CVE-2026-10054 affects Eclipse Theia (version 1.8.1 and later) where the browser backend exposes privileged terminal RPC over WebSocket endpoints (/services/shell-terminal, /services/terminals/:id) without service-level authentication. The WebSocket origin validation in the @theia/core module is fail-open, accepting connections if the Origin header is missing or if no THEIA_HOSTS allowlist is configured, which is the default setting. Additionally, the Socket.IO integration replaces the real Origin header with a client-supplied fix-origin header that can be controlled or omitted by an attacker. Consequently, a malicious web page from a foreign origin can open the WebSocket namespace, create terminals, attach to terminal data channels, execute arbitrary OS commands, and read their output. This vulnerability affects both local developer environments and hosted or tunneled deployments lacking strong external authentication. A fix is being developed to enforce same-origin validation by default, remove trust in the fix-origin header, require a SameSite=Strict; HttpOnly connection-token cookie for HTTP and WebSocket access, and sanitize shell terminal creation options.

An attacker can exploit this vulnerability by hosting a malicious web page that, when visited by a user running a vulnerable Eclipse Theia instance, can open WebSocket connections to privileged terminal services. This allows the attacker to create terminals, execute arbitrary operating system commands, and read their output, leading to full confidentiality, integrity, and availability compromise of the affected system. Both local and hosted deployments without strong external authentication are at risk.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. A fix is in development that will enforce same-origin validation by default, remove reliance on the client-supplied fix-origin header, require HTTP and WebSocket access to be gated by a SameSite=Strict; HttpOnly connection-token cookie, and sanitize shell terminal creation options. Until a fix is released, users should apply strong external authentication controls and avoid exposing Theia instances to untrusted networks or users.