How a 69 KB Photo Made TF2 Unplayable for 5 Days

April 13th an exploit to instantly crash every player in your server was found all you needed was a sign and a custom image it's patched now but how did this tiny image cause so many problems custom images have two restrictions when you apply one TF2 limits it to a 128 pixel square and two steam caps any uploads at 69 kiloby the image downscaling can be bypassed with hacks so you can upload an image of any size but the 69 kilobyte limit is enforced on the steam server so there's no way to get around that no matter what your image must fit within 69 kiloby which is weird because players who ran into this exploit reported out of memory crashes yet 69 kilobytes is a really really Far Cry from the 4 GB of RAM TF2 has but as it turns out it's actually very close the problem is a compression Gap steam stores the images as PNG but TF2 needs them as raw data if your phone number was all ones when you pass it out you just tell people it's 10 ones you wouldn't say one 10 times yet when you type it out on your phone you need to press one 10 times this is PNG and raw data PNG is an image format that reduces its file size with tricks like that if you open paint and save a monocolor PNG the file size is Tiny yet if you use BMP a raw data format which has no optimization or compression you end up with a much bigger file this is what happened if you create a massive 1 bit PNG that borders the 69 kilobyte limit in raw data it unfolds to 2.2 GB when you encounter the sign TF2 downloads the PNG off steam and decodes it into raw data when decoding is done TF2 tries to find 2 GB of RAM to put that raw data in realizes it can't and dies this got everyone on the server because normally it's smarter to process the image ahead of time instead of waiting for the guy to pull it out then have TF2 slowly download and process it right then you do it in the background in advance so by the time you see it it's all ready to go just not so good when there's a crash exploit since this hinges on TF2 dying trying to store 2 GB of raw data into RAM if you block the image from downloading in the first place image processing can't happen and you would be safe this was a temporary fix where you could block the server steam stores the custom images it also means that if 64-bit was released it would have just been random lag spikes since there will be more than enough room for a random 2 GB to be allocated this is how it looks on the beta version of 64-bit where the exploit wasn't patched yet it's just a black Square after the Patch exploit obors just render as nothing sprays work differently so this exploit didn't affect them but besides signs this would have also worked on any of the other decal tool items since animated pngs can be stored as do pngs you can also hack TF2 and force it to upload animated pngs to theme that even retains the animation data it shows up animated on websites but TF2 renders custom images in a static format so in game it doesn't move and just uses the first frame but it turned out that the police officer was a huge Team Fortress fan so you know he's like oh yeah I totally understand it's like let me go get my sign

Channel: shounic

Views: 330,231

Keywords: tf2, team, fortress, gameplay

Id: dkjDzQIyrj8

Length: 4min 26sec (266 seconds)

Published: Sat Apr 27 2024