Reverse engineering the obfuscated TikTok VM

The fascinating process of reverse engineering this VM is detailed here.

TikTok uses a custom virtual machine (VM) as part of its obfuscation and security layers. This project includes tools to:

Deobfuscate webmssdk.js that has the virtual machine.

Decompile TikTok’s virtual machine instructions into readable form.

Script Inject Replace webmssdk.js with the deobfuscated VM injector.

Sign URLs Generate signed URLs which can be used to perform auth-based requests eg. Post comments.

Is calling a massive embedded JS obfuscator a "VM" a bit of a stretch? Ultimately it's not translating anything to a lower-level language.

Still, I had no idea. This is really taking JS obfuscation to the next level.

One kind of wonders, what is the purpose of that level of obfuscation? The naive take is that obfuscation is usually to protect intellectual property... but this is client-side code that wouldn't give away anything about their secret sauce algorithm.

> Is calling a massive embedded JS obfuscator a "VM" a bit of a stretch? Ultimately it's not translating anything to a lower-level language.

From the Repo's README:

"TikTok is using a full-fledged bytecode VM, if you browse through it, it supports scopes, nested functions and exception handling. This isn't a typical VM and shows that it is definitely sophiscated."

But that's basically an emulator of a VM, isn't it? It's like rewriting the Flash AVM2 into JS... it's still running in JS whereas the original VM was C++. It could JIT compile stuff but only because it literally was reserving memory that could overflow, and (semi-technical take here) from that advantage, of being closer to the metal, flowed all of the flaws in AVM2 that precipitated most of Adobe's woes with Flash. A VM implant in a web page that uses a plugin like Java or Flash, to get around running browser-sandboxed code, which can take over physical memory, is far different from just emulating a VM in Javascript. I wouldn't call writing a ton of opcodes in JS, which resolved to JS functions, a "virtual machine", because it isn't reserving anything or doing anything that Javascript can't do. Someone correct me here if I'm wrong... this is just heavy-duty obfuscation.

Also, one major purpose of a VM is to improve performance over what's available in the browser. If you use that as a measurement, this clearly doesn't fit that goal.