[solatic]

> To play devil's advocate, they may not be worried about vulnerabilities in their code but rather vulnerabilities in their method of virus detection

This is an argument for factoring out the means of virus detection into a closed-source plugin/module, while opening the source of the rest of the code. Particularly since detection is presumably pure (i.e. functional programming notions of purity and referential transparency), and thus much less likely to be a source of vulnerabilities, compared to the rest of the client which actually interacts with the OS, disks/files, etc. and is therefore much more likely to be exploited. Because the vulnerability scanner would still be a closed-source binary blob, the public would need to trust the company that the blob is actually pure, but seeing that blob within the context of an open-source client which is handling I/O makes that trust easier.

Yes, it makes it easier for malware creators to test their creations against the closed-source module before releasing their malware into the wild. But sophisticated malware writers are already doing that, by installing the anti-virus client into a VM, updating it, disconnecting it from networks, then loading the malware into the VM and seeing if the malware is detected or not. So malware writers don't gain that much from the opening of the rest of the codebase (unless they succeed in finding vulnerabilities that the rest of the world doesn't), and the white-hat public gains a much more trustworthy security tool.