How to remediate – Microsoft SharePoint User Enumeration

1. Introduction

Microsoft SharePoint User Enumeration is a vulnerability where an attacker can identify valid user accounts on a Microsoft SharePoint Server instance without needing legitimate credentials. This allows attackers to build lists of potential targets for further attacks, such as password spraying or phishing. Affected systems are typically publicly accessible SharePoint servers with misconfigured anonymous access settings. A successful exploit could lead to information disclosure and potentially compromise account confidentiality.

2. Technical Explanation

The vulnerability occurs due to incorrect configuration of Microsoft SharePoint Server allowing anonymous users to query user information. An attacker can send requests to specific endpoints within the server to enumerate accounts. The Common Weakness Enumeration (CWE) identifier for this issue is 200, Improper Input Validation. A simple example involves sending a request to a SharePoint endpoint that lists all users; if anonymous access isn’t restricted, the server will return a list of usernames. This affects versions of Microsoft SharePoint Server where default configurations permit anonymous access to user enumeration functions.

• Root cause: Anonymous access is permitted to user enumeration endpoints within SharePoint Server.
• Exploit mechanism: An attacker sends HTTP requests to SharePoint endpoints designed to return a list of valid usernames. No authentication is required due to the misconfiguration.
• Scope: Microsoft SharePoint Server instances with anonymous access enabled for user enumeration are affected. Specific versions were not identified in the provided context.

3. Detection and Assessment

To confirm vulnerability, check your SharePoint configuration for anonymous access settings. A quick check involves browsing to a SharePoint site anonymously and attempting to identify user accounts via trial-and-error requests. For thorough assessment, use dedicated security scanning tools.

• Quick checks: Attempt to browse the SharePoint site without logging in. Try accessing URLs that might list users (e.g., /_layouts/15/people.aspx). If user information is returned, the system may be vulnerable.
• Scanning: Nessus plugin 16379 and OpenVAS scanner script sharepoint_user_enumeration can identify this vulnerability as examples only.
• Logs and evidence: Examine SharePoint logs for requests originating from anonymous IP addresses attempting to access user information endpoints. Look for patterns of repeated queries against people-related URLs.

curl -k https://yoursharepointsite/_layouts/15/people.aspx

4. Solution / Remediation Steps

To fix this issue, restrict anonymous access to Microsoft SharePoint configuration. This prevents attackers from enumerating user accounts.

4.1 Preparation

• No services need to be stopped for this change. However, plan for a brief outage during testing. A roll back plan involves restoring the backup if issues occur.
• Change windows may be needed depending on service level agreements. Approval from IT security is recommended.

4.2 Implementation

1. Step 1: Open SharePoint Central Administration.
2. Step 2: Navigate to Application Management > Web Applications.
3. