Block IPv6 SSRF bypass via ipv4_compat addresses (#153)

Adds ipv4_mapped? and ipv4_compat? checks to PrivateNetworkGuard.private_ip?
to block SSRF bypass attempts using IPv6 address formats like:
- ::ffff:169.254.169.254 (IPv4-mapped)
- ::169.254.169.254 (IPv4-compatible)

These formats could previously bypass the link_local? check since Ruby
treats them as IPv6 addresses, not IPv4.

Ref: HackerOne #3481701

lib/restricted_http/private_network_guard.rb

@@ -16,7 +16,7 @@ module RestrictedHTTP
 
     def private_ip?(ip)
       IPAddr.new(ip).then do |ipaddr|
-        ipaddr.private? || ipaddr.loopback? || ipaddr.link_local? || ipaddr.ipv4_mapped? || LOCAL_IP.include?(ipaddr)
+        ipaddr.private? || ipaddr.loopback? || ipaddr.link_local? || ipaddr.ipv4_mapped? || ipaddr.ipv4_compat? || LOCAL_IP.include?(ipaddr)
       end
     rescue IPAddr::InvalidAddressError
       true

test/lib/restricted_http/private_network_guard_test.rb

@@ -0,0 +1,89 @@
+require "test_helper"
+require "restricted_http/private_network_guard"
+
+class RestrictedHTTP::PrivateNetworkGuardTest < ActiveSupport::TestCase
+  test "private_ip? returns true for 'This' network (RFC1700)" do
+    assert_private_ip "0.0.0.0"
+    assert_private_ip "0.255.255.255"
+  end
+
+  test "private_ip? returns true for loopback addresses" do
+    assert_private_ip "127.0.0.0"
+    assert_private_ip "127.0.0.1"
+    assert_private_ip "127.255.255.255"
+  end
+
+  test "private_ip? returns true for RFC1918 private addresses" do
+    assert_private_ip "10.0.0.0"
+    assert_private_ip "10.255.255.255"
+    assert_private_ip "172.16.0.0"
+    assert_private_ip "172.31.255.255"
+    assert_private_ip "192.168.0.0"
+    assert_private_ip "192.168.255.255"
+  end
+
+  test "private_ip? returns true for link-local addresses" do
+    assert_private_ip "169.254.0.1"
+    assert_private_ip "169.254.169.254"  # AWS IMDS
+    assert_private_ip "169.254.255.255"
+  end
+
+  test "private_ip? returns false for public addresses" do
+    assert_not RestrictedHTTP::PrivateNetworkGuard.private_ip?("93.184.216.34")
+    assert_not RestrictedHTTP::PrivateNetworkGuard.private_ip?("8.8.8.8")
+  end
+
+  # IPv6 address format tests (SSRF bypass prevention)
+
+  test "private_ip? returns true for IPv4-mapped IPv6 addresses with private IPs" do
+    assert_private_ip "::ffff:192.168.1.1"
+    assert_private_ip "::ffff:10.0.0.1"
+    assert_private_ip "::ffff:172.16.0.1"
+  end
+
+  test "private_ip? returns true for IPv4-mapped IPv6 addresses with link-local IPs" do
+    assert_private_ip "::ffff:169.254.169.254"  # AWS metadata via mapped format
+  end
+
+  test "private_ip? returns true for IPv4-mapped IPv6 addresses even with public IPs" do
+    # Block all ipv4_mapped? since DNS never returns this format legitimately
+    assert_private_ip "::ffff:93.184.216.34"
+  end
+
+  test "private_ip? returns true for IPv4-compatible IPv6 addresses with private IPs" do
+    assert_private_ip "::192.168.1.1"
+    assert_private_ip "::10.0.0.1"
+  end
+
+  test "private_ip? returns true for IPv4-compatible IPv6 addresses with link-local IPs" do
+    assert_private_ip "::169.254.169.254"  # AWS metadata via compat format - the reported bypass
+  end
+
+  test "private_ip? returns true for IPv4-compatible IPv6 addresses even with public IPs" do
+    # Block all ipv4_compat? since DNS never returns this format legitimately
+    assert_private_ip "::93.184.216.34"
+  end
+
+  test "private_ip? returns true for invalid addresses" do
+    assert RestrictedHTTP::PrivateNetworkGuard.private_ip?("not-an-ip")
+    assert RestrictedHTTP::PrivateNetworkGuard.private_ip?("")
+  end
+
+  test "resolve raises Violation for private hostname" do
+    Resolv.stubs(:getaddress).returns("192.168.1.1")
+    assert_raises RestrictedHTTP::Violation do
+      RestrictedHTTP::PrivateNetworkGuard.resolve("private.example.com")
+    end
+  end
+
+  test "resolve returns IP for public hostname" do
+    Resolv.stubs(:getaddress).returns("93.184.216.34")
+    assert_equal "93.184.216.34", RestrictedHTTP::PrivateNetworkGuard.resolve("example.com")
+  end
+
+  private
+    def assert_private_ip(address)
+      assert RestrictedHTTP::PrivateNetworkGuard.private_ip?(address),
+        "Expected #{address} to be classified as private"
+    end
+end