imap/pop3/smtp: Added support for SASL authentication downgrades

Added support for downgrading the SASL authentication mechanism when the
decoding of CRAM-MD5, DIGEST-MD5 and NTLM messages fails. This enhances
the previously added support for graceful cancellation by allowing the
client to retry a lesser SASL mechanism such as LOGIN or PLAIN, or even
APOP / clear text (in the case of POP3 and IMAP) when supported by the
server.

1 files changed, 40 insertions, 2 deletions

diff --git a/lib/pop3.c b/lib/pop3.c
index 1554f09cd..c2c151a93 100644
--- a/lib/pop3.c
+++ b/lib/pop3.c
@@ -1161,14 +1161,52 @@ static CURLcode pop3_state_auth_cancel_resp(struct connectdata *conn,
                                             int pop3code,
                                             pop3state instate)
 {
+  CURLcode result = CURLE_OK;
   struct SessionHandle *data = conn->data;
+  struct pop3_conn *pop3c = &conn->proto.pop3c;
+  const char *mech = NULL;
+  char *initresp = NULL;
+  size_t len = 0;
+  pop3state state1 = POP3_STOP;
+  pop3state state2 = POP3_STOP;
 
   (void)pop3code;
   (void)instate; /* no use for this yet */
 
-  failf(data, "Authentication cancelled");
+  /* Remove the offending mechanism from the supported list */
+  pop3c->authmechs ^= pop3c->authused;
+
+  /* Calculate alternative SASL login details */
+  result = pop3_calc_sasl_details(conn, &mech, &initresp, &len, &state1,
+                                  &state2);
+
+  if(!result) {
+    /* Do we have any mechanisms left or can we fallback to another
+       authentication type? */
+    if(mech) {
+      /* Retry SASL based authentication */
+      result = pop3_perform_auth(conn, mech, initresp, len, state1, state2);
+
+      Curl_safefree(initresp);
+    }
+#ifndef CURL_DISABLE_CRYPTO_AUTH
+    else if((pop3c->authtypes & POP3_TYPE_APOP) &&
+            (pop3c->preftype & POP3_TYPE_APOP))
+      /* Perform APOP authentication */
+      result = pop3_perform_apop(conn);
+#endif
+    else if((pop3c->authtypes & POP3_TYPE_CLEARTEXT) &&
+            (pop3c->preftype & POP3_TYPE_CLEARTEXT))
+      /* Perform clear text authentication */
+      result = pop3_perform_user(conn);
+    else {
+      failf(data, "Authentication cancelled");
+
+      result = CURLE_LOGIN_DENIED;
+    }
+  }
 
-  return CURLE_LOGIN_DENIED;
+  return result;
 }
 
 /* For final responses in the AUTH sequence */