Tracked as CVE-2019-11932, the security flaw exists in the open source library named libpl_droidsonroids_gif.so, which is part of the android-gif-drawable package and is used by numerous Android applications when processing GIF files.
WhatsApp for Android was one of the impacted applications and Facebook patched it recently with the release of version 2.19.244. However, many other apps still use a vulnerable version of the library.
To exploit the flaw against WhatsApp, an attacker would have to send a malicious GIF file to a WhatsApp user. This would automatically trigger the security bug, as soon as the application generates a preview for the file in the WhatsApp Gallery.