diff --git a/README.md b/README.md
index 0d83cc0..84ecd7d 100644
--- a/README.md
+++ b/README.md
@@ -226,8 +226,9 @@ Key settings in more detail:
 
 - `access_control_allow_origin` — the origin a browser lets read pixa's
   responses, sent as the CORS `Access-Control-Allow-Origin` header: `*`, the
-  default, is any site; otherwise one origin, scheme and host only, such as
-  `https://example.com`. Anything else aborts startup
+  default, is any site; otherwise one origin: scheme, host and optional port,
+  exactly as the browser sends it, such as `https://example.com`. Anything
+  else aborts startup
 - `allowlist_hosts` — list of allowed upstream hosts
 - `blocked_networks` — list of CIDR ranges to refuse for SSRF protection,
   added to the always-enforced built-in ranges (loopback, private,
diff --git a/TODO.md b/TODO.md
index 0ffaae1..f546049 100644
--- a/TODO.md
+++ b/TODO.md
@@ -38,9 +38,9 @@ exhaustion
   server's write timeout and the per-request timeout); each has a
   `PIXA_` variable; durations are positive Go duration strings, the size a
   whole number of bytes up to 1 GiB, the origin `*` or one scheme, host
-  name or IP address and optional port; an invalid value aborts startup
-  naming the key and the value; documented in `config.example.yml` and
-  `README.md`.
+  and optional port, exactly as the browser sends it; an invalid value
+  aborts startup naming the key and the value; documented in
+  `config.example.yml` and `README.md`.
 - 2026-09-28 strip metadata from processed images (closes #82): every output is
   exported with govips' `StripMetadata`, so it carries no EXIF, XMP, IPTC or ICC
   profile; the image is first turned upright with `AutoRotate` (before sizes are
diff --git a/config.example.yml b/config.example.yml
index 40f1b4e..d86d366 100644
--- a/config.example.yml
+++ b/config.example.yml
@@ -78,7 +78,8 @@ downstream_timeout: 60s
 
 # The origin a browser lets read pixa's responses, sent as the CORS
 # Access-Control-Allow-Origin header: "*" (the default) is any site;
-# otherwise one origin, scheme and host only, such as https://example.com
+# otherwise one origin: scheme, host and optional port, exactly as the
+# browser sends it, such as https://example.com
 access_control_allow_origin: "*"
 
 # Maximum disk cache size in bytes. Explicit values are used exactly as
diff --git a/internal/config/config.go b/internal/config/config.go
index 1c17369..5c9504f 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -624,13 +624,9 @@ func (c *Config) validateUpstreamMaxResponseSize() error {
 	return nil
 }
 
-// validateAccessControlAllowOrigin checks that access_control_allow_origin
-// is "*" or one origin as browsers send it in the Origin header: a scheme,
-// a host name or IP address, and optionally a port from 1 to 65535, with
-// nothing after them. Anything else, such as a bare hostname or a trailing
-// slash, would match no request. A "*" is not allowed in a host name, so
-// https://*example.com is refused; the CORS middleware would read that
-// "*" as a pattern and let other sites read responses.
+// validateAccessControlAllowOrigin accepts "*" or an origin exactly as a browser
+// sends it: http or https, an IP address as netip writes it or a lowercase name
+// with a letter in its last part, and an optional port 1-65535, not the default.
 func (c *Config) validateAccessControlAllowOrigin() error {
 	origin := c.AccessControlAllowOrigin
 	if origin == "*" {
@@ -640,50 +636,54 @@ func (c *Config) validateAccessControlAllowOrigin() error {
 	errOrigin := fmt.Errorf("%s: value %q is %w",
 		settingName(keyAccessControlAllowOrigin), origin, errNotAnOrigin)
 
-	// url.Parse accepts an empty port, as in https://example.com:, and
-	// then reports no port, so a trailing colon is refused here.
 	parsed, err := url.Parse(origin)
-	if err != nil || parsed.Scheme+"://"+parsed.Host != origin ||
-		strings.HasSuffix(origin, ":") {
+	if err != nil {
 		return errOrigin
 	}
 
+	defaultPort := map[string]string{"http": "80", "https": "443"}[parsed.Scheme]
+	if defaultPort == "" {
+		return errOrigin
+	}
+
+	const letters = "abcdefghijklmnopqrstuvwxyz"
+
 	host := parsed.Hostname()
+	lastPart := host[strings.LastIndex(host, ".")+1:]
 
-	_, err = netip.ParseAddr(host)
-	if err != nil && !isHostName(host) {
+	addr, err := netip.ParseAddr(host)
+
+	switch {
+	case err == nil && addr.Is6():
+		host = "[" + addr.String() + "]"
+	case err == nil:
+		host = addr.String()
+	case strings.Trim(host, letters+"0123456789-.") != "": // a character other than these
+		return errOrigin
+	case !strings.ContainsAny(lastPart, letters):
 		return errOrigin
 	}
 
-	// A port is a 16-bit number, and 0 is not a port a browser sends.
-	if parsed.Port() != "" {
-		port, err := strconv.ParseUint(parsed.Port(), 10, 16)
-		if err != nil || port == 0 {
+	// The value must be exactly the origin rebuilt from its parts.
+	rebuilt := parsed.Scheme + "://" + host
+
+	port := parsed.Port()
+	if port != "" {
+		_, err := strconv.ParseUint(port, 10, 16)
+		if err != nil || port[0] == '0' || port == defaultPort {
 			return errOrigin
 		}
+
+		rebuilt += ":" + port
+	}
+
+	if rebuilt != origin {
+		return errOrigin
 	}
 
 	return nil
 }
 
-// isHostName reports whether host is not empty and has only ASCII
-// letters, digits, hyphens and dots.
-func isHostName(host string) bool {
-	if host == "" {
-		return false
-	}
-
-	for _, r := range host {
-		isLetterOrDigit := 'a' <= r && r <= 'z' || 'A' <= r && r <= 'Z' ||
-			'0' <= r && r <= '9'
-		if !isLetterOrDigit && r != '-' && r != '.' {
-			return false
-		}
-	}
-
-	return true
-}
-
 // validateAllowlistHost checks that an allowlist_hosts entry is a bare
 // hostname, optionally with a leading dot for suffix matching. URLs,
 // paths, and whitespace indicate a misconfigured entry. An entry with