Refuse [::], 0.0.0.0, IPv6 multicast and documentation space (closes #341)

On Linux a connection to the unspecified address [::] or 0.0.0.0
reaches the host's own loopback, and the SSRF guard let [::] through.
Both unspecified addresses now sit in alwaysBlockedNetworks, so an
allowlist reaches loopback only through an entry that covers a
loopback address, never through one that covers only 0.0.0.0 or ::;
::/128 joins the default blocklist beside 0.0.0.0/8. IPv6 multicast
(ff00::/8) and documentation space (2001:db8::/32) are refused by
default. Every default blocklist entry gets a one-line comment, and the
README, the rules above each list, the two pinning tests and the
allowlist refusal test follow.

Model: opus-5-5

internal/delivery/ssrf.go

@@ -37,8 +37,8 @@ var (
 		"blocked cloud metadata address",
 	)
 	errBlockedMetadata = errors.New(
-		"blocked link-local or cloud instance metadata " +
-			"address: ALLOWED_EGRESS_CIDRS cannot open it",
+		"blocked link-local, cloud instance metadata or " +
+			"unspecified address: ALLOWED_EGRESS_CIDRS cannot open it",
 	)
 	errInvalidScheme = errors.New(
 		"only http and https are allowed",
@@ -72,14 +72,17 @@ var blockedNetworks []*net.IPNet
 var blockedPublicNetworks []*net.IPNet
 
 // alwaysBlockedNetworks are the ranges no configuration can
-// open: the link-local blocks and the cloud instance metadata
-// endpoints that live outside them. Reaching one is credential
-// or user-data theft rather than delivery to an internal
-// service, so a supplied CIDR that covers such an address still
-// leaves it blocked.
+// open, so a supplied CIDR that covers one still leaves it
+// blocked. An entry is here for one of two reasons: it is a
+// metadata endpoint (the link-local blocks and the cloud
+// instance metadata endpoints that live outside them), or it is
+// an unspecified address. Reaching a metadata endpoint is
+// credential or user-data theft rather than delivery to an
+// internal service.
 //
-// Inclusion criterion — an address belongs here only if BOTH
-// hold, and every entry below satisfies both:
+// Inclusion criterion for metadata endpoints — one belongs here
+// only if BOTH hold, and every metadata entry below satisfies
+// both:
 //
 //  1. It is a fixed address assigned by the provider, or a
 //     range reserved by IANA — never one the operator chose.
@@ -90,8 +93,8 @@ var blockedPublicNetworks []*net.IPNet
 //     not cheaply rotated.
 //
 // Both halves are load-bearing, so use them to refuse a
-// candidate and say why. An endpoint disclosing only the
-// operator's own inventory (instance id, region, disks, NICs)
+// metadata candidate and say why. An endpoint disclosing only
+// the operator's own inventory (instance id, region, disks, NICs)
 // fails (2): letting a delivery target reach the operator's own
 // infrastructure is the feature ALLOWED_EGRESS_CIDRS exists to
 // provide. But (2) is not "IAM credentials only" either —
@@ -112,6 +115,15 @@ var blockedPublicNetworks []*net.IPNet
 // This is a criterion, not an enumeration of every metadata
 // address in existence.
 //
+// The unspecified addresses 0.0.0.0 and :: are here for a
+// separate reason: they disclose nothing, but no host can have
+// either, and on Linux a connection to one reaches this host's
+// own loopback. Listing them means an allowlist reaches loopback
+// only through an entry that covers a loopback address
+// (127.0.0.0/8, ::1/128, 0.0.0.0/0), never through one that
+// covers only 0.0.0.0 or :: (0.0.0.0/8, for example). Nothing
+// else lives at either address, so refusing them costs nothing.
+//
 // Every entry is either already in blockedNetworks — this list is
 // what makes it unconditional — or an alternate encoding of
 // 169.254.169.254 that Contains does not match against
@@ -131,23 +143,46 @@ var alwaysBlockedNetworks []*net.IPNet
 //nolint:gochecknoinits // init is the idiomatic way to parse CIDRs once at startup
 func init() {
 	blockedNetworks = mustParseCIDRs([]string{
+		// IPv4 loopback.
 		"127.0.0.0/8",
+		// RFC 1918 private network.
 		"10.0.0.0/8",
+		// RFC 1918 private network.
 		"172.16.0.0/12",
+		// RFC 1918 private network.
 		"192.168.0.0/16",
+		// IPv4 link-local.
 		"169.254.0.0/16",
+		// "This network", holding the IPv4 unspecified address 0.0.0.0.
 		"0.0.0.0/8",
+		// Carrier-grade NAT shared address space.
 		"100.64.0.0/10",
+		// IETF protocol assignments.
 		"192.0.0.0/24",
+		// IPv4 documentation (TEST-NET-1).
 		"192.0.2.0/24",
+		// Benchmarking.
 		"198.18.0.0/15",
+		// IPv4 documentation (TEST-NET-2).
 		"198.51.100.0/24",
+		// IPv4 documentation (TEST-NET-3).
 		"203.0.113.0/24",
+		// IPv4 multicast.
 		"224.0.0.0/4",
+		// Reserved, including the broadcast address.
 		"240.0.0.0/4",
+		// IPv6 loopback.
 		"::1/128",
+		// IPv6 unspecified address.
+		"::/128",
+		// IPv6 unique local addresses.
 		"fc00::/7",
+		// IPv6 link-local.
 		"fe80::/10",
+		// IPv6 multicast.
+		"ff00::/8",
+		// IPv6 documentation.
+		"2001:db8::/32",
 	})
 
