(Probably) fixed the sign exploit

src/main/java/net/minecraft/server/NetServerHandler.java

@@ -776,6 +776,17 @@ public class NetServerHandler extends NetHandler implements ICommandListener {
         // CraftBukkit start
         if (this.e.world.f(packet130updatesign.a, packet130updatesign.b, packet130updatesign.c)) {
             TileEntity tileentity = this.e.world.getTileEntity(packet130updatesign.a, packet130updatesign.b, packet130updatesign.c);
+            if (tileentity instanceof TileEntitySign) {
+                TileEntitySign sign = (TileEntitySign)tileentity;
+                if (!sign.fresh) {
+                    int x = packet130updatesign.a;
+                    int y = packet130updatesign.b;
+                    int z = packet130updatesign.c;
+                    server.getLogger().severe("Player " + getPlayer().getName() + "/" + getPlayer().getAddress().toString() + " just tried to change the sign text at " +
+                            x + "," + y + "," + z + " - very likely an exploit attempt. Recommend ban.");
+                    return;
+                }
+            }
             // CraftBukkit end
 
             int i;
@@ -817,6 +828,8 @@ public class NetServerHandler extends NetHandler implements ICommandListener {
                     for (int l = 0; l < 4; ++l) {
                         event.setLine(l, "");
                     }
+                } else {
+                    tileentitysign.fresh = false;
                 }
                 // CraftBukkit end
 

src/main/java/net/minecraft/server/TileEntitySign.java

@@ -0,0 +1,40 @@
+package net.minecraft.server;
+
+public class TileEntitySign extends TileEntity {
+
+    public String[] a = new String[] { "", "", "", ""};
+    public int b = -1;
+    public boolean fresh = true; // Craftbukkit
+
+    public TileEntitySign() {}
+
+    public void b(NBTTagCompound nbttagcompound) {
+        super.b(nbttagcompound);
+        nbttagcompound.a("Text1", this.a[0]);
+        nbttagcompound.a("Text2", this.a[1]);
+        nbttagcompound.a("Text3", this.a[2]);
+        nbttagcompound.a("Text4", this.a[3]);
+    }
+
+    public void a(NBTTagCompound nbttagcompound) {
+        fresh = false; // Craftbukkit
+        super.a(nbttagcompound);
+
+        for (int i = 0; i < 4; ++i) {
+            this.a[i] = nbttagcompound.i("Text" + (i + 1));
+            if (this.a[i].length() > 15) {
+                this.a[i] = this.a[i].substring(0, 15);
+            }
+        }
+    }
+
+    public Packet e() {
+        String[] astring = new String[4];
+
+        for (int i = 0; i < 4; ++i) {
+            astring[i] = this.a[i];
+        }
+
+        return new Packet130UpdateSign(this.e, this.f, this.g, astring);
+    }
+}