Fastjson 1.x CVE-2026-16723: Unauthenticated RCE Targeting Default Spring Boot Fat-Jars

Fastjson 1.x CVE-2026-16723: Unauthenticated RCE Targeting Default Spring Boot Fat-Jars 1. Basic Information Article Title : Unpatched Fastjson Vulnerability Exploited in Attacks Publisher : SecurityWeek Publication Date : July 28, 2026 Original Source : https://www.securityweek.com/unpatched-fastjson-vulnerability-exploited-in-attacks/ Related Sources : Alibaba Security Advisory: https://github.com/alibaba/fastjson2/wiki/Security-Advisory:-Remote-Code-Execution-in-fastjson-1.2.68%E2%80%931.2.83 Imperva: https://www.imperva.com/blog/imperva-customers-protected-against-cve-2026-16723-critical-fastjson-1-x-zero-day-rce/ FearsOff Technical Analysis: https://fearsoff.org/research/fastjson-1-2-83-rce Related Entities : CVE-2026-16723, Fastjson 1.2.68 to 1.2.83, Spring Boot executable fat-jar, Alibaba, Imperva, ThreatBook Severity : Critical 2. Summary This is an actively exploited vulnerability in end-of-life Fastjson 1.x used within Spring Boot fat-jars. If an attacker sends a crafted JSON request without authentication, it can reach remote code execution (RCE) with Java process permissions via external resource lookups, even when AutoType is not explicitly enabled. 3. Attack Flow The attacker scans for publicly exposed JSON-receiving endpoints. The attacker sends a crafted JSON payload containing @type . Fastjson 1.x type resolution logic treats the @JSONType annotation as a trust signal. The application bypasses AutoType restrictions and triggers a lookup to an attacker-controlled resource. Vulnerable Spring Boot fat-jar configurations reach the code execution path without external gadgets. Arbitrary code runs with the execution permissions of the Java application user. Inference : The attack may proceed to drop webshells, search for credentials, access cloud metadata, and deploy lateral movement tools. 4. Attacker Position and Execution Location The attacker sends HTTP(S) requests from the internet or an accessible internal network. The vulnerability is processed by