 	blockedPublicNetworks = mustParseCIDRs([]string{
@@ -207,6 +242,14 @@ func init() {
 		// allowlist from opening it.
 		"192.0.0.192/32",
 
+		// The unspecified addresses, each of which reaches this
+		// host's loopback on Linux.
+		//
+		// IPv4 unspecified address, inside the blocked 0.0.0.0/8.
+		"0.0.0.0/32",
+		// IPv6 unspecified address.
+		"::/128",
+
 		// 169.254.169.254 as an IPv4-compatible IPv6 address.
 		"::a9fe:a9fe/128",
 		// 169.254.169.254 behind the NAT64 well-known prefix.
@@ -343,8 +386,9 @@ func (g *Guard) allows(ip net.IP) bool {
 // The order is the policy:
 //
 //  1. alwaysBlockedNetworks is refused before the allowlist is
-//     consulted, so no configured CIDR reaches link-local or a
-//     cloud metadata endpoint at a non-public address.
+//     consulted, so no configured CIDR reaches link-local, a
+//     cloud metadata endpoint at a non-public address, or an
+//     unspecified address.
 //  2. The allowlist is consulted next, so a listed private
 //     network, or a listed public address on the default
 //     blocklist, becomes reachable.

internal/delivery/ssrf_allowlist_test.go

@@ -168,12 +168,13 @@ func TestGuardAllowlist_UnlistedPrivateStillRefused(t *testing.T) {
 
 // TestGuardAllowlist_MetadataAlwaysRefused is the load-bearing
 // case: cloud instance metadata endpoints are credential theft
-// rather than delivery to an internal service, so no allowlist
-// reaches one. Every guard below names a CIDR that covers its
-// target — including 0.0.0.0/0, ::/0, and the ordinary ULA and
-// CGNAT blocks an operator would really list — and the address
-// must stay refused anyway, on both the validation and the
-// delivery path.
+// rather than delivery to an internal service, and the
+// unspecified addresses 0.0.0.0 and :: reach this host's loopback
+// on Linux, so no allowlist reaches any of them. Every guard
+// below names a CIDR that covers its target — including
+// 0.0.0.0/0, ::/0, and the ordinary ULA and CGNAT blocks an
+// operator would really list — and the address must stay
+// refused anyway, on both the validation and the delivery path.
 func TestGuardAllowlist_MetadataAlwaysRefused(t *testing.T) {
 	t.Parallel()
 
@@ -219,15 +220,17 @@ type metadataAlwaysRefusedCase struct {
 }
 
 // metadataAlwaysRefusedCases enumerates every unconditionally
-// blocked address together with an allowlist entry that would
-// otherwise reach it. Split by family of address only to stay
-// under the function-length limit.
+// blocked address (link-local, the cloud metadata endpoints and
+// the unspecified addresses) together with an allowlist entry
+// that would otherwise reach it. Split by family of address only
+// to stay under the function-length limit.
 func metadataAlwaysRefusedCases() []metadataAlwaysRefusedCase {
 	cases := linkLocalRefusedCases()
 	cases = append(cases, ulaMetadataRefusedCases()...)
 	cases = append(cases, ipv4MetadataRefusedCases()...)
+	cases = append(cases, encodedMetadataRefusedCases()...)
 
