Showing posts from November, 2022

Detecting Indirect Syscalls from Userland, A Naive Approach.

Our goal is to detect indirect syscalls. Indirect syscalls have taken the red teaming community by storm due to the newfound ability of EDRs to detect direct syscalls. To understand how to catch them, we must first understand a bit about them. For example (have a look at SysWhispers for a pratical implmentation) mov r10, rcx mov rcx, <SSN> syscall ret Various individuals utilized instrumentation callbacks, as documented in these two blog posts.  Instrumentation callbacks are a post-op syscall hook that allows a user to execute a specified function before the kernel returns execution to the userland process. There's a great article on CodeProject which discusses this. The premise for these detections identifies where the syscall instruction comes from; if it is not NTDLL/win32u (the only 2 DLLs where syscall instructions should originate), then it is most likely a direct syscall. Indirect syscalls sought to resolve this by just going to the syscall, and what this means is onc