Fastjson zero-day exposes Java servers to remote attacks

Attackers are exploiting a critical vulnerability in Alibaba’s Fastjson library that can allow unauthenticated remote code execution on Java servers, raising urgent concerns for organisations running older versions of the widely deployed software component.

The flaw, tracked as CVE-2026-16723, affects Fastjson 1. x installations under specific configurations and carries a severity score of 9.0 out of 10. Successful exploitation can give an attacker control of a vulnerable application with the same privileges as the Java process, potentially enabling data theft, malware installation and movement across connected systems.

Fastjson is used to convert Java objects into JSON data and reconstruct objects from JSON input. It has been integrated into web applications, enterprise platforms and Spring Boot services across financial services, retail, logistics and public-sector systems. Its broad use means security teams may struggle to identify every affected application, particularly where the library is embedded inside third-party products.

The vulnerability has been observed in active attacks, with malicious requests reaching exposed systems shortly after technical details became public. Researchers have demonstrated that specially crafted JSON can manipulate Fastjson’s internal type-handling logic and trigger code execution without requiring valid credentials or user interaction.

The most serious risk involves versions 1.2.68 through 1.2.83 deployed in certain Spring Boot environments. Separate technical assessments indicate that a wider range of Fastjson 1. x releases may be vulnerable when SafeMode is disabled or dangerous deserialisation paths remain accessible.

The attack is particularly concerning because it does not always depend on traditional “gadget” classes. Java deserialisation exploits commonly require useful code sequences already present within an application’s dependencies. The Fastjson technique can bypass some of those constraints, increasing the number of systems that may be exposed.

No conventional patch was initially available for organisations maintaining Fastjson 1. x applications. Defenders have been advised to enable SafeMode, block or strictly validate untrusted JSON input, restrict access to vulnerable services and migrate to supported Fastjson 2. x releases where technically possible.

Security teams are also examining internet-facing Java services for evidence of exploitation. Indicators may include unusual outbound connections, unexpected child processes launched by Java, downloads of shell scripts or executable files, and new scheduled tasks or user accounts created after suspicious web requests.

The weakness adds to longstanding concern over insecure deserialisation in Java. Applications that transform attacker-controlled data directly into complex objects can unintentionally invoke methods or classes with dangerous behaviour. Similar flaws have repeatedly led to server compromises because vulnerable libraries are often deeply embedded and poorly documented.

Software inventories and dependency-management tools will be central to the response. Organisations may need to search compiled Java archives, container images and development repositories because Fastjson might not appear as a directly declared dependency. It can be pulled into an application through another package several levels down the software supply chain.

The disclosure emerged alongside other developments illustrating the expanding scope of digital threats. Satellite analysis of Myanmar’s Myawaddy region has identified at least 25 suspected online scam compounds that were built or significantly expanded during a six-month period, despite publicised operations by the military authorities.

The compounds are linked to organised criminal networks conducting cryptocurrency, investment and relationship scams. Many workers are believed to have been trafficked through fraudulent job advertisements and held under threats or physical violence. Scam-related losses across the Asia-Pacific region during 2025 were estimated at between $88.3 billion and $114.1 billion.

Criminal groups have adapted to raids by relocating personnel, rebuilding facilities and obtaining connectivity through foreign telecommunications providers. Their operations increasingly use artificial intelligence, encrypted communications, digital payment systems and specialised money-laundering networks.

Google has meanwhile introduced a unified naming system for cyber-threat actors following the integration of Mandiant and the company’s Threat Analysis Group. The system assigns tracked groups two-word cryptonyms designed to indicate a specific actor and its suspected country, motivation or operational category.

North Korea-linked groups will use names ending in “NEPTUNE”, while temporary clusters that have not been fully attributed will continue to receive identifiers beginning with UNC. Earlier names, external vendor labels and mappings to established threat databases will remain searchable to reduce confusion among security analysts.