Refuse local-use NAT64 addresses, and IPv4-compatible ones in blocked_networks (closes #299)

The check that keeps pixa from fetching internal addresses let through
64:ff9b:1::/48, the local-use NAT64 range, and matched an address in the
IPv4-compatible form (::/96) against blocked_networks only as IPv6, so
::203.0.113.5 passed with 203.0.113.0/24 listed. The local-use NAT64
range is now blocked next to 64:ff9b::/96, and an address in ::/96 is
refused when blocked_networks holds it as written or the IPv4 address it
holds. The mapped form stays matched as IPv4 only, as Go keeps plain
IPv4 addresses in that form; README.md says so.

Model: opus-5-5

README.md

@@ -694,7 +694,10 @@ Key settings in more detail:
   and generator pages. Default: empty
 - `blocked_networks` — list of CIDR ranges to refuse for SSRF protection, added
   to the always-enforced built-in ranges (loopback, private, link-local, CGNAT,
-  benchmark, NAT64, and the like); an invalid CIDR aborts startup
+  benchmark, NAT64, and the like); an invalid CIDR aborts startup. An IPv6
+  address in the IPv4-mapped form (`::ffff:a.b.c.d`) is matched only as the IPv4
+  address it holds, so an IPv6 range such as `::/0` refuses neither it nor a
+  plain IPv4 address
 - `trusted_proxies` — list of CIDR ranges of the reverse proxies in front of
   pixa. `X-Forwarded-For` is believed only when the direct peer falls inside one
   of these ranges; the logged and login-recorded client address is then the

TODO.md

@@ -32,17 +32,16 @@ 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 refuses an IPv6 address that holds an IPv4
-  address, in the IPv4-compatible form (`::/96`) or the mapped form
-  (`::ffff:a.b.c.d`), when `blocked_networks` contains either the address as
-  written or the IPv4 address it holds (closes #299). A source host such as
+  refuses `64:ff9b::/96`, and refuses an IPv6 address in the IPv4-compatible
+  form (`::/96`) when `blocked_networks` contains either the address as written
+  or the IPv4 address it holds (closes #299). A source host such as
   `[64:ff9b:1::a00:1]` was fetched, which on a network that runs NAT64 on that
-  range reaches the IPv4 address it holds, here `10.0.0.1`; with
+  range reaches the IPv4 address it holds, here `10.0.0.1`; and with
   `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.
+  `[::203.0.113.5]` was fetched. Both now answer 403. The mapped form
+  (`::ffff:a.b.c.d`) is still matched only as the IPv4 address it holds: Go
+  keeps a plain IPv4 address in that form, and an IPv6 entry such as `::/0` must
+  not refuse every IPv4 address.
 - 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

@@ -599,39 +599,32 @@ 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. An IPv6
-// address that holds an IPv4 address is refused when a prefix contains the
-// address as written or the IPv4 address it holds.
+// internal-range check or by one of the operator-supplied prefixes. An
+// IPv4-compatible address (in ::/96) is refused when a prefix contains the
+// address as written or the IPv4 address it holds. An IPv4-mapped address (in
+// ::ffff:0:0/96) is matched only as the IPv4 address it holds: Go keeps a
+// plain IPv4 address in that form too, and an IPv6 prefix such as ::/0 must
+// not refuse every IPv4 address.
 func isBlockedIP(ip net.IP, blocked []netip.Prefix) bool {
 	if isPrivateIP(ip) {
 		return true
 	}
 
-	asIPv6, asIPv4 := ipv6AndIPv4Forms(ip)
-	if !asIPv6.IsValid() {
+	addr, ok := netip.AddrFromSlice(ip)
+	if !ok {
 		return true
 	}
 
-	return slices.ContainsFunc(blocked, func(prefix netip.Prefix) bool {
-		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{}
-	}
+	addr = addr.Unmap()
 
+	// ip read as IPv4 where it is or holds an IPv4 address, otherwise the
+	// zero netip.Addr, which no prefix contains. Matching asIPv4 as well as
+	// addr refuses more only for an IPv4-compatible address.
 	asIPv4, _ := netip.AddrFromSlice(ipv4CompatibleToIPv4(ip).To4())
 
-	return asIPv6, asIPv4
+	return slices.ContainsFunc(blocked, func(prefix netip.Prefix) bool {
+		return prefix.Contains(addr) || prefix.Contains(asIPv4)
+	})
 }
 
 // ipv4CompatibleToIPv4 returns the IPv4 address that an IPv4-compatible IPv6

internal/server/source_url_internal_test.go

@@ -589,16 +589,18 @@ func TestImageRoutesRefuseIPv6SourceHostHoldingInternalIPv4Address(t *testing.T)
 }
 
 // TestImageRoutesCheckIPv4AddressInIPv6SourceHost requests ipv6SourcePath on
-// IPv6 source hosts that hold an IPv4 address, through a /v1/image/ URL with a
+// 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] and [::ffff:203.0.113.5] hold a public IPv4
-// address in the IPv4-compatible form (::/96) and in the IPv4-mapped form
-// (::ffff:0:0/96). Each answers 403 when blocked_networks lists
-// 203.0.113.0/24, and when it lists the range of its own form. When
-// 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.
+// answers 403. [::203.0.113.5] holds the public address 203.0.113.5 in the
+// IPv4-compatible form (::/96) and answers 403 when blocked_networks lists
+// 203.0.113.0/24 or ::/96. [::ffff:203.0.113.5] holds it in the IPv4-mapped
+// form (::ffff:0:0/96) and answers 403 when blocked_networks lists
+// 203.0.113.0/24. Go keeps the plain address 203.0.113.5 in the mapped form
+// too, and an IPv6 range must not refuse it: with ::/0 listed it is not
+// refused. Neither is [::203.0.113.5] with blocked_networks empty. Where a
+// host 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()
 
@@ -623,9 +625,9 @@ func TestImageRoutesCheckIPv4AddressInIPv6SourceHost(t *testing.T) {
 		{"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},
+		{"plain IPv4 address with ::/0 in blocked_networks",
+			"203.0.113.5",
+			[]netip.Prefix{netip.MustParsePrefix("::/0")}, false},
 		{"IPv4-compatible form of an address with blocked_networks empty",
 			ipv4CompatibleHost, nil, false},
 	}