diff --git a/ninjax-core/src/main/java/org/r10r/ninjax/core/NinjaSessionConverter.java b/ninjax-core/src/main/java/org/r10r/ninjax/core/NinjaSessionConverter.java
index 0d447ef..c812ffc 100644
--- a/ninjax-core/src/main/java/org/r10r/ninjax/core/NinjaSessionConverter.java
+++ b/ninjax-core/src/main/java/org/r10r/ninjax/core/NinjaSessionConverter.java
@@ -35,6 +35,17 @@ public NinjaSessionConverter(NinjaProperties ninjaProperties) {
         });
 
         byte[] decodedKey = Base64.getDecoder().decode(encodedSecret);
+
+        // HS256 requires a key of at least 256 bits (32 bytes). A shorter key (e.g. the
+        // 'changeme' demo default) would still "work" with this custom Jwts implementation but
+        // produces weak, forgeable tokens. Fail fast at startup instead of silently accepting it.
+        int MINIMUM_SECRET_LENGTH_IN_BYTES = 32;
+        if (decodedKey.length < MINIMUM_SECRET_LENGTH_IN_BYTES) {
+            throw new RuntimeException(String.format(
+                    "The secret '%s' in 'conf/application.conf' is too weak. HS256 requires at least %d bytes (256 bits) after Base64-decoding, but the configured secret decodes to %d bytes. Please generate a strong secret using 'mvn ninja:generateSecret'.",
+                    NinjaConstants.NINJA_APPLICATION_SECRET_KEY, MINIMUM_SECRET_LENGTH_IN_BYTES, decodedKey.length));
+        }
+
         this.secretKeyForSessionEncryption = new SecretKeySpec(decodedKey, 0, decodedKey.length, "HmacSHA256");
 
         this.sessionExpiryTimeInSeconds = ninjaProperties.get("application.session.expire_time_in_seconds").map(v -> Long.valueOf(v));
diff --git a/ninjax-core/src/test/java/org/r10r/ninjax/core/NinjaSessionConverterTest.java b/ninjax-core/src/test/java/org/r10r/ninjax/core/NinjaSessionConverterTest.java
new file mode 100644
index 0000000..5b73199
--- /dev/null
+++ b/ninjax-core/src/test/java/org/r10r/ninjax/core/NinjaSessionConverterTest.java
@@ -0,0 +1,88 @@
+package org.r10r.ninjax.core;
+
+import static com.google.common.truth.Truth.assertThat;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.Base64;
+import java.util.Map;
+import java.util.Optional;
+import org.junit.jupiter.api.Test;
+import org.r10r.ninjax.core.properties.NinjaProperties;
+
+public class NinjaSessionConverterTest {
+
+    /**
+     * Test double for NinjaProperties that serves a fixed map of properties instead of
+     * loading from conf/application.conf.
+     */
+    private static class FixedNinjaProperties extends NinjaProperties {
+        private final Map<String, String> values;
+
+        FixedNinjaProperties(Map<String, String> values) {
+            this.values = values;
+        }
+
+        @Override
+        public Optional<String> get(String propertyName) {
+            return Optional.ofNullable(values.get(propertyName));
+        }
+    }
+
+    private static String base64SecretOfLength(int numberOfBytes) {
+        return Base64.getEncoder().encodeToString(new byte[numberOfBytes]);
+    }
+
+    @Test
+    public void shouldRejectSecretShorterThan32Bytes() {
+        // given
+        NinjaProperties properties = new FixedNinjaProperties(
+                Map.of(NinjaConstants.NINJA_APPLICATION_SECRET_KEY, base64SecretOfLength(31)));
+
+        // when
+        RuntimeException exception = assertThrows(RuntimeException.class,
+                () -> new NinjaSessionConverter(properties));
+
+        // then
+        assertThat(exception.getMessage()).contains("too weak");
+    }
+
+    @Test
+    public void shouldRejectWeakChangemeDemoDefault() {
+        // given
+        NinjaProperties properties = new FixedNinjaProperties(
+                Map.of(NinjaConstants.NINJA_APPLICATION_SECRET_KEY, "changeme"));
+
+        // when
+        RuntimeException exception = assertThrows(RuntimeException.class,
+                () -> new NinjaSessionConverter(properties));
+
+        // then
+        assertThat(exception.getMessage()).contains("too weak");
+    }
+
+    @Test
+    public void shouldAcceptSecretOfAtLeast32Bytes() {
+        // given
+        NinjaProperties properties = new FixedNinjaProperties(
+                Map.of(NinjaConstants.NINJA_APPLICATION_SECRET_KEY, base64SecretOfLength(32)));
+
+        // when
+        NinjaSessionConverter converter = new NinjaSessionConverter(properties);
+
+        // then
+        assertThat(converter).isNotNull();
+    }
+
+    @Test
+    public void shouldFailWhenSecretIsMissing() {
+        // given
+        NinjaProperties properties = new FixedNinjaProperties(Map.of());
+
+        // when
+        RuntimeException exception = assertThrows(RuntimeException.class,
+                () -> new NinjaSessionConverter(properties));
+
+        // then
+        assertThat(exception.getMessage()).contains("Missing key");
+    }
+}
diff --git a/ninjax-demo-todo/src/main/java/conf/application.conf b/ninjax-demo-todo/src/main/java/conf/application.conf
index 6e7daa6..9a77022 100644
--- a/ninjax-demo-todo/src/main/java/conf/application.conf
+++ b/ninjax-demo-todo/src/main/java/conf/application.conf
@@ -1,5 +1,6 @@
 ninja.port=8081
-application.secret=changeme
+# Demo secret only. Generate your own for production with 'mvn ninja:generateSecret'.
+application.secret=RHwx0fWz73AlJx1fulfkrYKL5Yo7t8F1H8xUajByE28=
 application.session.expire_time_in_seconds=3600
 application.session.cookie.secure=false
 
