diff --git a/spec/mspec/lib/mspec/helpers/tmp.rb b/spec/mspec/lib/mspec/helpers/tmp.rb
index 4c0eddab75..437261a6d0 100644
--- a/spec/mspec/lib/mspec/helpers/tmp.rb
+++ b/spec/mspec/lib/mspec/helpers/tmp.rb
@@ -36,7 +36,18 @@ all specs are cleaning up temporary files:
 end
 
 def tmp(name, uniquify = true)
-  mkdir_p SPEC_TEMP_DIR unless Dir.exist? SPEC_TEMP_DIR
+  if Dir.exist? SPEC_TEMP_DIR
+    stat = File.stat(SPEC_TEMP_DIR)
+    if stat.world_writable? and !stat.sticky?
+      raise ArgumentError, "SPEC_TEMP_DIR (#{SPEC_TEMP_DIR}) is world writable but not sticky"
+    end
+  else
+    umask = File.umask
+    if (umask & 0002) == 0 # o+w
+      raise ArgumentError, "File.umask #=> #{umask.to_s(8)} (world-writable)"
+    end
+    mkdir_p SPEC_TEMP_DIR
+  end
 
   if uniquify and !name.empty?
     slash = name.rindex "/"