diff --git a/tools/shared.py b/tools/shared.py
index 485c91a80f76d..c7704376736e9 100644
--- a/tools/shared.py
+++ b/tools/shared.py
@@ -1098,6 +1098,11 @@ def load(self, args=[]):
       settings = re.sub(r'var ([\w\d]+)', r'self.attrs["\1"]', settings)
       exec settings
 
+      # Apply default values for settings that are configured from environment variables.
+      if os.environ.get('EMSCRIPTEN_STRICT') and int(os.environ.get('EMSCRIPTEN_STRICT')) != 0:
+        # The default value -s ERROR_ON_UNDEFINED_SYMBOLS=0 is deprecated. Use the default value 1 in strict mode.
+        self.attrs['ERROR_ON_UNDEFINED_SYMBOLS'] = 1
+
       # Apply additional settings. First -O, then -s
       for i in range(len(args)):
         if args[i].startswith('-O'):