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CVE-2026-55494 – Authentication Bypass (Unauthenticated Docker API Access) – Tugtainer Agent ≤ 1.30.

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Summary

CVE-2026-55494 is a critical (CVSS 9.8) improper access control vulnerability in the Tugtainer Agent, a self-hosted tool used to automate updates of Docker containers. When the agent’s AGENT_SECRET environment variable is left unconfigured, its request-signature verification logic silently skips authentication, allowing unauthenticated network attackers to reach privileged Docker management APIs. Public proof-of-concept exploit code is available, including a chain combining this flaw with a second vulnerability (CVE-2026-62308) to achieve root-level remote code execution on the host.

Technical details

  • Root cause: the verify_signature() function in the agent’s auth.py returns success and exits before performing any credential check when Config.AGENT_SECRET is empty or unset.
  • Trigger condition: any deployment of Tugtainer Agent that does not have AGENT_SECRET explicitly configured is left with its signature verification effectively disabled.
  • Attack vector: network-based, no authentication or user interaction required (CVSS AV:N, PR:N, UI:N).
  • Impact: unauthenticated access to Docker management endpoints — listing, inspecting, creating, starting, stopping, and removing containers, pulling/pruning images, and reading container logs/environment variables. Published exploit chains further leverage this access, combined with CVE-2026-62308, to spawn a container sharing the host’s PID namespace, gaining filesystem access via /proc/1/root and resulting in root-level remote code execution on the underlying host.

Affected software

  • Quenary Tugtainer Agent versions prior to 1.30.4 (confirmed vulnerable in versions including 1.30.2).
  • Fixed in version 1.30.4.

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

Mitigation and recommended actions

  • Immediate: upgrade Tugtainer Agent to version 1.30.4 or later, which requires AGENT_SECRET to be configured by default and requires an explicit override flag to run without authentication.
  • If immediate patching is not possible: ensure AGENT_SECRET is set to a strong, randomly generated value on every deployment, and restrict network access to the Tugtainer Agent’s API endpoints (e.g., via firewall rules or placing the service on an internal-only network) so it is not reachable from the internet or untrusted networks.
  • Review Docker socket permissions granted to the Tugtainer Agent, as the severity of exploitation depends on the privileges available to containers it manages.

References

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