fix(hostmatcher): block reserved IP ranges from external/private filters (#38039)

modules/hostmatcher/hostmatcher.go

@@ -8,6 +8,7 @@ import (
 	"path/filepath"
 	"slices"
 	"strings"
+	"sync"
 )
 
 // HostMatchList is used to check if a host or IP is in a list.
@@ -23,10 +24,64 @@ type HostMatchList struct {
 	ipNets []*net.IPNet
 }
 
-// MatchBuiltinExternal A valid non-private unicast IP, all hosts on public internet are matched
+// MatchBuiltinExternal A valid global-unicast IP that is neither private (see MatchBuiltinPrivate)
+// nor a reserved special-purpose range (see reservedIPNets); i.e. a routable host on the public internet.
 const MatchBuiltinExternal = "external"
 
-// MatchBuiltinPrivate RFC 1918 (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) and RFC 4193 (FC00::/7). Also called LAN/Intranet.
+// reservedIPNets are special-purpose ranges that net.IP.IsPrivate omits but that must not be
+// treated as public/external destinations (CGNAT, cloud metadata, IPv6 transition, etc.). We layer
+// these on top of net.IP.IsPrivate (RFC 1918 / RFC 4193) so future additions to Go's IsPrivate are
+// picked up automatically, while still covering the ranges it leaves out; otherwise the default
+// allow-list would let authenticated users reach cloud metadata, internal, and IPv6 transition
+// endpoints (SSRF), and a "private" block-list would fail to catch them.
+var reservedIPNets = sync.OnceValue(func() []*net.IPNet {
+	var nets []*net.IPNet
+	for _, cidr := range []string{
+		// IPv4
+		"100.64.0.0/10",    // RFC 6598 Carrier-Grade NAT
+		"168.63.129.16/32", // Azure WireServer metadata endpoint
+		"192.0.0.0/24",     // RFC 6890 IETF protocol assignments
+		"192.0.2.0/24",     // RFC 5737 TEST-NET-1
+		"192.88.99.0/24",   // RFC 7526 6to4 relay anycast (deprecated)
+		"198.18.0.0/15",    // RFC 2544 benchmarking
+		"198.51.100.0/24",  // RFC 5737 TEST-NET-2
+		"203.0.113.0/24",   // RFC 5737 TEST-NET-3
+		// IPv6
+		"100::/64",       // RFC 6666 discard-only
+		"64:ff9b::/96",   // RFC 6052 NAT64 (can embed IPv4 such as 169.254.169.254)
+		"64:ff9b:1::/48", // RFC 8215 local-use NAT64
+		"2001::/32",      // RFC 4380 Teredo tunneling (embeds IPv4)
+		"2001:10::/28",   // RFC 4843 ORCHID (deprecated)
+		"2001:20::/28",   // RFC 7343 ORCHIDv2
+		"2001:db8::/32",  // RFC 3849 documentation
+		"2002::/16",      // RFC 3056 6to4 (embeds IPv4)
+	} {
+		_, ipNet, err := net.ParseCIDR(cidr)
+		if err != nil {
+			panic("hostmatcher: invalid reserved CIDR " + cidr + ": " + err.Error())
+		}
+		nets = append(nets, ipNet)
+	}
+	return nets
+})
+
+// isPrivateIP reports whether ip falls in a private (net.IP.IsPrivate) or reserved special-purpose
+// range (see reservedIPNets) that must not be considered a public/external destination.
+func isPrivateIP(ip net.IP) bool {
+	if ip.IsPrivate() {
+		return true
+	}
+	for _, ipNet := range reservedIPNets() {
+		if ipNet.Contains(ip) {
+			return true
+		}
+	}
+	return false
+}
+
+// MatchBuiltinPrivate RFC 1918 (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) and RFC 4193 (FC00::/7),
+// plus the reserved special-purpose ranges in reservedIPNets (CGNAT, NAT64, cloud metadata, etc.).
+// Also called LAN/Intranet.
 const MatchBuiltinPrivate = "private"
 
