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CVE-2026-61891 – Unauthenticated Arbitrary File Read – Eclipse Theia before 1.74.0

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Summary

CVE-2026-61891 is an unauthenticated arbitrary file read (path traversal / missing authentication) vulnerability in Eclipse Theia. In browser-based deployments, HTTP file-download endpoints exposed by the @theia/filesystem backend convert a client-supplied URI directly to a filesystem path and stream the file without confining it to the workspace or enforcing the connection token. It is rated High severity (CVSS 7.5).

Technical details

  • Root cause: Two HTTP file-download endpoints (GET /file in file-download-endpoint.ts and the /files/ handler in file-download-handler.ts) convert a client-controlled URI directly into a filesystem path and stream the file without validating that the path stays within the workspace roots (CWE-22, CWE-36).
  • Contributing flaw: In browser (non-Electron) deployments the connection token is enforced only on WebSocket upgrades; the HTTP middleware in @theia/core re-issues the cookie and calls next() without rejecting tokenless HTTP requests, leaving these endpoints reachable without a valid token (CWE-306).
  • Trigger conditions: A browser-mode Theia backend reachable over the network; no authentication or user interaction required.
  • Attack vector: Network (remote, unauthenticated HTTP requests).
  • Impact: An attacker can read any file readable by the backend process, including files outside the opened workspace such as /etc/hosts, SSH keys, and tokens — resulting in disclosure of sensitive information (CWE-200).

Affected software

  • Eclipse Theia — all versions before 1.74.0 (browser/non-Electron deployments).
  • Not affected: Electron desktop deployments, which use a separate security token.

Severity

  • CVSS v3.1 Base Score: 7.5 (High)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Mitigation and recommended actions

  • Immediate: Upgrade to Eclipse Theia 1.74.0 or later.
  • If no patch can be applied: Restrict network access to the Theia backend (e.g., place it behind an authenticating reverse proxy, VPN, or firewall) so the HTTP file-download endpoints are not reachable by untrusted clients. Note that excluding the @theia/filesystem package does not remove endpoint exposure at runtime.

How IONIX identifies potentially affected assets

IONIX matches the following signals against data already collected when it crawled the asset; identifying the technology sends no request beyond that crawl.

  • Raw HTTP response body: id="theia-app", window.theia
  • URL of a loaded script: theia-<hex>.js

References

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1

Map your entire attack surface (continously)

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2

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3

Identify Potential External Exposures

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4

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IONIX transforms real-world PoCs into safe, non-intrusive test payloads that can be run in production environments without disruption. These simulations are precisely targeted to the systems that are vulnerable, ensuring rapid validation without unnecessary load.

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6

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Results are routed through integrations with ticketing, SOAR, and SIEM tools. Issues are written in plain language, bundled into remediation clusters, and prioritized based on asset criticality, exploitability, and blast radius. This shortens mean time to remediation (MTTR) and empowers teams to act with confidence.

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