This isn't how the legal system works. Most CP cases get prosecuted because the defendant solicits or shares CP with a minor of some other CP collectors, but let's imagine a situation where someone gets busted from something else and then investigators discover they have CP (real-world non-AI CP in this example).
Having it at all is a strict liability crime. If the defendant says malware put it on their computer and they don't know how or when it got there, that's called an affirmative defense - it's admissible as a claim, but the burden of proof for the claim is on the defendant. Otherwise you could just claim it was planted on your computer by ghosts or demons or space aliens. If the machine was infested with malware to the point of the browser being nearly inoperable and all sort so fother junk being present, a jury might buy it. But the defendant has to make some sort of showing to back up the claim. The whole thing about 'reasonable doubt' in criminal cases is not that something sounds possible or even plausible, but that you can support the claim with some mix of logic and empirical evidence like any other reasoned argument.
Having it at all is a strict liability crime. If the defendant says malware put it on their computer and they don't know how or when it got there, that's called an affirmative defense - it's admissible as a claim, but the burden of proof for the claim is on the defendant. Otherwise you could just claim it was planted on your computer by ghosts or demons or space aliens. If the machine was infested with malware to the point of the browser being nearly inoperable and all sort so fother junk being present, a jury might buy it. But the defendant has to make some sort of showing to back up the claim. The whole thing about 'reasonable doubt' in criminal cases is not that something sounds possible or even plausible, but that you can support the claim with some mix of logic and empirical evidence like any other reasoned argument.
https://www.law.cornell.edu/wex/affirmative_defense