fix(actions): handle empty deny list correctly (#9753)

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# Which Problems Are Solved

A customer reached out that after an upgrade, actions would always fail
with the error "host is denied" when calling an external API.
This is due to a security fix
(https://github.com/zitadel/zitadel/security/advisories/GHSA-6cf5-w9h3-4rqv),
where a DNS lookup was added to check whether the host name resolves to
a denied IP or subnet.
If the lookup fails due to the internal DNS setup, the action fails as
well. Additionally, the lookup was also performed when the deny list was
empty.

# How the Problems Are Solved

- Prevent DNS lookup when deny list is empty
- Properly initiate deny list and prevent empty entries

# Additional Changes

- Log the reason for blocked address (domain, IP, subnet)

# Additional Context

- reported by a customer
- needs backport to 2.70.x, 2.71.x and 3.0.0 rc

internal/actions/http_module_config.go

@@ -1,6 +1,8 @@
 package actions
 
 import (
+	"errors"
+	"fmt"
 	"net"
 	"reflect"
 	"strings"
@@ -60,6 +62,9 @@ func HTTPConfigDecodeHook(from, to reflect.Value) (interface{}, error) {
 }
 
 func NewHostChecker(entry string) (AddressChecker, error) {
+	if entry == "" {
+		return nil, nil
+	}
 	_, network, err := net.ParseCIDR(entry)
 	if err == nil {
 		return &HostChecker{Net: network}, nil
@@ -76,19 +81,39 @@ type HostChecker struct {
 	Domain string
 }
 
-func (c *HostChecker) Matches(ips []net.IP, address string) bool {
+type AddressDeniedError struct {
+	deniedBy string
+}
+
+func NewAddressDeniedError(deniedBy string) *AddressDeniedError {
+	return &AddressDeniedError{deniedBy: deniedBy}
+}
+
+func (e *AddressDeniedError) Error() string {
+	return fmt.Sprintf("address is denied by '%s'", e.deniedBy)
+}
+
+func (e *AddressDeniedError) Is(target error) bool {
+	var addressDeniedErr *AddressDeniedError
+	if !errors.As(target, &addressDeniedErr) {
+		return false
+	}
+	return e.deniedBy == addressDeniedErr.deniedBy
+}
+
+func (c *HostChecker) IsDenied(ips []net.IP, address string) error {
 	// if the address matches the domain, no additional checks as needed
 	if c.Domain == address {
-		return true
+		return NewAddressDeniedError(c.Domain)
 	}
 	// otherwise we need to check on ips (incl. the resolved ips of the host)
 	for _, ip := range ips {
 		if c.Net != nil && c.Net.Contains(ip) {
-			return true
+			return NewAddressDeniedError(c.Net.String())
 		}
 		if c.IP != nil && c.IP.Equal(ip) {
-			return true
+			return NewAddressDeniedError(c.IP.String())
 		}
 	}
-	return false
+	return nil
 }