Researchers have issued a warning about an advanced form of voice phishing called “Letscall” that specifically targets individuals in South Korea. The criminals behind Letscall employ a multi-step attack, deceiving victims into downloading malicious apps from a counterfeit Google Play Store website.
Once installed, the malware redirects incoming calls to a call center controlled by the attackers, where trained operators posing as bank employees extract sensitive information.
Letscall utilizes cutting-edge technologies like voice over IP (VOIP) and WebRTC to facilitate voice traffic routing. By employing Session Traversal Utilities for NAT (STUN) and Traversal Using Relays around NAT (TURN) protocols, including Google STUN servers, it ensures high-quality phone or video calls and bypasses NAT and firewall restrictions. The Letscall group consists of Android developers, designers, frontend and backend developers, as well as call operators specializing in voice social engineering attacks.
The malware operates in three stages, starting with a downloader app that prepares the victim’s device for the installation of powerful spyware. This spyware enables the rerouting of incoming calls to the attackers’ call center, where the final stage occurs.
In this stage, specific commands are executed, including those related to manipulating the address book, creating and removing contacts, and creating, modifying, and removing filters that determine which calls are intercepted.
Whats sets Letscall apart is its use of advanced evasion techniques, incorporating obfuscation during the initial download and employing complex naming structures in ZIP file directories. It intentionally corrupts the manifest to confuse and bypass security systems.
Criminals have developed automated systems that call victims and play pre-recorded messages to deceive them further. By combining mobile phone infections with vishing techniques, attackers can request micro-loans in victims’ names, assuring them of suspicious activities and redirecting calls to their centers.
Although the threat is currently limited to South Korea, researchers warn that there are no technical barriers preventing the expansion of these attackers to other regions. This form of vishing attack highlights the evolving tactics of criminals and their ability to exploit technology for malicious purposes.
The Letscall group showcases intricate knowledge of Android security and voice routing technologies, underscoring the importance of vigilance and robust security measures.
