There are a myriad of messaging clients available on smartphones these days, including the more popular Facebook Messenger as well others such as Telegram which has found itself blocked from use in Russia. While these services do offer expanded capabilities compared to good old SMS which has been a staple capability of mobile phones for decades, that hasn't meant that they have been infallible as far as bugs are concerned. For instance, back in February, it was found that receiving a message with a single character from the Indian Telugu alphabet would crash multiple apps on iOS, including WhatsApp, as well as the iOS Springboard.
Now, according to a claim on Reddit, another bug has been found in WhatsApp that will cause the app to lock up, crash back to the launcher, or take down the whole mobile operating system. If a specially crafted message, found on Pastebin, is sent to a device, it can manifest in the aforementioned behavior, thanks to tens of thousands of invisible "left-to-right" and "right-to-left" control characters placed between the emoji and the final quotation marks.
While sending this nefarious message to others is certainly not condoned, you should also exercise caution if attempting to view the seemingly innocent looking line of text on Pastebin out of mere curiosity as it may also hang the browser tab or cause the browser itself to become unresponsive. Furthermore, victims don't even have to interact with the message to suffer inconvenience. At present, WhatsApp has yet to acknowledge the bug but it goes without saying that an update should be forthcoming to address the issue, if not already released.
Of course, it's not the first time that WhatsApp has fallen foul of such flaws, with the app found to be susceptible to crashing when it received 4,000 smileys according to a discovery towards the end of 2015.