-	return append(cases, encodedMetadataRefusedCases()...)
+	return append(cases, unspecifiedRefusedCases()...)
 }
 
 // linkLocalRefusedCases covers the link-local blocks, including
@@ -367,6 +370,23 @@ func encodedMetadataRefusedCases() []metadataAlwaysRefusedCase {
 	}
 }
 
+// unspecifiedRefusedCases covers the unspecified addresses, each
+// of which reaches this host's loopback on Linux.
+func unspecifiedRefusedCases() []metadataAlwaysRefusedCase {
+	return []metadataAlwaysRefusedCase{
+		{
+			name:   "IPv4 unspecified address under 0.0.0.0/0",
+			allow:  allowAllIPv4,
+			target: "http://0.0.0.0:8080/hook",
+		},
+		{
+			name:   "IPv6 unspecified address under ::/0",
+			allow:  allowAllIPv6,
+			target: "http://[::]:8080/hook",
+		},
+	}
+}
+
 // TestGuardAllowlist_PublicUnaffected asserts the allowlist does
 // not narrow anything: public addresses were reachable before it
 // existed and stay reachable, whether or not a list is set.
@@ -524,6 +544,10 @@ func TestAlwaysBlockedNetworks_PinnedSet(t *testing.T) {
 		// Oracle Cloud Classic metadata, inside the blocked
 		// 192.0.0.0/24.
 		"192.0.0.192/32",
+		// The IPv4 and IPv6 unspecified addresses, each of
+		// which reaches this host's loopback on Linux.
+		"0.0.0.0/32",
+		"::/128",
 		// 169.254.169.254 as an IPv4-compatible IPv6 address.
 		"::a9fe:a9fe/128",
 		// 169.254.169.254 behind the NAT64 well-known prefix.
@@ -556,7 +580,8 @@ func TestDefaultBlocklist_PinnedSet(t *testing.T) {
 		{cidr: "172.16.0.0/12", reopenable: true},
 		{cidr: "192.168.0.0/16", reopenable: true},
 		{cidr: linkLocalIPv4, reopenable: false},
-		{cidr: "0.0.0.0/8", reopenable: true},
+		// Its first address, 0.0.0.0, is in the unconditional set.
+		{cidr: "0.0.0.0/8", reopenable: false},
 		{cidr: "100.64.0.0/10", reopenable: true},
 		{cidr: "192.0.0.0/24", reopenable: true},
 		{cidr: "192.0.2.0/24", reopenable: true},
@@ -566,8 +591,11 @@ func TestDefaultBlocklist_PinnedSet(t *testing.T) {
 		{cidr: "224.0.0.0/4", reopenable: true},
 		{cidr: "240.0.0.0/4", reopenable: true},
 		{cidr: "::1/128", reopenable: true},
+		{cidr: "::/128", reopenable: false},
 		{cidr: "fc00::/7", reopenable: true},
 		{cidr: "fe80::/10", reopenable: false},
+		{cidr: "ff00::/8", reopenable: true},
+		{cidr: "2001:db8::/32", reopenable: true},
 		{cidr: "168.63.129.16/32", public: true, reopenable: true},
 	}
 

internal/delivery/ssrf_test.go

@@ -101,6 +101,42 @@ func TestValidateTargetURL_Blocked(t *testing.T) {
 	}
 }
 
+// TestDefaultGuard_RefusesUnspecifiedMulticastAndDocumentation
+// covers the unspecified addresses and the IPv6 multicast and
+// documentation ranges: with no allowlist set, each is refused
+// both when a target is created and when a delivery dials it.
+func TestDefaultGuard_RefusesUnspecifiedMulticastAndDocumentation(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	guard := delivery.NewTestGuard()
+
+	targets := []string{
+		// The unspecified addresses. On Linux a connection to
+		// either reaches this host's loopback.
+		"http://0.0.0.0:8080/hook",
+		"http://[::]:8080/hook",
+		// IPv6 multicast, all nodes.
+		"http://[ff02::1]/hook",
+		// IPv6 documentation.
+		"http://[2001:db8::1]/hook",
+	}
+
+	for _, target := range targets {
+		t.Run(target, func(t *testing.T) {
+			t.Parallel()
+
+			require.Error(t,
+				guard.ValidateTargetURL(context.Background(), target),
+				"%s must be refused at target creation", target,
+			)
+
+			assertDialRefused(t, guard, target)
+		})
+	}
+}
+
 func TestValidateTargetURL_Allowed(t *testing.T) {
 	t.Parallel()