Broomstick

Description

Oyster is a backdoor malware written in C++ that first appeared in July 2023. It allows for remote sessions, supporting tasks such as file transfer and command-line processing. This malware has been used by numerous threat actors as a tool to facilitate ransomware intrusions. The distribution of Oyster has likely occurred through various methods, as suggested by the build identifiers found in examined samples. Additionally, Oyster is capable of collecting basic system data and communicates with a command-and-control (C2) server. It can execute commands via cmd.exe and run additional files. In August 2024, a new version of Oyster was discovered that featured a new command-and-control (C2) communication protocol format. This 2024 version contained plaintext strings and lacked code obfuscation, suggesting it was still in development. In contrast to the 2024 version, the new 2025 Oyster version does not send C2 messages in plaintext, instead reintroducing the substitution cipher that was present in earlier versions of Oyster.