diff -urp Mail-SpamAssassin-3.4.2.old/lib/Mail/SpamAssassin/Conf.pm Mail-SpamAssassin-3.4.2/lib/Mail/SpamAssassin/Conf.pm
--- Mail-SpamAssassin-3.4.2.old/lib/Mail/SpamAssassin/Conf.pm	2020-06-15 19:10:21.700917582 +0200
+++ Mail-SpamAssassin-3.4.2/lib/Mail/SpamAssassin/Conf.pm	2020-06-15 19:07:59.045897164 +0200
@@ -3414,6 +3414,20 @@ internally, and should not be used.
     setting => 'priority',
     is_priv => 1,
     type => $CONF_TYPE_HASH_KEY_VALUE,
+    code => sub {
+      my ($self, $key, $value, $line) = @_;
+      my ($rulename, $priority) = split(/\s+/, $value, 2);
+      unless (defined $priority) {
+        return $MISSING_REQUIRED_VALUE;
+      }
+      unless ($rulename =~ IS_RULENAME) {
+        return $INVALID_VALUE;
+      }
+      unless ($priority =~ /^-?\d+$/) {
+        return $INVALID_VALUE;
+      }
+      $self->{priority}->{$rulename} = $priority;
+    }
   });
 
 =back
diff -urp Mail-SpamAssassin-3.4.2.old/lib/Mail/SpamAssassin/Constants.pm Mail-SpamAssassin-3.4.2/lib/Mail/SpamAssassin/Constants.pm
--- Mail-SpamAssassin-3.4.2.old/lib/Mail/SpamAssassin/Constants.pm	2020-06-15 19:10:21.701917596 +0200
+++ Mail-SpamAssassin-3.4.2/lib/Mail/SpamAssassin/Constants.pm	2020-06-15 19:07:59.045897164 +0200
@@ -43,7 +43,7 @@ BEGIN {
 	HARVEST_DNSBL_PRIORITY MBX_SEPARATOR
 	MAX_BODY_LINE_LENGTH MAX_HEADER_KEY_LENGTH MAX_HEADER_VALUE_LENGTH
 	MAX_HEADER_LENGTH ARITH_EXPRESSION_LEXER AI_TIME_UNKNOWN
-	CHARSETS_LIKELY_TO_FP_AS_CAPS MAX_URI_LENGTH RULENAME_RE
+	CHARSETS_LIKELY_TO_FP_AS_CAPS MAX_URI_LENGTH RULENAME_RE IS_RULENAME
   );
 
   %EXPORT_TAGS = (
@@ -404,5 +404,7 @@ use constant CHARSETS_LIKELY_TO_FP_AS_CA
 
 # Allowed rulename format
 use constant RULENAME_RE => qr([_a-zA-Z][_a-zA-Z0-9]{0,127});
+# Exact match
+use constant IS_RULENAME => qr/^${\(RULENAME_RE)}$/;
 
 1;