Match blocked_networks against both forms of an address holding IPv4

isBlockedIP refuses an address when a blocked_networks entry contains
it as an IPv6 address or contains the IPv4 address it holds, for the
mapped (::ffff:0:0/96) and the IPv4-compatible (::/96) form alike, so
an IPv6 entry refuses everything it refused on next. ipv6AndIPv4Forms
gives the two forms. Go keeps a plain IPv4 address in the mapped form,
so an entry covering ::ffff:0:0/96, such as ::/0, now refuses every
IPv4 address too.

Model: opus-5-5

TODO.md

@@ -32,13 +32,17 @@ P2: security: per-origin rate limiting
 
 - 2026-10-16 P1 security bug: the check that keeps pixa from fetching internal
   addresses refuses the local-use NAT64 range (`64:ff9b:1::/48`, RFC 8215) as it
-  refuses `64:ff9b::/96`, and matches an IPv6 address in the IPv4-compatible
+  refuses `64:ff9b::/96`, and refuses an IPv6 address that holds an IPv4
-  form (`::/96`) against `blocked_networks` by the IPv4 address it holds, as it
+  address, in the IPv4-compatible form (`::/96`) or the mapped form
-  already matched the mapped form (`::ffff:a.b.c.d`) (closes #299). A source
+  (`::ffff:a.b.c.d`), when `blocked_networks` contains either the address as
-  host such as `[64:ff9b:1::a00:1]` was fetched, which on a network that runs
+  written or the IPv4 address it holds (closes #299). A source host such as
-  NAT64 on that range reaches the IPv4 address it holds, here `10.0.0.1`; and
+  `[64:ff9b:1::a00:1]` was fetched, which on a network that runs NAT64 on that
-  with `203.0.113.0/24` in `blocked_networks`, `[::ffff:203.0.113.5]` was
+  range reaches the IPv4 address it holds, here `10.0.0.1`; with
-  refused but `[::203.0.113.5]` was fetched. Both now answer 403.
+  `203.0.113.0/24` in `blocked_networks`, `[::ffff:203.0.113.5]` was refused but
+  `[::203.0.113.5]` was fetched; and with `::ffff:0:0/96` listed,
+  `[::ffff:203.0.113.5]` was fetched. All now answer 403. Go keeps a plain IPv4
+  address in the mapped form, so an IPv6 entry covering `::ffff:0:0/96`, such as
+  `::/0`, now refuses every IPv4 address too.
 - 2026-10-16 P1 security bug: the check that keeps pixa from fetching internal
   addresses judges an IPv6 address in the IPv4-compatible form (`::/96`), such
   as `::127.0.0.1` or `::10.0.0.1`, by the IPv4 address in its last four bytes

internal/httpfetcher/httpfetcher.go

@@ -599,26 +599,41 @@ func isPrivateIP(ip net.IP) bool {
 }
 
 // isBlockedIP reports whether ip is refused, either by the built-in
-// internal-range check or by one of the operator-supplied prefixes.
+// internal-range check or by one of the operator-supplied prefixes. An IPv6
+// address that holds an IPv4 address is refused when a prefix contains the
+// address as written or the IPv4 address it holds.
 func isBlockedIP(ip net.IP, blocked []netip.Prefix) bool {
 	if isPrivateIP(ip) {
 		return true
 	}
 
-	// An IPv4-compatible or IPv4-mapped IPv6 address is matched as the IPv4
+	asIPv6, asIPv4 := ipv6AndIPv4Forms(ip)
-	// address it holds
+	if !asIPv6.IsValid() {
-	addr, ok := netip.AddrFromSlice(ipv4CompatibleToIPv4(ip))
-	if !ok {
 		return true
 	}
 
-	addr = addr.Unmap()
-
 	return slices.ContainsFunc(blocked, func(prefix netip.Prefix) bool {
-		return prefix.Contains(addr)
+		return prefix.Contains(asIPv6) || prefix.Contains(asIPv4)
 	})
 }
 
