Reject unpatched clients on patched server & warn about unpatched servers

src/main/java/net/minecraftforge/fml/client/ClientHooks.java

@@ -128,6 +128,11 @@ public class ClientHooks
             if (fmlver > FMLNetworkConstants.FMLNETVERSION) {
                 extraReason = "fml.menu.multiplayer.clientoutdated";
             }
+
+            if (!packet.getForgeData().isPatchAdvertised()) {
+                extraReason = "fml.menu.multiplayer.serverunpatched";
+            }
+
             target.forgeData = new ExtendedServerListData("FML", extraServerMods.isEmpty() && fmlNetMatches && channelsMatch && modsMatch, mods.size(), extraReason);
         } else {
             target.forgeData = new ExtendedServerListData("VANILLA", NetworkRegistry.canConnectToVanillaServer(),0, null);
@@ -143,11 +148,19 @@ public class ClientHooks
         switch (target.forgeData.type) {
             case "FML":
                 if (target.forgeData.isCompatible) {
+                    // HACK: Allow connections to unpatched servers, but show a warning
+                    if (target.forgeData.extraReason != null && target.forgeData.extraReason.equals("fml.menu.multiplayer.serverunpatched")) {
+                        idx = 96;
+                        tooltip = ForgeI18n.parseMessage("fml.menu.multiplayer.incompatible.extra", ForgeI18n.parseMessage(target.forgeData.extraReason));
+                    } else {
                         idx = 0;
                         tooltip = ForgeI18n.parseMessage("fml.menu.multiplayer.compatible", target.forgeData.numberOfMods);
+                    }
                 } else {
                     idx = 16;
                     if(target.forgeData.extraReason != null) {
+                        if (target.forgeData.extraReason.equals("fml.menu.multiplayer.serverunpatched"))
+                            idx = 96;
                         String extraReason = ForgeI18n.parseMessage(target.forgeData.extraReason);
                         tooltip = ForgeI18n.parseMessage("fml.menu.multiplayer.incompatible.extra", extraReason);
                     } else {

src/main/java/net/minecraftforge/fml/network/FMLNetworkConstants.java

@@ -34,7 +34,8 @@ public class FMLNetworkConstants
 {
     public static final String FMLNETMARKER = "FML";
     public static final int FMLNETVERSION = 2;
-    public static final String NETVERSION = FMLNETMARKER + FMLNETVERSION;
+    // HACK: Unpatched servers won't see this leading 0 - but we will.
+    public static final String NETVERSION = FMLNETMARKER + "0" + FMLNETVERSION;
     public static final String NOVERSION = "NONE";
 
     static final Marker NETWORK = MarkerManager.getMarker("FMLNETWORK");

src/main/java/net/minecraftforge/fml/network/FMLStatusPing.java

@@ -67,6 +67,7 @@ public class FMLStatusPing {
     private transient Map<ResourceLocation, Pair<String, Boolean>> channels;
     private transient Map<String, String> mods;
     private transient int fmlNetworkVer;
+    private transient boolean patched;
     public FMLStatusPing() {
         this.channels = NetworkRegistry.buildChannelVersionsForListPing();
         this.mods = new HashMap<>();
@@ -74,12 +75,20 @@ public class FMLStatusPing {
                     mods.put(modid, mc.getCustomExtension(ExtensionPoint.DISPLAYTEST).
                             map(Pair::getLeft).map(Supplier::get).orElse(FMLNetworkConstants.IGNORESERVERONLY)));
         this.fmlNetworkVer = FMLNetworkConstants.FMLNETVERSION;
+        this.patched = true;
     }
 
     private FMLStatusPing(Map<ResourceLocation, Pair<String, Boolean>> deserialized, Map<String,String> modMarkers, int fmlNetVer) {
         this.channels = ImmutableMap.copyOf(deserialized);
         this.mods = modMarkers;
         this.fmlNetworkVer = fmlNetVer;
+        this.patched = false;
+    }
+    private FMLStatusPing(Map<ResourceLocation, Pair<String, Boolean>> deserialized, Map<String,String> modMarkers, int fmlNetVer, boolean p) {
+        this.channels = ImmutableMap.copyOf(deserialized);
+        this.mods = modMarkers;
+        this.fmlNetworkVer = fmlNetVer;
+        this.patched = p;
     }
 
     public static class Serializer {
@@ -96,7 +105,8 @@ public class FMLStatusPing {
                         collect(Collectors.toMap(jo -> JSONUtils.getString(jo, "modId"), jo->JSONUtils.getString(jo, "modmarker")));
 
                 final int remoteFMLVersion = JSONUtils.getInt(forgeData, "fmlNetworkVersion");
-                return new FMLStatusPing(channels, mods, remoteFMLVersion);
+
+                return new FMLStatusPing(channels, mods, remoteFMLVersion, JSONUtils.hasField(forgeData, "keithPatch"));
             } catch (JsonSyntaxException e) {
                 LOGGER.debug(NETWORK, "Encountered an error parsing status ping data", e);
                 return null;
@@ -125,6 +135,8 @@ public class FMLStatusPing {
             });
             obj.add("mods", modTestValues);
             obj.addProperty("fmlNetworkVersion", forgeData.fmlNetworkVer);
+            if (forgeData.patched)
+                obj.addProperty("keithPatch", true);
             return obj;
         }
     }
@@ -141,4 +153,8 @@ public class FMLStatusPing {
         return fmlNetworkVer;
     }
 
+    public boolean isPatchAdvertised() {
+        return patched;
+    }
+
 }

src/main/java/net/minecraftforge/fml/server/ServerLifecycleHooks.java

@@ -155,6 +155,12 @@ public class ServerLifecycleHooks
             final ConnectionType connectionType = ConnectionType.forVersionFlag(packet.getFMLVersion());
             final int versionNumber = connectionType.getFMLVersionNumber(packet.getFMLVersion());
 
+            // HACK: Check for missing leading 0. If it's not there, we know the client isn't patched.
+            if (connectionType == ConnectionType.MODDED && packet.getFMLVersion().substring(FMLNetworkConstants.FMLNETMARKER.length()).charAt(0) != '0') {
+                rejectConnection(manager, connectionType, "Your client appears to be vulnerable to CVE-2021-44228, 45046, 45105, and/or 44832. To connect to this server, you must install ~keith's patched Forge version: https://bytes.keithhacks.cyou/keith/ForgePatch/releases/");
+                return false;
+            }
+
             if (connectionType == ConnectionType.MODDED && versionNumber != FMLNetworkConstants.FMLNETVERSION) {
                 rejectConnection(manager, connectionType, "This modded server is not network compatible with your modded client. Please verify your Forge version closely matches the server. Got net version "+ versionNumber + " this server is net version "+FMLNetworkConstants.FMLNETVERSION);
                 return false;

src/main/resources/assets/forge/lang/en_us.json

@@ -34,6 +34,7 @@
   "fml.menu.multiplayer.extraservermods":"Server has additional mods that may be needed on the client",
   "fml.menu.multiplayer.modsincompatible":"Server mod list is not compatible",
   "fml.menu.multiplayer.networkincompatible":"Server network message list is not compatible",
+  "fml.menu.multiplayer.serverunpatched": "Server is unpatched, or not advertising keith/ForgePatch",
   "fml.menu.loadingmods": "{0,choice,0#No mods|1#1 mod|1<{0} mods} loaded",
   "fml.menu.notification.title": "Startup Notification",
   "fml.menu.accessdenied.title": "Server Access Denied",