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 issue — which was discovered by cybersecurity company ZecOps exec Zuk Avraham — lies with Apple’s Mail app, which leaves devices vulnerable to hackers, according to Reuters.

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.

Avraham found a malicious program was exploiting the bug as far back as January 2018, though he’s not sure who was behind the program. He said iPhone owners who were affected were sent a blank email message that crashed the app and forced a reset.

Owners didn’t even have to open the message for the crash to happen, according to The Wall Street Journal. The Mail app downloading it was enough. Hackers could then access the device’s photos, contact, and other data. The vulnerability also left the Mail app susceptible to hackers, including the ability to see private messages.

While Apple is largely viewed within the cybersecurity industry as having a high standard for digital security, any successful hacking technique against the iPhone could affect millions due to the device’s global popularity. In 2019, Apple said there were about 900 million iPhones in active use.