CVE-2025-55182, known as React4Shell, is a severe vulnerability discovered in React Server Components (RSC) and several derivative frameworks including Next.js, React Router RSC preview, Redwood SDK, Waku, and RSC plugins such as Vite and Parcel. The flaw resides in the unsafe deserialization of data streams within the Flight protocol, a lightweight HTTP-based streaming protocol used to transfer serialized component data between client and server. Vulnerable versions include React Server Components 19.0.0 through 19.2.0 and Next.js versions 15.0.4 through 16.0.6. An attacker can exploit this vulnerability by sending a crafted HTTP request that triggers execution of arbitrary code on the server with React privileges, without requiring authentication or user interaction. This can lead to complete server takeover, enabling attackers to access sensitive data, deploy web shells, or pivot within the network. The vulnerability is widespread, affecting tens of millions of websites, including high-profile services like Airbnb and Netflix, and is present in about 39% of cloud infrastructures running React-based applications. While no confirmed exploitation in the wild has been reported, proof-of-concept exploits with near 100% reliability are publicly available, increasing the risk of imminent mass exploitation. Cloud providers such as Akamai, AWS, Cloudflare, Google Cloud, and Vercel have issued WAF rules to mitigate attacks, but these are temporary measures. Comprehensive mitigation requires updating to patched versions of React Server Components and Next.js, restricting access to RSC endpoints, enhancing IP reputation filtering, rate limiting, and deploying endpoint detection and response (EDR) solutions to detect anomalous behavior post-exploitation. Organizations should also conduct thorough log analysis to identify suspicious requests and signs of post-exploitation activity such as secret reconnaissance and web shell installation. The vulnerability underscores the risks introduced by the relatively new server-side React components and the need for secure deserialization practices.

For European organizations, the impact of CVE-2025-55182 is potentially severe due to the widespread adoption of React and Next.js in web applications across industries including finance, e-commerce, media, and government services. Successful exploitation can lead to complete server compromise, resulting in data breaches, theft of sensitive information such as credentials and secrets, service disruption, and potential lateral movement within corporate networks. Given that many cloud-hosted services in Europe run vulnerable React components, the risk extends to cloud infrastructure and hybrid environments. The vulnerability's ease of exploitation without authentication or user interaction increases the likelihood of automated mass attacks, which could disrupt critical online services and damage organizational reputation. Additionally, compromised servers could be used as footholds for further attacks targeting European supply chains or critical infrastructure. The presence of proof-of-concept exploits and partial WAF mitigations means organizations delaying patching face heightened exposure. The impact is amplified in sectors with stringent data protection regulations like GDPR, where breaches can result in significant fines and legal consequences.

European organizations should immediately update all React Server Components packages to versions 19.0.1, 19.1.2, or 19.2.1 and Next.js to versions 15.1.9, 15.2.6, 15.3.6, 15.4.8, 15.5.7, or 16.0.7 to remediate the vulnerability. They must inventory all applications using React RSC and Next.js to identify vulnerable instances, including indirect dependencies in derivative frameworks. Deploy and verify activation of WAF rules from cloud providers or third-party vendors to provide immediate protection, but do not rely solely on these. Restrict access to RSC endpoints by implementing strict IP whitelisting, reputation-based filtering, and rate limiting, especially for internal services. Enhance monitoring by deploying EDR/EPP solutions on servers hosting RSC components to detect anomalous process execution and post-exploitation behaviors such as secret reconnaissance and web shell deployment. Conduct thorough log analysis for suspicious HTTP requests indicative of exploitation attempts. Rotate all secrets, API keys, and credentials stored or accessible on affected servers as a precautionary measure. Educate development teams on secure deserialization practices and review application architectures to minimize exposure of server-side components. Finally, establish incident response plans tailored to this vulnerability to enable rapid containment and remediation if exploitation is detected.