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[#28] Play Any Mission on Any Map Exploit (How I Got Valve to Update MvM)

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October 22, 2021: Added checks to prevent loading an invalid .pop file
Thanks again to Eric and the TF Team!

You can play any mission on any map by manually calling the vote for the correct popfile in console.

Be careful, not all missions work on all maps. The mission must be fully compatible with the map in order for it to be winnable. Otherwise you may encounter unavoidable softlocks such as bots not spawning or even tanks instantly deploying their bombs.

In order to check whether a mission is compatible with a map, you must note down all of a missions' spawnbot "Where" and tank "StartingPathTrackNode" values. Then, compare them to a candidate map's available bot spawns and tank nodes. Generally, the spawnbots are more often to be compatible than tank nodes. Maps that have many redundant spawn names are very beneficial for having compatible popfiles, such as mvm_powerplant.

Valve fixed this, but why does it still work?

The reason is because it used to be a very dangerous exploit that could crash the server's game logic if done properly. The patch was removing the possibilities of voting for those particular cases. Before, it could be done on all Valve MvM matchmaking and community MvM servers to softlock them except for Mann Up servers because you can't change missions on that due to the game's loot logic. Once again, thank you to Eric and the TF team for fixing the vulnerability I discovered, which could have led to the abusive targeting of all bootcamp and community MvM servers.

Note: You cannot use this exploit to gain progress in community tours. One of the first things I did was to ensure that community tours were patched.

How does this exploit work?

Now, whenever a vote for changemission is called, there is an intermediate step to verify that the target popfile is valid.* If it is valid then it will allow the vote to be held. Once successful, the following command is executed `tf_mvm_popfile %pop`, which should be very familiar to mission makers as it is the command to change missions. What is not easily known is that the command forces the popfile to be loaded; there are no compatibility assessments done. The difference between this command and the gui vote menu is that the vote menu selectively pulls popfiles with the same map prefix, which happens to also be the primary key for gauging mission compatibility.

* A popfile is considered valid if and only if a wave schedule block exists within the targeted popfile. This was Valve's patch that I submitted, making sure to preserve the freedom of playing any mission on any map.

You can experience the softlock yourself by forcefully loading an invalid popfile in your own listen server. You will see that the mission countdown occurs but no enemies will ever spawn, indicating a softlock. Restarting the mission or changing the popfile again will not fix this either. The only way to fix it is to change the map, which reinitializes the entire game logic and is essentially soft-restarting the server.

Shown in this demonstration is playing ghost town endurance on coal town.

Coal Town
The score board is shown to see that the initial mission is Crash Course on coal town.
The vote menu is shown to see that ghost town endurance is not a valid choice to vote for.
The command `callvote changemission mvm_ghost_town` is typed in console to pass a vote.
After passing, the wave is started and the score board is shown to see that the mission is now ghost town endurance.

Decoy
The score board is shown to see that the initial mission is Doe's Drill on decoy.
The vote menu is shown to see that Bone Shaker is not a valid choice to vote for.
The command `callvote changemission mvm_bigrock_advanced2` is typed in console to pass a vote.
After passing, the wave is started and the score board is shown to see that the mission is now Bone Shaker.

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