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CVE-2026-88899 – Remote File Path Traversal / Sandbox Escape – knowns before v0.31.0

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Summary

CVE-2026-88899 is a critical external control of file path vulnerability (CWE-73) affecting the knowns npm package prior to version 0.31.0. The /api/opencode proxy endpoint fails to validate the x-opencode-directory request header, allowing unauthenticated remote attackers to redirect the embedded OpenCode agent’s file operations to arbitrary directories on the host filesystem. The issue is rated CRITICAL, with a CVSS v3.1 base score of 9.8 and a CVSS v4.0 base score of 9.3.

Technical details

  • Root cause: The proxyOpenCode() function in internal/server/server.go uses fail-open logic — it only substitutes the server’s safe activeRoot working directory when the client-supplied value is empty. If an attacker supplies a value, it is passed through unmodified to the underlying OpenCode daemon.
  • Trigger conditions: An attacker sends an HTTP request to the knowns server’s /api/opencode endpoint with a crafted x-opencode-directory header (or ?directory query parameter) pointing outside the intended project root (e.g., /root, /etc, ~/.ssh).
  • Attack vector: Network-based, requires no authentication and no user interaction; the vulnerable endpoint listens by default on port 4141.
  • Impact: The OpenCode agent initializes file operations at the attacker-specified path with the daemon’s OS-level privileges, enabling reading of sensitive files (SSH keys, credentials, .env files), writing malicious content (e.g., to shell profiles or cron jobs), and deletion/corruption of arbitrary files — resulting in full compromise of confidentiality, integrity, and availability.

Affected software

  • knowns (npm package, vendor: knowns-dev) — all versions prior to 0.31.0
  • Fixed in version 0.31.0, which removed the embedded OpenCode proxy entirely as part of a Chat UI removal

Severity

  • CVSS v3.1 Base Score: 9.8 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  • (A CVSS v4.0 score of 9.3, Critical, is also assigned: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N)

Mitigation and recommended actions

  • Immediate: Upgrade knowns to version 0.31.0 or later, where the embedded OpenCode proxy endpoint has been removed.
  • If immediate patching is not possible:
    • Restrict network access to the knowns server (default port 4141) to trusted hosts only, using firewall rules or network segmentation.
    • Do not expose the knowns server directly to the internet.
    • Monitor for HTTP requests to /api/opencode containing unexpected x-opencode-directory header values or directory query parameters referencing paths outside the intended project root.
    • No official patch exists for versions prior to 0.31.0 — upgrading is the only complete remediation.

References

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