diff --git a/.changeset/clever-rats-rush.md b/.changeset/clever-rats-rush.md
new file mode 100644
index 0000000000..c34427f8f5
--- /dev/null
+++ b/.changeset/clever-rats-rush.md
@@ -0,0 +1,5 @@
+---
+'@backstage/backend-defaults': patch
+---
+
+Sensitive internal fields on `BackstageCredentials` objects are now defined as read-only properties in order to minimize risk of leakage.
diff --git a/packages/backend-defaults/src/entrypoints/auth/helpers.test.ts b/packages/backend-defaults/src/entrypoints/auth/helpers.test.ts
new file mode 100644
index 0000000000..8454aa3ecc
--- /dev/null
+++ b/packages/backend-defaults/src/entrypoints/auth/helpers.test.ts
@@ -0,0 +1,67 @@
+/*
+ * Copyright 2024 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import {
+  createCredentialsWithNonePrincipal,
+  createCredentialsWithServicePrincipal,
+  createCredentialsWithUserPrincipal,
+} from './helpers';
+
+describe('credentials', () => {
+  it('should be created', () => {
+    expect(createCredentialsWithServicePrincipal('my-service')).toEqual({
+      $$type: '@backstage/BackstageCredentials',
+      version: 'v1',
+      principal: {
+        type: 'service',
+        subject: 'my-service',
+      },
+    });
+
+    expect(
+      createCredentialsWithUserPrincipal('user:default/mock', 'my-token'),
+    ).toEqual({
+      $$type: '@backstage/BackstageCredentials',
+      version: 'v1',
+      principal: {
+        type: 'user',
+        userEntityRef: 'user:default/mock',
+      },
+    });
+
+    expect(createCredentialsWithNonePrincipal()).toEqual({
+      $$type: '@backstage/BackstageCredentials',
+      version: 'v1',
+      principal: {
+        type: 'none',
+      },
+    });
+  });
+
+  it('should not include tokens when serialized', () => {
+    expect(
+      JSON.stringify(
+        createCredentialsWithServicePrincipal('my-service', 'my-token'),
+      ),
+    ).not.toMatch(/my-token/);
+
+    expect(
+      JSON.stringify(
+        createCredentialsWithUserPrincipal('user:default/mock', 'my-token'),
+      ),
+    ).not.toMatch(/my-token/);
+  });
+});
diff --git a/packages/backend-defaults/src/entrypoints/auth/helpers.ts b/packages/backend-defaults/src/entrypoints/auth/helpers.ts
index eebe45eb76..dad36670b9 100644
--- a/packages/backend-defaults/src/entrypoints/auth/helpers.ts
+++ b/packages/backend-defaults/src/entrypoints/auth/helpers.ts
@@ -28,16 +28,23 @@ export function createCredentialsWithServicePrincipal(
   token?: string,
   accessRestrictions?: BackstagePrincipalAccessRestrictions,
 ): InternalBackstageCredentials<BackstageServicePrincipal> {
-  return {
-    $$type: '@backstage/BackstageCredentials',
-    version: 'v1',
-    token,
-    principal: {
-      type: 'service',
-      subject: sub,
-      accessRestrictions,
+  return Object.defineProperty(
+    {
+      $$type: '@backstage/BackstageCredentials',
+      version: 'v1',
+      principal: {
+        type: 'service',
+        subject: sub,
+        accessRestrictions,
+      },
     },
-  };
+    'token',
+    {
+      enumerable: false,
+      configurable: true,
+      value: token,
+    },
+  );
 }
 
 export function createCredentialsWithUserPrincipal(
@@ -45,16 +52,23 @@ export function createCredentialsWithUserPrincipal(
   token: string,
   expiresAt?: Date,
 ): InternalBackstageCredentials<BackstageUserPrincipal> {
-  return {
-    $$type: '@backstage/BackstageCredentials',
-    version: 'v1',
-    token,
-    expiresAt,
-    principal: {
-      type: 'user',
-      userEntityRef: sub,
+  return Object.defineProperty(
+    {
+      $$type: '@backstage/BackstageCredentials',
+      version: 'v1',
+      expiresAt,
+      principal: {
+        type: 'user',
+        userEntityRef: sub,
+      },
     },
-  };
+    'token',
+    {
+      enumerable: false,
+      configurable: true,
+      value: token,
+    },
+  );
 }
 
 export function createCredentialsWithNonePrincipal(): InternalBackstageCredentials<BackstageNonePrincipal> {