Portainer has Unauthenticated Restore Endpoint that Allows Admin Takeover on Uninitialized Instances (CVE-2026-55761)

Portainer's /api/restore endpoint is intentionally unauthenticated to allow restoring backups before the first admin account is created. However, this endpoint remains accessible for a five-minute window after each startup when the instance is uninitialized. During this window, an unauthenticated attacker with network access can exploit the endpoint to replace the Portainer database with a crafted backup archive containing attacker-controlled credentials, gaining full administrative access. The same unauthenticated setup window exposes the /api/users/admin/init endpoint, allowing direct creation of the first admin account. The vulnerability affects Portainer versions >=2.39.0 <2.39.4 and >=2.40.0 <2.43.0. The fix gates both endpoints to require authentication or disable access after initialization. The vulnerability is rated high severity due to the potential for full admin takeover and subsequent control over Docker hosts and Kubernetes clusters managed by Portainer.

Successful exploitation allows an unauthenticated attacker to gain full administrative access to a Portainer instance during its five-minute initialization window before the first admin account is created. Although the instance is new and contains no confidential data or workloads at this time, the attacker can control the Portainer database and thus obtain credentials and API access to all Docker hosts, Kubernetes clusters, and edge agents registered with the instance. This can lead to root-equivalent control over the underlying hosts and clusters managed by Portainer. The impact is limited to uninitialized instances reachable by the attacker within the setup window.

A patch is available and included in Portainer versions 2.39.4 and 2.43.0. Users should upgrade to these or later versions to remediate the vulnerability. Until upgrading, administrators can mitigate risk by provisioning the administrator account at deployment using the --admin-password or --admin-password-file options to eliminate the uninitialized state. Additionally, restricting network access to the Portainer API before completing initial setup and completing the initial setup immediately after deployment reduce the attack window. Administrators should audit existing deployments for unauthorized admin accounts if the instance was accessible before setup completion. These workarounds do not replace applying the official fix.