Next.js, a popular React framework for building full-stack web applications, includes a development mode feature called 'next dev' that supports Hot Module Replacement (HMR) via WebSocket connections. In versions 16.0.1 through 16.1.6, a vulnerability (CVE-2026-27977) exists due to improper origin validation on these internal WebSocket endpoints. Specifically, when the Origin header is set to 'null'—a value common in privacy-sensitive or sandboxed browsing contexts—the server incorrectly treats this as a bypass of cross-site origin checks, even if the developer has configured 'allowedDevOrigins' to restrict connections. This flaw allows attacker-controlled web pages to establish WebSocket connections to the development server's HMR channel, potentially enabling them to observe or interfere with development-time WebSocket traffic. The vulnerability is confined to development mode and does not affect production builds. The fix, introduced in version 16.1.7, ensures that 'Origin: null' is subjected to the same origin-allowance checks as other origins, closing the bypass. Mitigation prior to upgrading involves not exposing the development server to untrusted networks and blocking WebSocket upgrade requests with 'Origin: null' at the network proxy level. No known active exploits have been reported, and the CVSS 4.0 base score is 2.3, indicating low severity due to the limited attack surface, requirement for user interaction, and lack of impact on production environments.

The impact of this vulnerability is primarily limited to development environments using vulnerable versions of Next.js. If an attacker can reach the development server from a malicious web page, they may connect to the internal WebSocket endpoint used for HMR, potentially allowing them to observe or manipulate development-time WebSocket traffic. This could lead to leakage of sensitive development information or interference with the developer's workflow. However, since this only affects development mode and not production deployments, the risk to end-users and live applications is minimal. Organizations that expose their development servers to untrusted networks or allow remote access without proper controls are at higher risk. The vulnerability does not allow code execution or compromise of production data, limiting its overall impact. Nevertheless, it could facilitate reconnaissance or targeted attacks against developers or internal testing environments, which might be leveraged in broader attack campaigns.

To mitigate this vulnerability, organizations should upgrade Next.js to version 16.1.7 or later, where the origin validation issue is fixed. Until upgrading is feasible, developers must ensure that the development server ('next dev') is not exposed to untrusted or public networks. Network-level controls should be implemented to block WebSocket upgrade requests to the '/_next/webpack-hmr' endpoint when the Origin header is 'null'. This can be done via reverse proxies or firewalls capable of inspecting WebSocket handshake headers. Additionally, developers should configure the 'allowedDevOrigins' setting carefully to restrict allowed origins and avoid default permissive configurations. Monitoring development environment access logs for suspicious WebSocket connection attempts can also help detect exploitation attempts. Finally, educating development teams about the risks of exposing dev servers externally and enforcing secure development environment practices will reduce exposure.