email: Added initial support for cancelling authentication

Should a client application fail to decode an authentication message
received from a server, or not support any of the parameters given by
the server in the message, then the authentication phrase should be
cancelled gracefully by the client rather than simply terminating the
connection.

The authentication phrase should be cancelled by simply sending a '*'
to the server, in response to erroneous data being received, as per
RFC-3501, RFC-4954 and RFC-5034.

This patch adds the necessary state machine constants and appropriate
response handlers in order to add this functionality for the CRAM-MD5,
DIGEST-MD5 and NTLM authentication mechanisms.

1 files changed, 21 insertions, 1 deletions

diff --git a/lib/pop3.c b/lib/pop3.c
index 7fc755e12..f4dc5d1b3 100644
--- a/lib/pop3.c
+++ b/lib/pop3.c
@@ -405,6 +405,7 @@ static void state(struct connectdata *conn, pop3state newstate)
     "AUTH_NTLM",
     "AUTH_NTLM_TYPE2MSG",
     "AUTH_XOAUTH2",
+    "AUTH_CANCEL",
     "AUTH_FINAL",
     "APOP",
     "USER",
@@ -1182,7 +1183,22 @@ static CURLcode pop3_state_auth_xoauth2_resp(struct connectdata *conn,
   return result;
 }
 
-/* For final responses to the AUTH sequence */
+/* For AUTH cancellation responses */
+static CURLcode pop3_state_auth_cancel_resp(struct connectdata *conn,
+                                            int pop3code,
+                                            pop3state instate)
+{
+  struct SessionHandle *data = conn->data;
+
+  (void)pop3code;
+  (void)instate; /* no use for this yet */
+
+  failf(data, "Authentication cancelled");
+
+  return CURLE_LOGIN_DENIED;
+}
+
+/* For final responses in the AUTH sequence */
 static CURLcode pop3_state_auth_final_resp(struct connectdata *conn,
                                            int pop3code,
                                            pop3state instate)
@@ -1404,6 +1420,10 @@ static CURLcode pop3_statemach_act(struct connectdata *conn)
       result = pop3_state_auth_xoauth2_resp(conn, pop3code, pop3c->state);
       break;
 
+    case POP3_AUTH_CANCEL:
+      result = pop3_state_auth_cancel_resp(conn, pop3code, pop3c->state);
+      break;
+
     case POP3_AUTH_FINAL:
       result = pop3_state_auth_final_resp(conn, pop3code, pop3c->state);
       break;