A newly disclosed research project has revealed what may be the largest user‑data exposure ever recorded. Despite that, the exposure occured entirely within the boundaries of responsible academic inquiries. Security researchers from the University of Vienna and SBA Research uncovered a critical flaw in WhatsApp’s contact discovery system that allowed them to enumerate more than 3.5 billion accounts. Their findings show that WhatsApp, one of the most widely used communication services in the world, responded to billions of automated queries with virtually no rate‑limiting, enabling a rapid sweep of user metadata.
A Simple Mechanism With Global Consequences
The weakness hinged on WhatsApp’s basic feature of checking whether a phone number belongs to an active user. To test this at scale, the researchers generated tens of billions of potential numbers using tools based on Google’s libphonenumber library and fed them into the standard WhatsApp desktop client. Instead of throttling or blocking the flood of incoming lookups, WhatsApp’s backend responded continuously, allowing more than 100 million number checks per hour from a single IP address. Because WhatsApp attaches public profile information to these lookups—including profile pictures if the user has set them as visible—the researchers were able to compile a sweeping dataset that touched nearly half the planet’s population.
What Information Was Exposed
Although message content remained protected by WhatsApp’s end‑to‑end encryption, the metadata returned by the servers still painted a detailed picture of global user behavior. For each valid account, WhatsApp revealed the phone number, public keys, profile image if it was set to “public,” “about” text if visible, and timestamps associated with the account. From these elements, the researchers inferred additional attributes, including operating system type, estimated account age, and whether the user had connected companion devices. More than half of all enumerated accounts exposed a profile photo, and nearly a third displayed an “about” message—information that can be sensitive when aggregated at scale.
Meta’s Response
The research team adhered to strict ethical guidelines, reporting their findings to Meta before publishing and deleting all collected data afterward. Meta confirmed that no malicious exploitation of this enumeration method had been detected and credited the study with helping verify and strengthen their anti‑scraping defenses, which have now been enhanced with rate limits and tighter profile‑visibility controls. WhatsApp emphasized that message content remained untouched throughout the study thanks to end‑to‑end encryption, but acknowledged that protecting metadata poses its own challenges.
Conclusion
This research reveals how a simple flaw in WhatsApp’s contact discovery system led to the exposure of data from 3.5 billion users. While message content remained secure thanks to end-to-end encryption, large amounts of public data, such as profile pictures, phone numbers, and "about" text, were easily accessed. The researchers responsibly reported the issue to Meta, which has since improved its security measures to prevent future exploitation. This incident highlights the importance of securing all types of user data, not just messages, and shows how easily vulnerabilities can be exploited when proper safeguards aren't in place.
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