diff --git a/winsup/cygwin/ChangeLog b/winsup/cygwin/ChangeLog
index 6397bf00b..4ecf66dfc 100644
--- a/winsup/cygwin/ChangeLog
+++ b/winsup/cygwin/ChangeLog
@@ -1,3 +1,12 @@
+2006-10-13  Corinna Vinschen  <corinna@vinschen.de>
+
+	* autoload.cc (Wow64DisableWow64FsRedirection): Define.
+	(Wow64RevertWow64FsRedirection): Define.
+	* security.cc (cygsuba_installed): New shared variable to store result
+	of cygsuba.dll installation test.
+	(subauth): Check if cygsuba.dll has been installed and registered
+	before issuing the (sub)authentication.
+
 2006-10-12  Corinna Vinschen  <corinna@vinschen.de>
 
 	* security.cc (create_token): Fix condition for source identifier.
diff --git a/winsup/cygwin/autoload.cc b/winsup/cygwin/autoload.cc
index 25ef3d62f..63a83d80a 100644
--- a/winsup/cygwin/autoload.cc
+++ b/winsup/cygwin/autoload.cc
@@ -521,6 +521,8 @@ LoadDLLfuncEx (SetHandleInformation, 12, kernel32, 1)
 LoadDLLfuncEx (SetProcessWorkingSetSize, 12, kernel32, 1)
 LoadDLLfuncEx (SignalObjectAndWait, 16, kernel32, 1)
 LoadDLLfuncEx (SwitchToThread, 0, kernel32, 1)
+LoadDLLfuncEx (Wow64DisableWow64FsRedirection, 4, kernel32, 1)
+LoadDLLfuncEx (Wow64RevertWow64FsRedirection, 4, kernel32, 1)
 
 LoadDLLfunc (SHGetDesktopFolder, 4, shell32)
 
diff --git a/winsup/cygwin/security.cc b/winsup/cygwin/security.cc
index 8080d3027..a0bcba33b 100644
--- a/winsup/cygwin/security.cc
+++ b/winsup/cygwin/security.cc
@@ -982,6 +982,20 @@ out:
   return primary_token;
 }
 
+extern "C"
+{
+  BOOL WINAPI Wow64DisableWow64FsRedirection (PVOID *);
+  BOOL WINAPI Wow64RevertWow64FsRedirection (PVOID);
+};
+
+static enum
+{
+  not_tested,
+  not_installed,
+  installed
+} cygsuba_installed __attribute__((section (".cygwin_dll_common"), shared))
+  = not_tested;
+
 int subauth_id = 255;
 
 HANDLE
@@ -1016,6 +1030,48 @@ subauth (struct passwd *pw)
   HANDLE user_token = NULL;
   HANDLE primary_token = INVALID_HANDLE_VALUE;
 
+  /* Check to see if cygsuba.dll has been registered and is present.  The
+     idea here is to avoid authentication failure messages in the security
+     event log for each logon attempt if cygsuba.dll hasn't been installed.
+     The test is only made once per DLL life time, since installing and
+     registering the subauthentication DLL requires reboot anyway. */
+  if (cygsuba_installed == not_installed)
+    {
+      debug_printf ("subauth not installed, exit subauth");
+      return INVALID_HANDLE_VALUE;
+    }
+  else if (cygsuba_installed == not_tested)
+    {
+      char auth_path[CYG_MAX_PATH];
+
+      cygsuba_installed = not_installed;
+      __small_sprintf (auth_path, "/proc/registry/HKEY_LOCAL_MACHINE/SYSTEM/"
+				  "CurrentControlSet/Control/Lsa/MSV1_0/Auth%d",
+				  subauth_id);
+      if (access (auth_path, F_OK))
+	{
+	  debug_printf ("%s doesn't exist, exit subauth", auth_path);
+	  return INVALID_HANDLE_VALUE;
+	}
+      /* On 64 bit systems the dll must be installed into the *real* system32
+	 directory so we have to switch off file system redirection. */
+      PVOID old_fsredir;
+      DWORD attr = INVALID_FILE_ATTRIBUTES;
+      Wow64DisableWow64FsRedirection (&old_fsredir);
+      if (GetSystemDirectory (auth_path, CYG_MAX_PATH))
+	{
+	  strcat (auth_path, "\\cygsuba.dll");
+	  attr = GetFileAttributes (auth_path);
+	}
+      Wow64RevertWow64FsRedirection (old_fsredir);
+      if (attr == INVALID_FILE_ATTRIBUTES)
+	{
+	  debug_printf ("%s doesn't exist, exit subauth", auth_path);
+	  return INVALID_HANDLE_VALUE;
+	}
+      cygsuba_installed = installed;
+    }
+
   push_self_privilege (SE_TCB_PRIV, true);
 
   /* Register as logon process. */