Fixed Windows-specific crash when multiple threads are used (#152).

PACKAGING.md

@@ -2,12 +2,12 @@
 
 An executable can only be made on a Windows host because the PyInstaller tool (https://www.pyinstaller.org/) does not support cross-compilation.
 
-1.) Install Python v3.9.x from https://www.python.org/.  To make life easier, check the option to add Python to the PATH environment variable.
+1.) Install Python v3.11.x from https://www.python.org/.  To make life easier, check the option to add Python to the PATH environment variable.
 
-2.) Using pip, install pyinstaller and colorama:
+2.) Using pip, install pyinstaller, colorama, and idna:
 
 ```
-    pip install pyinstaller colorama
+    pip install -U pyinstaller colorama idna
 ```
 
 3.) Install Cygwin (https://www.cygwin.com/).

src/ssh_audit/ssh_audit.py

@@ -61,6 +61,9 @@ from ssh_audit.ssh_socket import SSH_Socket
 from ssh_audit.utils import Utils
 from ssh_audit.versionvulnerabilitydb import VersionVulnerabilityDB
 
+
+no_idna_workaround = False
+
 # Only import colorama under Windows.  Other OSes can natively handle terminal colors.
 if sys.platform == 'win32':
     try:
@@ -69,6 +72,14 @@ if sys.platform == 'win32':
     except ImportError:
         pass
 
+    # This is a workaround for a Python bug that causes a crash on Windows when multiple threads are used (see https://github.com/python/cpython/issues/73474).  Importing the idna module and using it in a no-op seems to fix the issue.  Otherwise, if idna isn't available at run-time, force single threaded scans.
+    try:
+        import idna  # noqa: F401
+
+        ''.encode('idna')
+    except ImportError:
+        no_idna_workaround = True
+
 
 def usage(uout: OutputBuffer, err: Optional[str] = None) -> None:
     retval = exitcodes.GOOD
@@ -753,8 +764,15 @@ def process_commandline(out: OutputBuffer, args: List[str], usage_cb: Callable[.
             aconf.policy_file = a
         elif o in ('-T', '--targets'):
             aconf.target_file = a
+
+            # If we're on Windows, and we can't use the idna workaround, force only one thread to be used (otherwise a crash would occur).
+            if no_idna_workaround:
+                print("\nWARNING: the idna module was not found on this system, thus only single-threaded scanning will be done (this is a workaround for this Windows-specific crash: https://github.com/python/cpython/issues/73474).  Multi-threaded scanning can be enabled by installing the idna module (pip install idna).\n")
+                aconf.threads = 1
         elif o == '--threads':
             aconf.threads = int(a)
+            if no_idna_workaround:
+                aconf.threads = 1
         elif o in ('-L', '--list-policies'):
             aconf.list_policies = True
         elif o == '--lookup':