Apple Inc is planning to fix a flaw that a security firm said may have left more than half a billion iPhones vulnerable to hackers.

The bug, which also exists on iPads, was discovered by ZecOps, a San Francisco-based mobile security forensics company, while it was investigating a sophisticated cyberattack against a client that took place in late 2019.

Zuk Avraham, ZecOps' chief executive, said he found evidence the vulnerability was exploited in at least six cybersecurity break-ins.

An Apple spokesman acknowledged that a vulnerability exists in Apple’s software for email on iPhones and iPads, known as the Mail app, and that the company had developed a fix, which will be rolled out in a forthcoming update on millions of devices it has sold globally.