Possession of CSAM is a strict liability crime in most jurisdictions.
On this topic, the Supreme Court has ruled in Dickerson v US that, in all cases, to avoid First Amendment conflicts, all child pornography statutes must be interpreted with at least a "reckless disregard" standard.
Here is a typical criminal definition, from Minnesota, where a defendant recently tried to argue that the statute was strict liability and therefore unconstitutional, and that argument was rejected by the courts because it is clearly written to require knowledge and intent:
> Subd. 4. Possession prohibited. (a) A person who possesses a pornographic work or a computer disk or computer or other electronic, magnetic, or optical storage system ․ containing a pornographic work, knowing or with reason to know its content and character, is guilty of a felony․
Now, Dickerson rightfully lost and it's appropriate that SCOTUS rejected his case because he was involved in child porn production, not posession, so he can't rely on the Ferber precedent. He had the opportunity to ask the underage person in question their age, and chose not to, which would meet the reckless disregard standard anyway.