diff --git a/src/include/netif/ppp/ppp.h b/src/include/netif/ppp/ppp.h
index d819a9f3..de740ec7 100644
--- a/src/include/netif/ppp/ppp.h
+++ b/src/include/netif/ppp/ppp.h
@@ -423,7 +423,12 @@ struct ppp_pcb_s {
  ************************/
 
 /*
- * Set auth helper, optional, you can either fill ppp_pcb->settings.
+ * WARNING: For multi-threads environment, all ppp_set_* functions most
+ * only be called while the PPP is in the dead phase (i.e. disconnected).
+ */
+
+/*
+ * Set PPP authentication.
  *
  * Warning: Using PPPAUTHTYPE_ANY might have security consequences.
  * RFC 1994 says:
diff --git a/src/include/netif/ppp/pppapi.h b/src/include/netif/ppp/pppapi.h
index c5974e1e..913d93f7 100644
--- a/src/include/netif/ppp/pppapi.h
+++ b/src/include/netif/ppp/pppapi.h
@@ -47,11 +47,6 @@ extern "C" {
 struct pppapi_msg_msg {
   ppp_pcb *ppp;
   union {
-    struct {
-      u8_t authtype;
-      const char *user;
-      const char *passwd;
-    } setauth;
 #if PPP_NOTIFY_PHASE
     struct {
       ppp_notify_phase_cb_fn notify_phase_cb;
@@ -109,7 +104,6 @@ struct pppapi_msg {
 
 /* API for application */
 err_t pppapi_set_default(ppp_pcb *pcb);
-err_t pppapi_set_auth(ppp_pcb *pcb, u8_t authtype, const char *user, const char *passwd);
 #if PPP_NOTIFY_PHASE
 err_t pppapi_set_notify_phase_callback(ppp_pcb *pcb, ppp_notify_phase_cb_fn notify_phase_cb);
 #endif /* PPP_NOTIFY_PHASE */
diff --git a/src/netif/ppp/pppapi.c b/src/netif/ppp/pppapi.c
index 8dc54f80..32b479df 100644
--- a/src/netif/ppp/pppapi.c
+++ b/src/netif/ppp/pppapi.c
@@ -81,40 +81,6 @@ pppapi_set_default(ppp_pcb *pcb)
 }
 
 
-/**
- * Call ppp_set_auth() inside the tcpip_thread context.
- */
-static err_t
-pppapi_do_ppp_set_auth(struct tcpip_api_call_data *m)
-{
-  struct pppapi_msg *msg = (struct pppapi_msg *)m;
-
-  ppp_set_auth(msg->msg.ppp, msg->msg.msg.setauth.authtype,
-               msg->msg.msg.setauth.user, msg->msg.msg.setauth.passwd);
-  return ERR_OK;
-}
-
-/**
- * Call ppp_set_auth() in a thread-safe way by running that function inside the
- * tcpip_thread context.
- */
-err_t
-pppapi_set_auth(ppp_pcb *pcb, u8_t authtype, const char *user, const char *passwd)
-{
-  err_t err;
-  PPPAPI_VAR_DECLARE(msg);
-  PPPAPI_VAR_ALLOC(msg);
-
-  PPPAPI_VAR_REF(msg).msg.ppp = pcb;
-  PPPAPI_VAR_REF(msg).msg.msg.setauth.authtype = authtype;
-  PPPAPI_VAR_REF(msg).msg.msg.setauth.user = user;
-  PPPAPI_VAR_REF(msg).msg.msg.setauth.passwd = passwd;
-  err = tcpip_api_call(pppapi_do_ppp_set_auth, &PPPAPI_VAR_REF(msg).call);
-  PPPAPI_VAR_FREE(msg);
-  return err;
-}
-
-
 #if PPP_NOTIFY_PHASE
 /**
  * Call ppp_set_notify_phase_callback() inside the tcpip_thread context.