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Portainer missing authorization on custom template file endpoint, which exposes template content

Moderate severity GitHub Reviewed Published May 10, 2026 in portainer/portainer

Package

gomod github.com/portainer/portainer (Go)

Affected versions

>= 2.33.0, < 2.33.8
>= 2.39.0, < 2.39.1

Patched versions

2.33.8
2.39.1

Description

Summary

A missing authorization vulnerability in the Custom Template file endpoint (GET /api/custom_templates/{id}/file) allows any authenticated user to read the file content of any custom template by enumerating sequential integer IDs, bypassing Resource Control access restrictions. Template files may contain environment-specific values such as connection strings, API tokens, or registry credentials that administrators would not expect standard users to read.

Severity

Medium

CWE-862 — Missing Authorization

Exploitation requires an authenticated user account and at least one custom template to exist. Template files are returned verbatim and may contain embedded credentials.

Affected Versions

The vulnerability exists in every Portainer release since custom templates were introduced — the customTemplateFile handler has never performed an authorization check.

Fixes are included in the following releases:

Branch First vulnerable Fixed in
2.33.x (LTS) 2.33.0 2.33.8
2.39.x (LTS) 2.39.0 2.39.1

Portainer 2.40.0 and later are not affected — the fix was already on develop when the 2.40.x STS line branched. Portainer LTS branches receive fixes for 6 months plus a 3-month overlap after the next LTS ships. All releases prior to 2.33.0 are end-of-life and will not receive a fix; users on EOL versions should upgrade to a supported release.

Workarounds

There is no runtime configuration that blocks the vulnerable endpoint directly. Administrators who cannot immediately upgrade can reduce exposure by:

  • Avoiding storing secrets in custom templates until the patched release is deployed. Move sensitive configuration values to Portainer environment variables or an external secret store.
  • Reviewing existing custom templates for embedded secrets. Assume any secret previously stored in a custom template on an unpatched instance has been exposed to every authenticated user and rotate accordingly.

Neither replaces the fix.

Affected Code

The customTemplateFile handler in api/http/handler/customtemplates/customtemplate_file.go (lines 30-53) retrieves a custom template by its numeric ID and returns the file content without performing any authorization check.

All other custom template endpoints properly verify access:

Endpoint Method Authorization Check
/api/custom_templates/{id} GET (inspect) userCanEditTemplate() + UserCanAccessResource()
/api/custom_templates/{id} PUT (update) userCanEditTemplate()
/api/custom_templates/{id} DELETE userCanEditTemplate()
/api/custom_templates GET (list) FilterAuthorizedCustomTemplates()
/api/custom_templates/{id}/file GET None

Vulnerable code (customtemplate_file.go:30-53):

func (handler *Handler) customTemplateFile(w http.ResponseWriter, r *http.Request) *httperror.HandlerError {
    customTemplateID, _ := request.RetrieveNumericRouteVariableValue(r, "id")
    customTemplate, _ := handler.DataStore.CustomTemplate().Read(portainer.CustomTemplateID(customTemplateID))
    // NO AUTHORIZATION CHECK
    fileContent, _ := handler.FileService.GetFileContent(customTemplate.ProjectPath, entryPath)
    return response.JSON(w, &fileResponse{FileContent: string(fileContent)})
}

Secure reference (customtemplate_inspect.go:50-75):

canEdit := userCanEditTemplate(customTemplate, securityContext)
hasAccess := authorization.UserCanAccessResource(securityContext.UserID, teamIDs, resourceControl)
if !canEdit && !hasAccess {
    return httperror.Forbidden("Access denied to resource", httperrors.ErrResourceAccessDenied)
}

Impact

Any authenticated user (including the lowest-privilege standard user) can read the file content of every custom template in the instance. Custom templates commonly contain Docker Compose configuration, which may include environment-specific secrets such as database connection strings, API tokens, or registry credentials.

Timeline

  • 2026-02-11: Reported via GitHub Security Advisory by duddnr0615k.
  • 2026-03-04: Fix merged to develop and cherry-picked to release/2.39.
  • 2026-03-19: 2.39.1 released with fix.
  • 2026-03-25: 2.40.0 released with fix already present from branch cut.
  • 2026-05-07: 2.33.8 released.

Credit

  • duddnr0615k — identified and reported the missing authorization check on the custom template file endpoint.

References

@predlac predlac published to portainer/portainer May 10, 2026
Published to the GitHub Advisory Database May 14, 2026
Reviewed May 14, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required Low
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N

EPSS score

Weaknesses

Missing Authorization

The product does not perform an authorization check when an actor attempts to access a resource or perform an action. Learn more on MITRE.

CVE ID

CVE-2026-44884

GHSA ID

GHSA-cqpq-2fgr-8mvc

Source code

Credits

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