+// ipv6AndIPv4Forms returns ip as an IPv6 address and, when it holds an IPv4
+// address in the IPv4-mapped form (::ffff:0:0/96) or the IPv4-compatible form
+// (::/96), that IPv4 address. Otherwise the IPv4 form is the zero netip.Addr,
+// which no prefix contains. Go keeps a plain IPv4 address in the mapped form,
+// so the IPv6 form of 203.0.113.5 is ::ffff:203.0.113.5. An ip that is not 4
+// or 16 bytes long gives two zero addresses.
+func ipv6AndIPv4Forms(ip net.IP) (netip.Addr, netip.Addr) {
+	asIPv6, ok := netip.AddrFromSlice(ip.To16())
+	if !ok {
+		return netip.Addr{}, netip.Addr{}
+	}
+
+	asIPv4, _ := netip.AddrFromSlice(ipv4CompatibleToIPv4(ip).To4())
+
+	return asIPv6, asIPv4
+}
+
 // ipv4CompatibleToIPv4 returns the IPv4 address that an IPv4-compatible IPv6
 // address (in ::/96, such as ::127.0.0.1) holds in its last four bytes, and
 // any other address unchanged.

internal/server/source_url_internal_test.go

@@ -592,14 +592,21 @@ func TestImageRoutesRefuseIPv6SourceHostHoldingInternalIPv4Address(t *testing.T)
 // IPv6 source hosts that hold an IPv4 address, through a /v1/image/ URL with a
 // signature and through an encrypted URL, with the fetcher pixad uses.
 // [64:ff9b:1::a00:1], in the local-use NAT64 range (64:ff9b:1::/48, RFC 8215),
-// answers 403. [::203.0.113.5] holds a public IPv4 address in the
+// answers 403. [::203.0.113.5] and [::ffff:203.0.113.5] hold a public IPv4
-// IPv4-compatible form (::/96). It answers 403 when blocked_networks lists
+// address in the IPv4-compatible form (::/96) and in the IPv4-mapped form
-// 203.0.113.0/24. When blocked_networks is empty it is not refused: the
+// (::ffff:0:0/96). Each answers 403 when blocked_networks lists
-// fetcher tries to connect to it, finds nothing there, and the request fails
+// 203.0.113.0/24, and when it lists the range of its own form. When
-// with another status.
+// blocked_networks is empty [::203.0.113.5] is not refused: the fetcher tries
+// to connect to it, finds nothing there, and the request fails with another
+// status.
 func TestImageRoutesCheckIPv4AddressInIPv6SourceHost(t *testing.T) {
 	t.Parallel()
 
+	const (
+		ipv4CompatibleHost = "[::203.0.113.5]"
+		ipv4MappedHost     = "[::ffff:203.0.113.5]"
+	)
+
 	tests := []struct {
 		name            string
 		sourceHost      string
@@ -608,10 +615,19 @@ func TestImageRoutesCheckIPv4AddressInIPv6SourceHost(t *testing.T) {
 	}{
 		{"local-use NAT64 form of 10.0.0.1", "[64:ff9b:1::a00:1]", nil, true},
 		{"IPv4-compatible form of an address in blocked_networks",
-			"[::203.0.113.5]",
+			ipv4CompatibleHost,
 			[]netip.Prefix{netip.MustParsePrefix("203.0.113.0/24")}, true},
+		{"IPv4-compatible form with ::/96 in blocked_networks",
+			ipv4CompatibleHost,
+			[]netip.Prefix{netip.MustParsePrefix("::/96")}, true},
+		{"IPv4-mapped form of an address in blocked_networks",
+			ipv4MappedHost,
+			[]netip.Prefix{netip.MustParsePrefix("203.0.113.0/24")}, true},
+		{"IPv4-mapped form with ::ffff:0:0/96 in blocked_networks",
+			ipv4MappedHost,
+			[]netip.Prefix{netip.MustParsePrefix("::ffff:0:0/96")}, true},
 		{"IPv4-compatible form of an address with blocked_networks empty",
-			"[::203.0.113.5]", nil, false},
+			ipv4CompatibleHost, nil, false},
 	}
 
 	for _, tt := range tests {