SafeMode is a Fastjson hardening setting that disables AutoType and trims parser features that can be abused when handling untrusted JSON. In practice, it tells the library to be more conservative about what object types it will accept and how it will interpret input. That reduces the chance that a crafted payload can steer parsing into dangerous classes or unexpected behaviors.
In cyber security, SafeMode matters because JSON parsers often sit directly on the request path of web applications. If an attacker can send malicious JSON to a vulnerable Fastjson deployment, parser features can become an entry point for code execution or other unsafe behavior. SafeMode is a defensive mitigation, not a full replacement for patching or removing risky versions, but it can lower exposure while teams migrate to safer releases and validate how their applications process attacker-controlled input.