diff --git a/internal/imgcache/urlparser.go b/internal/imgcache/urlparser.go
index d109025..467c8d9 100644
--- a/internal/imgcache/urlparser.go
+++ b/internal/imgcache/urlparser.go
@@ -3,6 +3,7 @@ package imgcache
 import (
 	"errors"
 	"fmt"
+	"net/netip"
 	"net/url"
 	"regexp"
 	"strconv"
@@ -50,11 +51,13 @@ var sizeFormatRegex = regexp.MustCompile(
 // percentEscapeRegex matches one percent-escape, such as "%2e"
 var percentEscapeRegex = regexp.MustCompile(`%[0-9A-Fa-f]{2}`)
 
-// hostRegex matches the host of an image URL: a host name or an IPv4 address
-// (letters, digits, "-", "_" and "."), or an IPv6 address in brackets, either
-// one optionally followed by ":" and a port, which may be empty
+// hostRegex matches the forms the host of an image URL may take: a host name
+// or an IPv4 address (letters, digits, "-", "_" and "."), or an IPv6 address
+// in brackets (hex digits, ":" and "."), either one optionally followed by ":"
+// and a port, which may be empty. Its one group holds the text in brackets,
+// which parseImageComponents also checks is an IPv6 address.
 var hostRegex = regexp.MustCompile(
-	`^([A-Za-z0-9_.-]+|\[[0-9A-Fa-f:.]+\])(:[0-9]*)?$`)
+	`^(?:[A-Za-z0-9_.-]+|\[([0-9A-Fa-f:.]+)\])(?::[0-9]*)?$`)
 
 // ParsedURL contains the parsed components of an image proxy URL.
 type ParsedURL struct {
@@ -147,14 +150,22 @@ func parseImageComponents(remainder string) (*ParsedURL, error) {
 		return nil, ErrMissingHost
 	}
 
-	// Go's URL parser, which the allowlist check and the fetcher use, reads a
-	// host that hostRegex does not match as a different host, or refuses it:
-	// what comes before an "@" is a user name and password, and what comes
-	// after a "?" or "#" is a query or a fragment
-	if !hostRegex.MatchString(host) {
+	// Only the forms hostRegex describes are accepted. A "?", "#" or "@" in
+	// the host would make Go's URL parser, which the allowlist check and the
+	// fetcher use, read a different host from the one allowlisted and cached
+	hostMatch := hostRegex.FindStringSubmatch(host)
+	if hostMatch == nil {
 		return nil, fmt.Errorf("%w: %q", ErrInvalidHost, host)
 	}
 
+	// Go's URL parser refuses brackets around anything but an IPv6 address
+	if inBrackets := hostMatch[1]; inBrackets != "" {
+		addr, err := netip.ParseAddr(inBrackets)
+		if err != nil || !addr.Is6() {
+			return nil, fmt.Errorf("%w: %q", ErrInvalidHost, host)
+		}
+	}
+
 	// Host names do not depend on letter case, so the allowlist, the upstream
 	// connection limits, the cache and the signature see one spelling of each
 	// host