A massive database of Mexican voter records was made publicly accessible on the internet, a US security researcher has discovered.
The names, addresses, dates of birth and voter ID numbers of 87 million Mexicans appeared to be listed in the cache.
It was discovered by Chris Vickery, who had been browsing unsecured databases, with a security tool called Shodan.
The voter data has since been taken offline.
"When I opened it up in my database, viewer I saw names, obvious addresses and identifying numbers. I started Googling the addresses to see where they were," Mr Vickery, a security researcher for software firm MacKeeper, told the BBC.
"All the addresses turned out to be in Mexico. I thought, 'This is a Mexican voter database - it has to be.'"
Mr Vickery had made his discovery on 14 April, he said, and initially had trouble reaching an official to warn about the leak.
After mentioning the database during a talk at Harvard last week, a Mexican who happened to be in the audience helped to authenticate the data.
"He was able to authenticate his father's entry in the database - he said, 'Oh my God, that's my address, that's everything,'" said Mr Vickery.
A journalist, also present at the talk, helped Mr Vickery to inform the Mexican National Electoral Institute, which organises federal elections in the country.
The institute has since released a statement in Spanish about the data's publication online.
"A copy of the electoral register has been found in a file storage site of the US company Amazon," it reads.
"An internal investigation has been launched and the case has been reported to the special prosecutor for electoral crimes."
Once the authorities had been notified, steps were taken to remove the information from Amazon's cloud servers. This was done by 22 April.
"All [Amazon Web Services] security features and networks continue to operate as designed," Amazon said in a statement.
"On April 21, AWS was notified that an unsecured database containing sensitive information was being hosted on the AWS cloud and was publicly accessible via the internet.
"We then notified the customer by both email and phone.
"As of 01:00 [local time] on 22 April, this database was no longer publicly accessible."
Mr Vickery said he had since been invited to Mexico as a guest of the government and planned to accept the invitation if his wife was able to join him on the trip.
"The embassy told me I was on the cover of every major Mexican newspaper on Saturday," he said.
"I want to get my hands on one of those newspapers."
Recently, the details of 70 million voters in the Philippines were reported to have leaked online.
And in December last year, Mr Vickery found a cache of data on 191 million US voters after a database was made accessible via the web.
Speaking on the dangers of this sort of data being made public, Mr Vickery said it could sometimes be used by scammers.
"They call up old people and tell them they have a virus," he said.
The scammers then give the victims instructions that result in malware being installed on their computers.
"It's a huge problem over here," Mr Vickery said.
And in Mexico in particular, he said - where up to 100,000 people are kidnapped every year - data on people's home addresses could be considered particularly sensitive.
Alex Cruz Farmer at security company NSFocus IB said: "This is a significant breach, and what makes it worse is that the data was being held outside of Mexico."
Mr Cruz Farmer said data-governance rules in the country forbade exporting people's personal information outside the country without their permission.
"As Mr Vickery has quite rightly raised, the concern over what the data could be used for is extremely distressing," he added.