Refuse an image URL host that no URL host can be

Only @ was refused in the host of a /v1/image/ URL, 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: it passed the allowlist and each text
after the ? was cached under keys of its own. The host must now be a
host name or an IPv4 address (letters, digits, -, _ and .) or an IPv6
address in brackets, either one optionally followed by : and a port;
anything else is refused with 400. An IPv6 address without brackets,
which needed a signature, is refused too.

Model: opus-5-5

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
 

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

internal/imgcache/urlparser.go

@@ -50,6 +50,12 @@ 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
+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 +147,12 @@ 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)
+	// 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) {
+		return nil, fmt.Errorf("%w: %q", ErrInvalidHost, host)
 	}
 
 	// Host names do not depend on letter case, so the allowlist, the upstream