Allow worker to load from a URL instead of a `blob`

[billyvg]

The issue with using blobs to load our Worker is that stricter CSPs will not allow it. You cannot control the "source" of the blob, so allowing our worker blob would allow all blobs.

Instead, the proposed solution would be to release the worker to our CDN and add an option to the replay SDK to load from the CDN instead of a blob. This would mean that users could then allow worker-src for our CDN.

[mydea]

I think the way forward for this is, as a first step, to probably land #6665.
Then the next step could be to make this release to the CDN.
And the final step is to figure out a way to opt in to either using this, or the blob - ideally with tree shaking (?). Maybe the default should be to use the CDN, and users can opt-in to use the blob. WDYT? cc @Lms24

[mydea]

OK, so we've been talking about this some more, and would propose the following changes:

1. We extract the worker into @sentry-internal/replay-worker (first step, no actual changes for usage)
2. We change the worker to output an actual JS file (not a string form)
3. We deploy this to the CDN
4. We make the replay package use the CDN version exclusively. When that fails, we fall back to no compression

Advantages:

• CSP works better. Users can whitelist the Sentry CDN specifically
• We reduce the size of the replay bundle, making the worker size impact async (the worker itself is already async, so usage should not be affected) & hopefully reducing LCP as well

If there are no objections against this, we'll start work on this cc @Lms24 @bruno-garcia ?

[bruno-garcia]

My impression is that this seems rather complex. Relying on the CDN might always be a failure mode in some scenarios, so we'll be relying on the fallback to no compression anyway but always trying to fetch the resource from the CDN.
Or maybe I'm just overthinking here.

On the other hand the advantages seem to pay off the added complexity.

How impactful is this issue right now? Does this affect a large number of users?

[billyvg]

Right now? I would say not too impactful, but this is just a guess. we would have to see if we can determine the # of uncompressed vs compressed segments.

However, I think this is a better solution and worth the complexity as it gives the users a more secure way to load our worker.

[mydea]

So, we have started working on this. However, there is a core question:

1. We could try to get this shipped before GA. This is rather unlikely to happen, as we'd need to really rush this.
2. We can ship it after GA, but it can kind of be argued that this is a breaking change? It shouldn't affect that many people, and we do have a good fallback (=we just use uncompressed payloads if the worker fails), but it could be a bit unexpected.
3. Or we wait for v8 to ship this change, but this is an undefined time horizon as of now.

What do you think, how should we approach this?

One alternative version for 2. would be to update the docs & changelog to indicate that you need to whitelist our CDN url for compression to work (even though the actual code hasn't landed yet), then we can do this change after GA and technically not be breaking. cc @billyvg @Lms24 @bruno-garcia

[billyvg]

Let's not try to rush this for GA, that's too risky. Waiting for v8 is too long of a wait. I think updating our docs to call this out for GA is a great idea and releasing the change asap after GA.

The security implications of allowing all blobs to be run in a worker are great enough to justify the potential breaking changes IMO.

[billyvg]

@mydea started a draft PR here: getsentry/sentry-docs#6269

[mydea]

Yeah, I agree and this would also be my vote!

[Lms24]

As we talked about this yesterday, I also agree that it's okay to do this between GA and v8. Especially because it's "soft-breaking" in the sense that we'd fall back to sending uncompressed payloads.

[mydea]

So after some research, I figured out that actually it is impossible to serve a web worker from a different origin 😬 so we cannot serve this from a CDN.

The only thing that would be possible is to serve the worker from the CDN, and have a minimal wrapper served as a blob:

importScripts('https://our-cdn-url.com/replay-worker.js');

This should work, and would reduce our bundle size, but would not address the CSP issue - actually, it would make it worse because users would need to whitelist both the blob as well as the CDN URL.

[tj-kev]

Is there any update on the preferred solution for this? I have a strict CSP and cannot allow "blob:" as this could potentially open up areas of attack. I know there is a fallback but I am now reluctant to turn on CSP reporting to sentry as this will create a lot of entries.