CVE-2026-44884: Portainer Custom Template Information Disclosure Vulnerability

Technical summary

A missing authorization check in the GET /api/custom_templates/{id}/file endpoint fails to validate whether the authenticated user has Resource Control permissions to access a specific custom template. The endpoint accepts sequential integer IDs and returns file content without verifying access restrictions, allowing any authenticated user to bypass intended RBAC boundaries through ID enumeration. The vulnerability affects Portainer Community Edition from 2.33.0 up to (but not including) 2.33.8, and 2.39.0 up to (but not including) 2.39.1.

Business impact

Organizations deploying Portainer for multi-tenant or role-separated container management face information disclosure risk. Custom templates often embed environment-specific credentials, API tokens, registry secrets, and database connection strings. An insider or compromised standard user account can extract these templates, potentially gaining lateral movement capability or access to backend systems the template was designed to interact with. In regulated environments, unauthorized credential exposure may trigger compliance reporting obligations.

Affected systems

Portainer Community Edition versions 2.33.0 through 2.33.7 (inclusive) and version 2.39.0 are affected. Version 2.33.8 and 2.39.1 contain the fix. Deployments on Docker, Swarm, Kubernetes, or Azure Container Instances using vulnerable Portainer versions require immediate attention. Enterprise Edition status should be verified against vendor guidance.

Exploitability

Exploitation requires valid authentication—an attacker must have a legitimate Portainer login, though any privilege level suffices. Once authenticated, the attack is trivial: simple HTTP GET requests with incremented template IDs will retrieve files without user interaction. No special network access, uncommon tools, or timing constraints are needed. The CVSS score of 6.5 reflects the requirement for prior authentication but acknowledges the high confidentiality impact and ease of execution.

Remediation

Upgrade Portainer Community Edition to version 2.33.8 or 2.39.1 or later. Verify the deployed version matches your release series (2.33.x or 2.39.x) and confirm the patch version after upgrade. Review custom templates for sensitive data and consider rotating any credentials that may have been exposed during the vulnerable window. Audit access logs for suspicious enumeration of template IDs.

Patch guidance

Download and deploy Portainer Community Edition 2.33.8 (for users on the 2.33.x line) or 2.39.1 (for users on the 2.39.x line) from the official Portainer release repository. Test in a non-production environment first to confirm compatibility with your orchestration platform. Verify the fix by confirming the endpoint now enforces Resource Control checks; attempt to retrieve a custom template ID for which your test account lacks permission and expect a 403 Forbidden response. Document the patch timestamp for compliance records.

Detection guidance

Monitor for repeated GET requests to /api/custom_templates/{id}/file with sequential or rapidly changing ID parameters from a single authenticated user, especially from accounts with limited permissions. Inspect audit logs for template file access patterns inconsistent with user role. Implement API request logging with parameter capture to identify enumeration attempts. Consider restricting direct API access to known integration points and enforcing rate limits on template endpoint queries.

Why prioritize this

Although rated MEDIUM severity, this vulnerability warrants prompt patching because it directly exposes secrets and credentials in multi-user or multi-tenant deployments. The barrier to exploitation is low (authentication + simple enumeration), the impact is high (credential disclosure), and template files are frequently used to store sensitive configuration. Organizations with multiple users or roles in Portainer should treat this as high priority.

Risk score, explained

CVSS 6.5 reflects: Attack Vector = Network (endpoint is HTTP/HTTPS-accessible), Attack Complexity = Low (no special conditions), Privileges Required = Low (any authenticated user), User Interaction = None (direct API call), Scope = Unchanged (no privilege escalation beyond reading templates), Confidentiality = High (secrets exposed), Integrity = None, Availability = None. The score appropriately captures that this is a pure information disclosure flaw with a low execution bar but significant business consequence.

Frequently asked questions