AutoType is a Fastjson feature that can automatically resolve a JSON value into a Java class while parsing. Instead of treating input as plain data, the parser may use embedded type hints to decide which object to create. That convenience can be useful in trusted environments, but it is also security-sensitive because it expands the parser’s power over attacker-controlled input.
In cyber security, AutoType matters because deserialization bugs often begin when untrusted JSON is allowed to influence object selection. If a vulnerable application accepts a crafted payload, type resolution can help an attacker reach unexpected code paths, trigger unsafe gadgets, or bypass application assumptions about data shape. Defenders usually reduce this risk by disabling AutoType, using safer defaults, restricting accepted types, and treating all external JSON as untrusted. In practice, AutoType is one of the settings security teams review first when assessing Fastjson exposure.