diff --git a/internal/imgcache/service.go b/internal/imgcache/service.go
index c7706c5..01f0db3 100644
--- a/internal/imgcache/service.go
+++ b/internal/imgcache/service.go
@@ -7,9 +7,9 @@ import (
 	"fmt"
 	"io"
 	"log/slog"
-	"net"
 	"net/url"
 	"runtime/debug"
+	"strings"
 	"time"
 
 	"github.com/dustin/go-humanize"
@@ -220,11 +220,13 @@ func (s *Service) ValidateRequest(req *ImageRequest) error {
 	}
 
 	// An allowlist entry names no port, so a host followed by ":", with or
-	// without a port, needs a signature. This is checked here and not in
-	// allowlist.IsAllowed, which also matches referer_blocklist, where a
-	// Referer must be blocked whatever its port.
-	_, _, splitErr := net.SplitHostPort(parsedURL.Host)
-	hostHasPort := splitErr == nil
+	// without a port, needs a signature. The port is read from the parsed
+	// URL, as the fetcher reads it: in an IPv6 address written without
+	// brackets, such as 2001:db8::1:8443, it is what follows the last ":".
+	// This is checked here and not in allowlist.IsAllowed, which also matches
+	// referer_blocklist, where a Referer must be blocked whatever its port.
+	hostHasPort := parsedURL.Port() != "" ||
+		strings.HasSuffix(parsedURL.Host, ":")
 
 	if !hostHasPort && s.allowlist.IsAllowed(parsedURL) {
 		return nil