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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.

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

References

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Map your entire attack surface (continously)

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Not all CVEs matter. IONIX filters vulnerabilities by asking attacker-centric questions: Can it be reached from the internet? Does it require authentication? Is it being exploited in the wild? This dramatically reduces noise and focuses teams on threats that can actually be weaponized.

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.

5

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By combining context about software stack, versioning, exposure status, and reachability, IONIX ensures that only the right payloads are executed against the right assets, maximizing efficiency and minimizing risk.

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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