Client software could ask the testnet server to sign a mainnet tx, and since the server is blind, it wouldn't know the difference, but the Mercury CTO seems confident that no one is doing this yet:
Sure, if you’re vendor and there are plausible vulnerabilities affecting your soft, this can be very pragmatic to downside funds exposed. Giving time to people to deploy the fixes.
I don’t wish to sound too harsh on conduition here, I believe it’s great to have more folks doing vulnerabilities hunting in the ecosystem. On the other hand, in infosec rule of thumb is often to give 90 days to vendors. Unless there are clear hints that vendors do not wish to implement mitigations (or mitigations cannot be deployed easily).
I know 90 days can be a lot, so even if you think circumstances are worthy of less, in my opinion nothing displayed in the disclosure report warrants to give only 4 days to the vendors.
As of today, it’s indeed quite easy to go and burak a bitcoin L2. But I don’t think it is the culture we wish to nurture on the long-term in the ecosystem, if we wish seriously to take care of end-users financial wealth (or privacy). Failing to do so, that’s only going slowly towards the path were vulnerabilities are weaponized for other purposes...