence, the tipster included Mr. Bar-Zik’s own details from the voter registry, and those of his wife and son.

“It was spooky,” he said.

Mr. Netanyahu is in a fight for his political survival, under indictment on corruption charges and facing a third general election in less than a year, after the previous two failed to produce conclusive results.

He had encouraged supporters to download the Elector app. It was not clear if Likud or Elector had violated the government’s standards for keeping data private, or what kind of security testing the app had undergone.