CVE-2025-27601

Executive Summary

CVE-2025-27601 is an improper API access control vulnerability in Umbraco, a free and open-source .NET content management system. The flaw allows low-privilege, authenticated users to create and update data type information that should be restricted to users with access to the settings section. This vulnerability affects Umbraco API management package versions prior to 15.2.3 and 14.3.3. The issue has been patched in the mentioned versions, and no known workarounds are available. The CVSS score is 4.3 (Medium), and the EPSS score is 0.0%, indicating a low probability of exploitation in the wild.

Supply Chain & Package Security

Umbraco is a .NET CMS distributed via NuGet. The vulnerability exists in the API management package, which is a core component of the Umbraco ecosystem. Organizations using Umbraco should ensure they are using the patched versions (14.3.3 or 15.2.3) to mitigate the risk. The lack of a workaround emphasizes the need for immediate patching.

CVSS Breakdown & Threat Level

• CVSS Score: 4.3 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: Low (PR:L)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: Low (C:L)
• Integrity Impact: Low (I:L)
• Availability Impact: None (A:N)

The threat level is medium due to the requirement for authentication and the limited impact on confidentiality and integrity. However, the ability for low-privilege users to modify data type information could lead to further privilege escalation or data manipulation.

Deep Dive & Mechanics

The vulnerability stems from improper access control checks in the Umbraco API management package. Specifically, the API endpoints responsible for creating and updating data type definitions do not properly verify that the requesting user has the necessary permissions (i.e., access to the settings section). This allows authenticated users with low privileges to perform actions that should be restricted.

The flaw is likely due to missing or insufficient authorization checks in the controller or service layer. The API endpoints may rely on client-side checks or fail to validate user roles before processing requests. This can be exploited by sending crafted HTTP requests to the vulnerable endpoints.

Vulnerable vs. Patched Code

The exact code changes are not publicly available, but the patch likely involves adding authorization attributes or checks to the affected API endpoints. For example, in the vulnerable version, the code might look like:

[HttpPost]
public IActionResult CreateDataType(DataTypeModel model)
{
    // Missing authorization check
    var dataType = _dataTypeService.Create(model);
    return Ok(dataType);
}

In the patched version, an authorization check is added:

[HttpPost]
[Authorize(Policy = "SettingsAccess")]
public IActionResult CreateDataType(DataTypeModel model)
{
    var dataType = _dataTypeService.Create(model);
    return Ok(dataType);
}

The Authorize attribute ensures that only users with the SettingsAccess policy can access the endpoint.

Exploitation & Lab Setup

Red Team

To exploit this vulnerability, an attacker needs a valid low-privilege account on the Umbraco CMS. The following steps outline the exploitation process:

1. Authenticate: Obtain a valid session token or credentials for a low-privilege user.
2. Identify Vulnerable Endpoints: Use API documentation or reconnaissance to find endpoints like /umbraco/api/datatype/create or /umbraco/api/datatype/update.
3. Craft Request: Send a POST request to the vulnerable endpoint with the desired data type payload.

Example request using curl:

curl -X POST https://target.com/umbraco/api/datatype/create \
  -H "Authorization: Bearer <low_priv_token>" \
  -H "Content-Type: application/json" \
  -d '{"name": "MaliciousDataType", "propertyEditorAlias": "Umbraco.TextBox", "dataTypeSettings": {}}'

4. Verify: Check if the data type was created or updated without proper authorization.

Blue Team

To detect exploitation attempts, monitor for the following:

• API Access Logs: Look for requests to /umbraco/api/datatype/* from users without settings section access.
• Anomalous Data Type Changes: Monitor for creation or modification of data types by unexpected users.
• Failed Authorization Attempts: While the vulnerability bypasses authorization, any failed attempts may indicate scanning.

Indicators of Compromise (IOCs)

• Network IOCs:
  • POST requests to /umbraco/api/datatype/create or /umbraco/api/datatype/update from low-privilege users.
  • Unusual data type names or property editor aliases in API requests.
• System IOCs:
  • Unexpected data type entries in the Umbraco database.
  • Log entries showing data type modifications by users without settings access.

Detection Engineering

YARA Rule

rule CVE_2025_27601_Umbraco_API_Access_Control {
    meta:
        description = "Detects exploitation attempts for CVE-2025-27601"
        author = "Valters IT"
        date = "2025-03-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-27601"
    strings:
        $s1 = "/umbraco/api/datatype/create" ascii wide nocase
        $s2 = "/umbraco/api/datatype/update" ascii wide nocase
        $s3 = "dataTypeSettings" ascii wide nocase
    condition:
        any of ($s1, $s2) and $s3
}

Sigma Rule

title: CVE-2025-27601 - Umbraco API Access Control Bypass
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects attempts to exploit improper API access control in Umbraco
author: Valters IT
date: 2025-03-01
logsource:
    category: webserver
    product: iis
detection:
    selection:
        cs-uri-stem:
            - '/umbraco/api/datatype/create'
            - '/umbraco/api/datatype/update'
        cs-method: 'POST'
    condition: selection
falsepositives:
    - Legitimate admin actions
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-27601 - Umbraco API Access Control Bypass"; flow:to_server,established; content:"POST"; http_method; content:"/umbraco/api/datatype/create"; http_uri; nocase; content:"/umbraco/api/datatype/update"; http_uri; nocase; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Patching: Upgrade to Umbraco versions 14.3.3 or 15.2.3.
2. Access Control Review: Ensure that all API endpoints have proper authorization checks.
3. Monitoring: Implement logging and monitoring for API access to detect unauthorized attempts.
4. Least Privilege: Review user roles and permissions to ensure minimal access.
5. Web Application Firewall (WAF): Deploy WAF rules to block requests to vulnerable endpoints from unauthorized users.

Valters IT
Securing Digital Frontiers