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CVE-2026-86121 – Unauthenticated RCE / Authentication Bypass – Cua computer-server before 0.3.42

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Summary

CVE-2026-86121 is a critical authentication-bypass vulnerability (CWE-306: Missing Authentication for Critical Function) in Cua computer-server, the agent daemon used by the open-source trycua/cua project to let AI computer-use agents remotely control a desktop or container. In default/local deployments, the server binds to all network interfaces and silently disables authentication whenever the CONTAINER_NAME environment variable is not set, allowing any unauthenticated network-adjacent attacker to execute arbitrary commands, read/write arbitrary files, and obtain an interactive shell. The issue is rated CRITICAL, with a CVSS v3.1 base score of 9.8 (CVSS v4.0: 9.3).

Technical details

  • Root cause: The computer-server authentication check returns True (i.e., "allow access") unconditionally whenever the CONTAINER_NAME environment variable is unset, which is the default state for local/standalone (non-containerized) deployments. No API token or credential verification is enforced in this case.
  • Trigger conditions: The server also defaults to binding on 0.0.0.0 (all network interfaces) rather than localhost, so any host that can reach TCP port 8000 can interact with it — no special configuration or user interaction is required.
  • Attack vector: Network (AV:N) — a remote, unauthenticated attacker on the same network segment, or one who can otherwise route traffic to the exposed port, can connect directly.
  • Impact: Exposed endpoints include command execution (/cmd, shell-based), an unauthenticated WebSocket interface (/ws), interactive pseudo-terminal access (/pty*), and status/action enumeration endpoints (/status, /commands). Combined with a lack of path containment on file operations, this gives attackers full remote code execution and the ability to read or write sensitive filesystem paths, resulting in complete loss of confidentiality, integrity, and availability of the host or container running the service.

Affected software

  • Product: Cua computer-server (pkg:pypi/cua-computer-server), part of the trycua/cua project
  • Vulnerable versions: All versions prior to 0.3.42

Severity

  • CVSS v3.1 Base Score: 9.8 (Critical) — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  • CVSS v4.0 Score: 9.3 (Critical) — 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 cua-computer-server to version 0.3.42 or later, which changes the default network binding to 127.0.0.1 (localhost) and closes the authentication bypass path.
  • If unable to patch immediately:
    • Explicitly set the CONTAINER_NAME environment variable (or otherwise ensure the authentication path is enforced) so the service does not fall back to its permissive "no auth" mode.
    • Restrict network exposure of TCP port 8000 (and any associated WebSocket/PTY endpoints) using host firewall rules or security groups so it is reachable only from trusted, localhost, or internal management networks — never expose it directly to the internet.
    • Audit any existing deployments for signs of unauthorized command execution, unexpected file modifications, or unfamiliar SSH authorized_keys entries, since this flaw allows unauthenticated attackers to modify such files.
    • Require deployments to run computer-server inside properly isolated containers/network namespaces with strict inbound access control until the patched version is applied.

References

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