An attacker with appropriate JSON:API write permissions could potentially inject a malicious payload in certain rare circumstances, leading to PHP Object Injection. PHP Object Injection occurs when untrusted input is used to instantiate objects, which can allow an attacker to execute arbitrary code or manipulate application logic. This issue is exploitable only if a site uses an entity reference field type that stores a serialized property and has enabled write access for JSON:API, which is read-only by default.