If you aren't using Apache, the fix is in the configuration files:
Before applying any fix, you must identify the exact symptom. There are four primary failure modes when trying to view an SHTML file:
Or, if the server is running a vulnerable version of the mod_include module (common in older Apache versions), simply viewing a maliciously crafted SHTML file could allow the execution of system commands (Remote Code Execution - RCE).
chmod +x yourfile.shtml