diff --git a/ospd_openvas/preferencehandler.py b/ospd_openvas/preferencehandler.py
index bb8abced..138ad4d9 100644
--- a/ospd_openvas/preferencehandler.py
+++ b/ospd_openvas/preferencehandler.py
@@ -32,7 +32,7 @@
 OID_ESXI_AUTH = "1.3.6.1.4.1.25623.1.0.105058"
 OID_SNMP_AUTH = "1.3.6.1.4.1.25623.1.0.105076"
 OID_PING_HOST = "1.3.6.1.4.1.25623.1.0.100315"
-# TODO: check me, check me, check me 
+# TODO: check me, check me, check me
 OID_KRB5_AUTH = "1.3.6.1.4.1.25623.1.81.0"
 
 BOREAS_ALIVE_TEST = "ALIVE_TEST"
@@ -588,11 +588,15 @@ def build_credentials_as_prefs(self, credentials: Dict) -> List[str]:
             added to the redis KB.
         """
         cred_prefs_list = []
+        krb5_set = False
+        smb_set = False
         for credential in credentials.items():
             service = credential[0]
             cred_params = credentials.get(service)
             if not cred_params:
-                logger.warning("No credentials parameter found for service %s", service)
+                logger.warning(
+                    "No credentials parameter found for service %s", service
+                )
                 continue
             cred_type = cred_params.get('type', '')
             username = cred_params.get('username', '')
@@ -664,6 +668,12 @@ def build_credentials_as_prefs(self, credentials: Dict) -> List[str]:
                 )
             # Check servic smb
             elif service == 'smb':
+                if krb5_set:
+                    self.errors.append(
+                        "Kerberos and SMB credentials are mutually exclusive."
+                    )
+                    continue
+                smb_set = True
                 cred_prefs_list.append(
                     f'{OID_SMB_AUTH}:1:entry:SMB login:|||{username}'
                 )
@@ -671,13 +681,23 @@ def build_credentials_as_prefs(self, credentials: Dict) -> List[str]:
                     f'{OID_SMB_AUTH}:2:password:SMB password:|||{password}'
                 )
             elif service == 'krb5':
+                if smb_set:
+                    self.errors.append(
+                        "Kerberos and SMB credentials are mutually exclusive."
+                    )
+                    continue
+                krb5_set = True
                 realm = cred_params.get('realm', '')
                 if not realm:
-                    self.errors.append("Missing realm for Kerberos authentication.")
+                    self.errors.append(
+                        "Missing realm for Kerberos authentication."
+                    )
                     continue
                 kdc = cred_params.get('kdc', '')
                 if not kdc:
-                    self.errors.append("Missing KDC for Kerberos authentication.")
+                    self.errors.append(
+                        "Missing KDC for Kerberos authentication."
+                    )
                     continue
                 cred_prefs_list.append(
                     f'{OID_KRB5_AUTH}:1:entry:KRB5 login:|||{username}'
@@ -688,7 +708,7 @@ def build_credentials_as_prefs(self, credentials: Dict) -> List[str]:
                 cred_prefs_list.append(
                     f'{OID_KRB5_AUTH}:3:entry:KRB5 realm:|||{realm}'
                 )
-                #TODO: add multiple kdcs
+                # TODO: add multiple kdcs
                 cred_prefs_list.append(
                     f'{OID_KRB5_AUTH}:4:entry:KRB5 kdc:|||{kdc}'
                 )