GHSA-j3rv-43j4-c7qm: jackson-databind has a PolymorphicTypeValidator bypass via generic type parameters that allows arbitrary class instantiation

jackson-databind's PolymorphicTypeValidator (PTV) is the primary safety mechanism guarding polymorphic deserialization. When polymorphic typing is enabled and a type identifier contains generic parameters (i.e. the type ID string contains <), DatabindContext._resolveAndValidateGeneric() validates only the raw container class name (the substring before <) against the configured PTV. If the container type is approved, the method parses the full canonical type string via TypeFactory.constructFromCanonical() and returns the fully parameterized type without ever validating the nested type arguments against the PTV. The nested type arguments are then resolved, instantiated, and populated as beans during deserialization. An attacker who controls the type ID can therefore place a denied class as a generic type parameter of an allowed container — for example java.util.ArrayList<com.evil.Gadget> when only java.util.ArrayList is allow-listed. The container passes the PTV check; com.evil.Gadget is loaded via Class.forName(name, true, loader), instantiated, and its properties are set from attacker-controlled JSON. This completely bypasses an explicitly configured PTV allow-list. This is the same vulnerability class responsible for the historical sequence of jackson-databind deserialization CVEs; here it manifests as a validator bypass rather than a missing deny-list entry. Impact - Bypass of the PTV allow-list, including the recommended BasicPolymorphicTypeValidator configured with name-prefix allow rules. - Arbitrary class instantiation of any type assignable to the container's element/parameter position, with attacker-controlled property values (setter/field injection). - Potential unauthenticated remote code execution when a class with exploitable side effects (JNDI lookup, JDBC/connection-pool gadgets,TemplatesImpl-style loaders, etc.) is present on the classpath. Applications that accept untrusted JSON and rely on a configured PTV — the documented, security-conscious