 // MatchBuiltinLoopback 127.0.0.0/8 for IPv4 and ::1/128 for IPv6, localhost is included.
@@ -100,11 +155,11 @@ func (hl *HostMatchList) checkIP(ip net.IP) bool {
 	for _, builtin := range hl.builtins {
 		switch builtin {
 		case MatchBuiltinExternal:
-			if ip.IsGlobalUnicast() && !ip.IsPrivate() {
+			if ip.IsGlobalUnicast() && !isPrivateIP(ip) {
 				return true
 			}
 		case MatchBuiltinPrivate:
-			if ip.IsPrivate() {
+			if isPrivateIP(ip) {
 				return true
 			}
 		case MatchBuiltinLoopback:

modules/hostmatcher/hostmatcher_test.go

@@ -159,3 +159,58 @@ func TestHostOrIPMatchesList(t *testing.T) {
 	}
 	test(cases)
 }
+
+// TestReservedRanges ensures special-purpose ranges that net.IP.IsPrivate misses are kept out of the
+// "external" allow-list (the default for webhook delivery and repository migrations) and folded into
+// the "private" block-list, so they cannot be used for SSRF to metadata/internal endpoints.
+func TestReservedRanges(t *testing.T) {
+	external := ParseHostMatchList("", "external")
+	private := ParseHostMatchList("", "private")
+
+	// legitimate public destinations: external, not private
+	for _, ip := range []string{"8.8.8.8", "1.1.1.1", "2001:4860:4860::8888", "1000::1"} {
+		addr := net.ParseIP(ip)
+		assert.Truef(t, external.MatchIPAddr(addr), "public ip %s should be external", ip)
+		assert.Falsef(t, private.MatchIPAddr(addr), "public ip %s should not be private", ip)
+	}
+
+	// RFC 1918 / RFC 4193 private ranges (now folded into privateIPNets instead of net.IP.IsPrivate):
+	// not external, blockable as private. Includes range edges to guard the CIDR boundaries.
+	for _, ip := range []string{
+		"10.0.0.0", "10.255.255.255", // 10.0.0.0/8
+		"172.16.0.0", "172.31.255.255", // 172.16.0.0/12
+		"192.168.0.0", "192.168.255.255", // 192.168.0.0/16
+		"fc00::", "fdff:ffff:ffff:ffff:ffff:ffff:ffff:ffff", // fc00::/7
+	} {
+		addr := net.ParseIP(ip)
+		assert.Falsef(t, external.MatchIPAddr(addr), "private ip %s must not be external", ip)
+		assert.Truef(t, private.MatchIPAddr(addr), "private ip %s should match private block-list", ip)
+	}
+
+	// 172.32.0.0 is just outside 172.16.0.0/12: a public destination, not private
+	if addr := net.ParseIP("172.32.0.0"); assert.NotNil(t, addr) {
+		assert.True(t, external.MatchIPAddr(addr), "172.32.0.0 should be external")
+		assert.False(t, private.MatchIPAddr(addr), "172.32.0.0 should not be private")
+	}
+
+	// reserved ranges that IsPrivate does not cover: not external, but blockable as private
+	for _, ip := range []string{
+		"100.64.0.1",         // CGNAT
+		"100.127.255.254",    // CGNAT
+		"168.63.129.16",      // Azure WireServer
+		"192.0.2.1",          // TEST-NET-1
+		"198.18.0.1",         // benchmarking
+		"198.51.100.1",       // TEST-NET-2
+		"203.0.113.1",        // TEST-NET-3
+		"192.88.99.1",        // 6to4 relay anycast
+		"64:ff9b::1",         // NAT64
+		"64:ff9b::a9fe:a9fe", // NAT64 embedding 169.254.169.254
+		"2001::1",            // Teredo
+		"2002::1",            // 6to4
+		"2001:db8::1",        // documentation
+	} {
+		addr := net.ParseIP(ip)
+		assert.Falsef(t, external.MatchIPAddr(addr), "reserved ip %s must not be external", ip)
+		assert.Truef(t, private.MatchIPAddr(addr), "reserved ip %s should match private block-list", ip)
+	}
+}