This would basically allow advertisers to match customers with appropriate ads by tallying their own list of email addresses and phone numbers with ones the user has provided to Twitter.
The company maintained that it never shared "personal data" with partners or other outsiders and that it had resolved the problem as of September 17th.
Twitter didn't know how many people might have been affected, though, and was reporting this primarily to be "transparent" about what happened.
Your email address or phone number - whichever you used for two-factor authentication or security purposes.
"We're very sorry this happened and are taking steps to make sure we don't make a mistake like this again", the San Francisco-based company said. If they were, it would need to be reported to the company's appropriate data protection registrar in the European Union with the company subject to a major fine - and the risk of a class-action lawsuit.
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Twitter repeatedly apologizes for this transgression in its statement, but it's bound to impact trust between its users and the social media platform.
Twitter is only supposed to use phone numbers for two-factor authentication, but it appears to have been unintentionally used for more.
Twitter explained when a user provided their number for the security feature, Tailored Audiences and Partner Audiences may have used that data inadvertently.
Twitter has revealed a number of additional data-security incidents this year.
This language, in which Twitter calls the error, "the issue that allowed this to occur", is way too passive.
Hackers in August this year broke into Twitter CEO Jack Dorsey's account and posted a flurry of rogue tweets, including racial slurs. "We are clear about how we use the information we collect, including the contact information that people upload or add to their own accounts".