Program: 153272
Project: 153272
Buggy source:
static String serverLogin(DataInputStream in,DataOutputStream out,PassthroughConnection ptc,KillableThread thread,boolean loginToProxy,ClientInfo clientInfo){
  ptc.printLogMessage("String_Node_Str");
  Packet02Handshake CtSHandshake=new Packet02Handshake(out,ptc,thread);
  CtSHandshake.setUsername(ptc.clientInfo.getUsername());
  if (CtSHandshake.packetId == null || CtSHandshake.write(out,ptc,thread,false) == null) {
    return "String_Node_Str";
  }
  Packet02Handshake StCHandshake=new Packet02Handshake(in,ptc,thread);
  if (StCHandshake.packetId == null || StCHandshake.read(in,ptc,thread,true,null) == null) {
    return "String_Node_Str";
  }
  Packet01Login clientLogin=new Packet01Login(out,ptc,thread);
  if (!loginToProxy) {
    clientLogin.setVersion(Globals.getClientVersion());
  }
 else {
    clientLogin.setVersion(Globals.getFakeVersion());
  }
  clientLogin.setUsername(ptc.clientInfo.getUsername());
  clientLogin.setDimension((byte)0);
  clientLogin.setMapSeed(0);
  if (clientLogin.packetId == null || clientLogin.write(out,ptc,thread,false) == null) {
    return "String_Node_Str";
  }
  if (loginToProxy) {
    PacketFFKick fakeKick=new PacketFFKick(in,ptc,thread);
    if (fakeKick.packetId == null || fakeKick.read(in,ptc,thread,true,null) == null) {
      return "String_Node_Str";
    }
    if (!fakeKick.getMessage().equals("String_Node_Str")) {
      return "String_Node_Str";
    }
    String hashString=StCHandshake.getUsername();
    String password=Globals.getPassword();
    String toHash=hashString + password;
    String reply=sha1Hash(toHash).substring(0,24);
    Packet02Handshake hashReply=new Packet02Handshake(out,ptc,thread);
    hashReply.setUsername(reply);
    if (hashReply.packetId == null || hashReply.write(out,ptc,thread,false) == null) {
      return "String_Node_Str";
    }
    Packet02Handshake hostnameForwarding=new Packet02Handshake(out,ptc,thread);
    hostnameForwarding.setUsername(clientInfo.getHostname());
    if (hostnameForwarding.packetId == null || hostnameForwarding.write(out,ptc,thread,false) == null) {
      return "String_Node_Str";
    }
    if (!Globals.isQuiet()) {
      ptc.printLogMessage("String_Node_Str" + clientInfo.getHostname());
    }
  }
  return null;
}

Fixed version (ground truth):
static String serverLogin(DataInputStream in,DataOutputStream out,PassthroughConnection ptc,KillableThread thread,boolean loginToProxy,ClientInfo clientInfo){
  if (loginToProxy) {
    ptc.printLogMessage("String_Node_Str");
  }
 else {
    ptc.printLogMessage("String_Node_Str");
  }
  Packet02Handshake CtSHandshake=new Packet02Handshake(out,ptc,thread);
  CtSHandshake.setUsername(ptc.clientInfo.getUsername());
  if (CtSHandshake.packetId == null || CtSHandshake.write(out,ptc,thread,false) == null) {
    return "String_Node_Str";
  }
  Packet02Handshake StCHandshake=new Packet02Handshake(in,ptc,thread);
  if (StCHandshake.packetId == null || StCHandshake.read(in,ptc,thread,true,null) == null) {
    return "String_Node_Str";
  }
  Packet01Login clientLogin=new Packet01Login(out,ptc,thread);
  if (!loginToProxy) {
    clientLogin.setVersion(Globals.getClientVersion());
  }
 else {
    clientLogin.setVersion(Globals.getFakeVersion());
  }
  clientLogin.setUsername(ptc.clientInfo.getUsername());
  clientLogin.setDimension((byte)0);
  clientLogin.setMapSeed(0);
  if (clientLogin.packetId == null || clientLogin.write(out,ptc,thread,false) == null) {
    return "String_Node_Str";
  }
  if (loginToProxy) {
    PacketFFKick fakeKick=new PacketFFKick(in,ptc,thread);
    if (fakeKick.packetId == null || fakeKick.read(in,ptc,thread,true,null) == null) {
      return "String_Node_Str";
    }
    if (!fakeKick.getMessage().equals("String_Node_Str")) {
      return "String_Node_Str";
    }
    String hashString=StCHandshake.getUsername();
    String password=Globals.getPassword();
    String toHash=hashString + password;
    String reply=sha1Hash(toHash).substring(0,16);
    Packet02Handshake hashReply=new Packet02Handshake(out,ptc,thread);
    hashReply.setUsername(reply);
    if (hashReply.packetId == null || hashReply.write(out,ptc,thread,false) == null) {
      return "String_Node_Str";
    }
    Packet02Handshake hostnameForwarding=new Packet02Handshake(out,ptc,thread);
    hostnameForwarding.setUsername(clientInfo.getHostname());
    if (hostnameForwarding.packetId == null || hostnameForwarding.write(out,ptc,thread,false) == null) {
      return "String_Node_Str";
    }
    if (!Globals.isQuiet()) {
      ptc.printLogMessage("String_Node_Str" + clientInfo.getHostname());
    }
  }
  return null;
}