Ignore Process Interrupts

What the technique is

Adversaries may evade defensive mechanisms by executing commands that hide from process interrupt signals. Many operating systems use signals to deliver messages to control process behavior. Command interpreters often include specific commands/flags that ignore errors and other hangups, such as when the user of the active session logs off. These interrupt signals may also be used by defensive tools and/or analysts to pause or terminate specified running processes. Adversaries may invoke processes using nohup, PowerShell -ErrorAction SilentlyContinue, or similar commands that may be immune to hangups. This may enable malicious commands and malware to continue execution through system events that would otherwise terminate its execution, such as users logging off or the termination of its C2 network connection. Hiding from process interrupt signals may allow malware to continue execution, but unlike Trap this does not establish Persistence since the process will not be re-invoked once actually terminated.

What MITRE says you would watch

• AN0181

  Execution of processes using nohup or shell redirection to ignore SIGHUP and continue running after session termination. Defender perspective: correlation between commands including nohup, disowned jobs, or & suffix with continued process execution after parent terminal exit.

• AN0182

  PowerShell or script execution with parameters that suppress errors or ignore user interrupts, such as -ErrorAction SilentlyContinue. Defender perspective: detecting discrepancies between suppressed error arguments and continued execution behavior.

• AN0183

  Use of nohup, disown, or AppleScript constructs to suppress process interrupts. Defender perspective: commands containing nohup or hidden background tasks (osascript with persistent execution) correlated with processes surviving user logouts.