diff --git a/ninjax-demo-todo/src/test/resources/conf/application.conf b/ninjax-demo-todo/src/test/resources/conf/application.conf
index 1f03122..cc263a1 100644
--- a/ninjax-demo-todo/src/test/resources/conf/application.conf
+++ b/ninjax-demo-todo/src/test/resources/conf/application.conf
@@ -1,6 +1,6 @@
 
 # Set in test: ninja.port=
-application.secret=changeme
+application.secret=RHwx0fWz73AlJx1fulfkrYKL5Yo7t8F1H8xUajByE28=
 application.session.expire_time_in_seconds=3600
 application.session.cookie.secure=false
 
diff --git a/ninjax-jetty-demo-todo/src/main/java/conf/application.conf b/ninjax-jetty-demo-todo/src/main/java/conf/application.conf
index 6e7daa6..9a77022 100644
--- a/ninjax-jetty-demo-todo/src/main/java/conf/application.conf
+++ b/ninjax-jetty-demo-todo/src/main/java/conf/application.conf
@@ -1,5 +1,6 @@
 ninja.port=8081
-application.secret=changeme
+# Demo secret only. Generate your own for production with 'mvn ninja:generateSecret'.
+application.secret=RHwx0fWz73AlJx1fulfkrYKL5Yo7t8F1H8xUajByE28=
 application.session.expire_time_in_seconds=3600
 application.session.cookie.secure=false
 
diff --git a/ninjax-jetty-demo-todo/src/test/resources/conf/application.conf b/ninjax-jetty-demo-todo/src/test/resources/conf/application.conf
index 1f03122..cc263a1 100644
--- a/ninjax-jetty-demo-todo/src/test/resources/conf/application.conf
+++ b/ninjax-jetty-demo-todo/src/test/resources/conf/application.conf
@@ -1,6 +1,6 @@
 
 # Set in test: ninja.port=
-application.secret=changeme
+application.secret=RHwx0fWz73AlJx1fulfkrYKL5Yo7t8F1H8xUajByE28=
 application.session.expire_time_in_seconds=3600
 application.session.cookie.secure=false