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CVE-2026-12609 – Unauthenticated Path Traversal / Arbitrary File Read – Eclipse Theia 1.66.0 through

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Summary

CVE-2026-12609 is an unauthenticated path traversal vulnerability in Eclipse Theia’s @theia/plugin-ext backend. A remote attacker can read arbitrary files accessible to the Theia backend process, resulting in a loss of confidentiality. It affects Eclipse Theia versions 1.66.0 through 1.73.1 and is rated HIGH (CVSS 7.5).

Technical details

  • Root cause: The /hostedPlugin/:pluginId/:path(*) endpoint resolves the requested file path without validating that it remains within the plugin directory (CWE-22, Improper Limitation of a Pathname to a Restricted Directory).
  • Trigger conditions: An attacker sends percent-encoded traversal sequences (%2e%2e%2f, decoding to ../) in the path to escape the plugin directory.
  • Attack vector: Network-based and unauthenticated — no privileges and no user interaction are required.
  • Impact: Retrieval of arbitrary files readable by the Theia backend process (high confidentiality impact; no integrity or availability impact).

Affected software

  • Eclipse Theia versions 1.66.0 through 1.73.1 (inclusive).

Severity

  • CVSS v3.1 base score: 7.5 (HIGH)
  • Vector string: 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, which contains the fix.
  • If no patch can be applied: Restrict network access to the Theia backend so the affected endpoint is not reachable by untrusted clients until the upgrade can be performed.

References

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