Kaspersky Lab researchers have uncovered attacks being carried out by a new piece of malware using a zero-day vulnerability in the Telegram Desktop app.

The vulnerability was used to deliver multipurpose malware, which depending on the computer can be used either as a backdoor or as a tool to deliver mining software. According to the research, the vulnerability has been actively exploited since March 2017 for the cryptocurrency mining functionality, including Monero, Zcash, etc.

According to the research, the Telegram zero-day vulnerability was based on the RLO (right-to-left override) Unicode method. It is generally used for coding languages that are written from right to left, like Arabic or Hebrew. Besides that, however, it can also be used by malware creators to mislead users into downloading malicious files disguised, for example, as images.

Attackers used a hidden Unicode character in the file name that reversed the order of the characters, thus renaming the file itself. As a result, users downloaded hidden malware which was then installed on their computers. Kaspersky Lab reported the vulnerability to Telegram and the zero-day flaw has not since been observed in messenger's products.

During their analysis, Kaspersky Lab experts identified several scenarios of zero-day exploitation in the wild by threat actors. Firstly, the vulnerability was exploited to deliver mining malware. By using the victim's PC computing power, cybercriminals have been creating different types of cryptocurrency including Monero, Zcash, Fantomcoin and others. Moreover, while analyzing a threat actor's servers, Kaspersky Lab researchers found archives containing a Telegram local cache that had been stolen from victims.

Secondly, upon successful exploitation of the vulnerability, a backdoor that used the Telegram API as a command and control protocol was installed, resulting in the hackers gaining remote access to the victim's computer. After installation, it started to operate in a silent mode, which allowed the threat actor to remain unnoticed in the network and execute different commands including the further installation of spyware tools.

The artefacts discovered during the research indicate Russian origins of cybercriminals, Kaspersky added.