Hackers in the Russian underground exploited a Telegram Zero-Day vulnerability to deliver malware

Security researcher Alexey Firsh at Kaspersky Lab last discovered a Telegram zero-day in the desktop Windows version that was exploited in attacks in the wild.

Security researcher Alexey Firsh at Kaspersky Lab last discovered a zero-day vulnerability in the desktop Windows version of the popular Telegram instant messaging app.

The bad news is that the Telegram zero-day flaw was being exploited by threat actors in the wild to deliver cryptocurrency miners for Monero and ZCash.

According to the expert, hackers have actively exploited the vulnerability since at least March 2017. Attackers tricked victims into downloading cryptocurrency miners or to establish a backdoor.

“In October 2017, we learned of a vulnerability in Telegram Messenger’s Windows client that was being exploited in the wild. It involves the use of a classic right-to-left override attack when a user sends files over the messenger service.” reads the analysis of the expert.

The flaw is related to the way Telegram Windows client handles the RLO (right-to-left override) Unicode character (U+202E), which is used for any language that uses a right to left writing mode, like Arabic or Hebrew.

The attackers used a hidden RLO Unicode character in the file name that reversed the order of the characters, in this way the file name could be renamed. In a real attack scenario, then the attackers sent the file to the target recipient.

The crooks craft a malicious code to be sent in a message, let assume it is a JS file that is renamed as follows:

evil.js -> photo_high_re*U+202E*gnp.js  (— *U+202E* is the RLO character)

The RLO character included in the file name is used by an attacker to display the string gnp.js in reverse masquerading the fact that the file is a js and tricking the victims into believing that it is a harmless .png image.

According to the researcher, the flaw was known only in the Russia crime community, it was not triggered by other crooks.