diff --git a/README.md b/README.md
index 4fc97e9..85d08c0 100644
--- a/README.md
+++ b/README.md
@@ -575,7 +575,11 @@ pixa lower-cases the host of a `/v1/image/` URL before anything else uses it, so
 `CDN.Example.com` is matched, fetched, cached and counted against
 `upstream_connections_per_host` as `cdn.example.com`. Such a URL whose host has
 a port, as in `cdn.example.com:8443`, or a `:` with no port, needs a signature
-even when the host is allowlisted. A host holding `@` is refused with 400.
+even when the host is allowlisted. The host must be a host name or an IPv4
+address (letters, digits, hyphens, underscores and dots) or an IPv6 address in
+brackets, either one optionally followed by `:` and a port. Any other host, such
+as one holding `@`, or a `?`, `#`, `%`, `\` or space written percent-encoded, is
+refused with 400.
 
 ### Configuration
 
diff --git a/TODO.md b/TODO.md
index 0ef9f6b..6b6417e 100644
--- a/TODO.md
+++ b/TODO.md
@@ -30,6 +30,17 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-15 the host of a `/v1/image/` URL must be a host name or an IPv4
+  address (letters, digits, hyphens, underscores and dots) or an IPv6 address in
+  brackets, either one optionally followed by `:` and a port, and any other host
+  is refused with 400 (closes #275): only `@` was refused, so a `?` written as
+  `%3F` gave the host `cdn.example.com?x=1`, which Go's URL parser reads as
+  `cdn.example.com` with a query. Such a URL passed the allowlist, and each text
+  after the `?` fetched the host's root path and cached the answer under keys of
+  its own. A `#`, `%`, `\` or space written percent-encoded is refused the same
+  way, and so is an IPv6 address without brackets, which needed a signature
+  before. Underscores are kept because `allowlist_hosts` accepts host names
+  holding them.
 - 2026-10-15 the README says how a source URL with a query is written into an
   image URL, and the image route accepts it (closes #255): the query follows the
   source path after `%3F`. pixa now reads the image URL's path decoded once, so
diff --git a/internal/handlers/image_errors_internal_test.go b/internal/handlers/image_errors_internal_test.go
index fbbcf6f..eac170f 100644
--- a/internal/handlers/image_errors_internal_test.go
+++ b/internal/handlers/image_errors_internal_test.go
@@ -264,3 +264,79 @@ func TestHandleImage_AllowlistOrSignature(t *testing.T) {
 		})
 	}
 }
+
+// TestHandleImage_InvalidHost_Returns400 sends image URLs whose host no URL
+// host can be, and checks that each is refused with 400 naming the host as
+// decoded: allowlistedHost with a character after it that a URL host cannot
+// hold, percent-encoded, an IPv6 address without brackets, and brackets
+// around something that is not an IPv6 address. A "?" or "#" after
+// allowlistedHost would otherwise end the host when the source URL is read,
+// so the request would pass the allowlist and be fetched and cached under a
+// host that is not the one fetched.
+func TestHandleImage_InvalidHost_Returns400(t *testing.T) {
+	t.Parallel()
+
+	photos := newPhotoFetcher(t, allowlistedHost)
+
+	tests := []struct {
+		name        string
+		writtenHost string
+		decodedHost string
+	}{
+		{"? written as %3F", allowlistedHost + "%3Fx=1", allowlistedHost + "?x=1"},
+		{"# written as %23", allowlistedHost + "%23x", allowlistedHost + "#x"},
+		{"% written as %25", allowlistedHost + "%253F", allowlistedHost + "%3F"},
+		{`\ written as %5C`, allowlistedHost + "%5Cx", allowlistedHost + `\x`},
+		{"space written as %20", allowlistedHost + "%20", allowlistedHost + " "},
+		{"IPv6 address without brackets", "2001:db8::1", "2001:db8::1"},
+		{"IPv4 address in brackets", "[203.0.113.5]", "[203.0.113.5]"},
+		{"no address in brackets", "[abc]", "[abc]"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			rec := sendGet(t, newImageRoute(t, photos), photoURL(tt.writtenHost))
+			checkErrorBody(t, rec, http.StatusBadRequest,
+				fmt.Sprintf("invalid image URL: invalid source host: %q",
+					tt.decodedHost))
+		})
+	}
+}
+
+// TestHandleImage_HostForms_Served checks that the image route serves, with
+// a valid signature, the image at photoPath on a host name, an IPv4 address,
+// an IPv6 address in brackets, and a host followed by ":" with or without a
+// port.
+func TestHandleImage_HostForms_Served(t *testing.T) {
+	t.Parallel()
+
+	hosts := []string{
+		allowlistedHost,
+		"img_1.example.com",
+		"203.0.113.5",
+		"[2001:db8::1]",
+		allowlistedHost + ":8443",
+		"[2001:db8::1]:8443",
+		allowlistedHost + ":",
+	}
+
+	photos := newPhotoFetcher(t, hosts...)
+	exp := time.Now().Add(time.Hour)
+
+	for _, host := range hosts {
+		t.Run(host, func(t *testing.T) {
+			t.Parallel()
+
+			sig := photoSignature(testSigningKey, host, exp)
+
+			rec := sendGet(t, newImageRoute(t, photos),
+				photoURLWithSig(host, sig, exp))
+			if rec.Code != http.StatusOK {
+				t.Errorf("status = %d, want %d: %s",
+					rec.Code, http.StatusOK, rec.Body)
+			}
+		})
+	}
+}
diff --git a/internal/imgcache/urlparser.go b/internal/imgcache/urlparser.go
index 1794d1f..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"
@@ -50,6 +51,14 @@ var sizeFormatRegex = regexp.MustCompile(
 // percentEscapeRegex matches one percent-escape, such as "%2e"
 var percentEscapeRegex = regexp.MustCompile(`%[0-9A-Fa-f]{2}`)
 
+// 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]*)?$`)
+
 // ParsedURL contains the parsed components of an image proxy URL.
 type ParsedURL struct {
 	// Host is the source origin host (e.g., "cdn.example.com")
@@ -141,10 +150,20 @@ func parseImageComponents(remainder string) (*ParsedURL, error) {
 		return nil, ErrMissingHost
 	}
 
-	// A URL reads what comes before "@" as a user name and password, and the
-	// rest as the host
-	if strings.Contains(host, "@") {
-		return nil, fmt.Errorf("%w: %q holds @", ErrInvalidHost, 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