WASM has strong tried and proven sandboxing. We basically can build on nearly 30 years of experience. The decoders don't need a lot of access, they can basically be pure functions.
If this will pan out security-wise I don't know. I'm more worried that it will be so slow that no one will use it. Interesting idea, though, and I can see applications outside of the "big data" realm this apparently targets.
OOM killing in WebAssembly is trivial, since it’s all in a growable linear memory. All the runtimes I’m aware of have a simple maximum memory setting, and they’ll trap any allocation requests after that point.
Attack is not just on file format itself. Based on the function signature it's possible for a single decoder to generate infinite bytestream - makes a lot of headache to reader implementation - implementing STRLEN is no longer trivial question.
Either engines should put some limit (e.g. VARCHAR(2000) to enforce length to be limited to 2000, but there are some other engines supporting unlimited BLOBs), or decoder should give a hint what is the maximum length it will yield. Unfortunately current research level project does not have such considerations implemented yet...
For images, it makes sense: people dealing with 16k x 16k PNGs are uncommon. Give them an error message that tells them the setting to bump. But what should be the threshold for "big data"? I'm sure it will follow Zipf's Law, but the tail will be fatter.
WASM implementations are fairly mature now, but if there was e.g. an image file format with embedded WASM that needed to execute before you could view it, it would become the new low-hanging-fruit target for 0-click RCEs - whether it's exploiting the WASM engine itself or some other attack surface that's influenceable via it (See also, the FORCEDENTRY JBIG2 exploit).
That exploit targeted an integer overflow in a bespoke Apple sandboxing mechanism. Bespoke sandboxing mechanisms have weird bugs.
Not that Wasm engines don't have bugs, but the whole point is to have an extremely solid, well-specified and efficient implementation of a widely accepted bytecode format. We can scope down the capabilities given to any program to a minimal set.
Bugs are near-inevitable, and mitigations are the last line of defence. Scripting engines are excellent for bypassing mitigations (iiuc in the case of the FORCEDENTRY exploit, it was used for adjusting ASLR'd offsets).
As a random example that's an area of personal interest to me, I know of 3 distinct methods of achieving userland ROP execution of the Nintendo Switch 2, and all three rely on the (ab)use of a scripting engine (even if they aren't a vulnerability in the scripting engine itself).
But seriously, if your format requires extensibility to the point that it embeds a bytecode, especially a Turing-complete bytecode, what format are you going to choose? Just design a new one? That's how you end up with a scripting engine with three ROP exploits.
> WASM has strong tried and proven sandboxing. We basically can build on nearly 30 years of experience. The decoders don't need a lot of access, they can basically be pure functions.
I've heard that kind of sentiment many times before. It's not a good (thought-terminating) mindset to have for any secure software.
There are several WASM implementations, WASM is just a format. "Pure functions" are pure at a superficial level. Many people say that they don't mutate global state, but they do ... it's just hidden. The decoders "not needing a lot of access" doesn't matter if the WASM engine is pwned through arbitrary code execution inside the environment, or if it's contorted to bypass the access control you are mentioning through various side-effects.
> The decoders don't need a lot of access, they can basically be pure functions
They don't currently either do they? It's the tight coupling of the interface layer no? I'm not sure this would be faster, or more secure so reliability might be the best usecase?
All WASM can do is transfer bag of bytes between module runtime and host. So yes, so yeah it can just decode into a buffer. Even you use wasm components to give it I/O, you can still make these go to buffer.
If this will pan out security-wise I don't know. I'm more worried that it will be so slow that no one will use it. Interesting idea, though, and I can see applications outside of the "big data" realm this apparently targets.