diff --git a/include/webfuse/adapter/authenticate.h b/include/webfuse/adapter/authenticate.h
index e11e300..0146768 100644
--- a/include/webfuse/adapter/authenticate.h
+++ b/include/webfuse/adapter/authenticate.h
@@ -31,7 +31,7 @@ struct wf_credentials;
 /// \see wf_server_protocol_add_authenticator
 //------------------------------------------------------------------------------
 typedef bool wf_authenticate_fn(
-    struct wf_credentials * credentials,
+    struct wf_credentials const * credentials,
     void * user_data);
 
 #ifdef __cplusplus
diff --git a/test/webfuse/mocks/mock_authenticator.cc b/test/webfuse/mocks/mock_authenticator.cc
index 35648ed..4aee1bb 100644
--- a/test/webfuse/mocks/mock_authenticator.cc
+++ b/test/webfuse/mocks/mock_authenticator.cc
@@ -23,17 +23,17 @@ void set_authenticator(size_t i, Authenticator * authenticator)
     g_authenticators[i] = authenticator;
 }
 
-bool authenticate(struct wf_credentials * creds, void * user_data)
+bool authenticate(struct wf_credentials const * creds, void * user_data)
 {
     return g_authenticators[0]->authenticate(creds, user_data);
 }
 
-bool authenticate_1(struct wf_credentials * creds, void * user_data)
+bool authenticate_1(struct wf_credentials const * creds, void * user_data)
 {
     return g_authenticators[1]->authenticate(creds, user_data);
 }
 
-bool authenticate_2(struct wf_credentials * creds, void * user_data)
+bool authenticate_2(struct wf_credentials const * creds, void * user_data)
 {
     return g_authenticators[2]->authenticate(creds, user_data);
 }
diff --git a/test/webfuse/mocks/mock_authenticator.hpp b/test/webfuse/mocks/mock_authenticator.hpp
index bb38be3..ac7f2bf 100644
--- a/test/webfuse/mocks/mock_authenticator.hpp
+++ b/test/webfuse/mocks/mock_authenticator.hpp
@@ -12,22 +12,22 @@ class Authenticator
 public:
     virtual ~Authenticator() { }
     virtual bool authenticate(
-        struct wf_credentials * credentials,
+        struct wf_credentials const * credentials,
         void * user_data) = 0;
 };
 
 class MockAuthenticator: public Authenticator
 {
 public:
-    MOCK_METHOD2(authenticate, bool (struct wf_credentials * credentials, void * user_data));
+    MOCK_METHOD2(authenticate, bool (struct wf_credentials const * credentials, void * user_data));
 };
 
 void set_authenticator(Authenticator * authenticator);
 void set_authenticator(size_t index, Authenticator * authenticator);
 
-bool authenticate(struct wf_credentials * creds, void * user_data);
-bool authenticate_1(struct wf_credentials * creds, void * user_data);
-bool authenticate_2(struct wf_credentials * creds, void * user_data);
+bool authenticate(struct wf_credentials const * creds, void * user_data);
+bool authenticate_1(struct wf_credentials const * creds, void * user_data);
+bool authenticate_2(struct wf_credentials const * creds, void * user_data);
 
 }
 
diff --git a/test/webfuse/tests/adapter/test_server_config.cc b/test/webfuse/tests/adapter/test_server_config.cc
index b9687be..ceec97d 100644
--- a/test/webfuse/tests/adapter/test_server_config.cc
+++ b/test/webfuse/tests/adapter/test_server_config.cc
@@ -20,7 +20,7 @@ wf_mountpoint * create_mountpoint(
 }
 
 bool authenticate(
-    wf_credentials * credentials,
+    wf_credentials const * credentials,
     void * user_data)
 {
     (void) credentials;
diff --git a/test/webfuse/tests/integration/test_lowlevel.cc b/test/webfuse/tests/integration/test_lowlevel.cc
index 050b23a..0c49192 100644
--- a/test/webfuse/tests/integration/test_lowlevel.cc
+++ b/test/webfuse/tests/integration/test_lowlevel.cc
@@ -37,7 +37,7 @@ wf_test_integration_lowlevel_on_disconnected(
 
 bool
 wf_test_integration_lowlevel_authenticate(
-    struct wf_credentials * credentials,
+    struct wf_credentials const * credentials,
     void * )
 {
     char const * username = wf_credentials_get(credentials, "username");