MUGEN fighting games’ strongest characters can break a computer

While most “who would win” battles take place in the unfettered realm of the imagination, in 1999 a group of anonymous software developers released a little program called MUGEN that lets you make them a little more real.

At first glance, MUGEN looks like one of the zillions of lousy Street Fighter 2 ripoffs that clogged arcades in the 1990s. But when you start it up, you discover that the game only has one character: a nondescript martial artist named Kung Fu Man. That’s because MUGEN isn’t exactly a game — it’s a construction kit for fighting games, assembling tools to create characters from the ground up. And boy, have people created characters.

With a few free downloads, you can have Peter Griffin face off against Jake from Avatar, or Jake from Adventure Time, or Jake from State Farm. Tens of thousands of MUGEN characters exist, spanning every franchise you can think of. All of the Power Rangers? Sure. All of the Ninja Turtles? Of course. Caillou? There are like eight different versions of Caillou.

One eternal question spans all of pop culture: “Who would win?” This week we have answers. Prepare yourself for Polygon’s Who Would Win Week.

You know when you’re arguing “who would win” there’s always someone who tries to bend the rules to get the outcome they want? MUGEN gives these folks their time to shine by letting them make increasingly overpowered characters that can lay waste to your everyday Kung Fu Man without breaking a sweat. The community has a name for these creations: “cheapies.”

It’s pretty easy to make a strong character in a fighting game. Give them powerful, fast, far-reaching attacks and let them cook. And there are plenty of MUGEN characters that fit that description. A Ronald McDonald edit named “Dark Donald” is one of the most notorious, as is the dreaded “Omega Tom Hanks,” who can fill the screen with damaging DVDs of the actor’s famous films. But a cheapie takes things to a whole different level. You’re not meant to be able to play against them at all, let alone win.

The first shot was fired by a character designer who goes by “Ironcommando” with the release of “A-Bomb” in 2008. This character is about as simple as you can get — just a static image of a cartoon bomb, apparently drawn in MS Paint. Pressing any attack button or getting hit by anything triggers his sole offensive move — a massive, screen-filling explosion that cannot be blocked and kills any foe.

This would seem like a pretty definitive play for the most powerful fighting game character ever. But what would come to be known as the Cheapie Wars were just getting started, and things would only get more ridiculous from there.

How do you beat that? It wasn’t long until character creators found a way. MUGEN supports “reversal” moves — commands that, if they are timed correctly, negate the opponent’s move and counter them with an attack of their own. So a reversal character would simply trigger A-Bomb’s explosion, reverse it automatically, and win.

One of the next entrants in the nascent Cheapie Wars was “F1,” named after the key that, when pressed in MUGEN’s debug mode, instantly killed the fighter on the second-player side. When the round began, F1’s artificial intelligence would have it access the console and execute that command, immediately winning the fight without having to worry about reversals. Then, of course, other creators found ways to prevent the F1 key from being recognized, removing that vulnerability from their own cheapies.

One of the things that separates MUGEN from other games with character creation tools is just how few guardrails there are. When you create a wrestler in a WWE 2K game, you can’t make them 50 feet tall and able to crush the entire ring under their boots. Your Dark Souls adventurer isn’t going to be so buff that the game bows before them. But MUGEN doesn’t restrict creativity in order to preserve a sense of balance. And that has led to some wild decisions in the race to create the most powerful character of all time.

As new techniques would be discovered, cheapie creators would immediately use them to decimate the previous most powerful characters and stand atop the mountain. These coding techniques and hacks were used to divide cheapies into “tiers.” The starter tier was named “nuke” after A-Bomb — characters that don’t do anything outside of the game’s basic ruleset to instantly deplete their opponent’s life.

Image: K. Thor Jensen for Polygon

Soon afterward came the “null tier” cheapies, which were the first to exploit MUGEN’s code itself. Character creators discovered a bug that allowed for character code to access variables and properties outside of itself by overflowing a buffer with blank “null” controllers. By targeting a specific overflow point, these cheapies could simply switch the opponent’s “alive” property to “false,” killing them immediately and ending the round.

Because they bypass the core interaction mechanics of MUGEN, a cheapie battle is over the moment it starts. These fights take place in the fraction of a second in between “Round 1 — fight” and “KO.” To create new and more powerful cheapies, designers had to find methods that would deploy before their opponent’s. Enterprising hackers dug deep into how MUGEN packaged and loaded characters to find faster and better exploits.

The Wild West nature of this process led to some extreme concepts. A cheapie category called “secretary tier” involved the character hiding a prog.exe file in a temp folder to manipulate the data coming into and out of MUGEN. “Postman tier” cheapies do something similar, but with a .bat file. In addition to winning the rounds, some of these characters use custom code to overwrite the data of their opponents, making them instantly die in future fights.

More and more methods came to light for cheapie construction. “Omed tier.” “Whale tier.” “Hypernull tier.” The wars waged fast and furious over MUGEN fan forums, with some even instituting new rules to block the sharing of particularly nefarious characters.

So where does this all end? In theory, with the introduction of “dragon-tier cheapies,” which allegedly use MUGEN to not only run external code but actual malware, causing the computer they are run on to brick itself and become unusable. That’s a pretty definitive win condition, and it’s hard to imagine any character that could top it.

I wanted to test these mighty PC-destroying warriors out for myself, so I grabbed an old laptop and installed MUGEN onto it. Then I dug through forums and archives to download as many cheapies as I could find, with bizarre names like “END_White-Day” and “Undefined Universe.” Once I had a roster built up, I set up a tournament of champions from as many different tiers as I could find.

It was enlightening, and a little nerve-wracking. I couldn’t get my hands on any dragon-tier cheapies, and some in the community claim they don’t exist. Offering malware-infected files for download is generally frowned upon by most web hosts, after all. I was able to get cheapies from many of the other tiers — postman, null, secretary — and these fighters did a number on both MUGEN and each other when I had them play matches. Rounds ended before they started, or extended into infinity with combo counters ratcheting into the triple digits but neither competitor falling. Menus and displays were scrambled and broken, the program locked up or crashed to desktop several times, and the whole tournament collapsed in a volcano of nightmarish code.

By 2023, Windows has patched out most of the vulnerabilities that allowed the Cheapie Wars to escalate to such a degree. MUGEN itself is still widely in use, though, and enterprising hackers are still developing new and ornate ways to break it. So maybe the next cheapie will be Skynet itself, able to gain sentience and finally settle the “who would win” question once and for all.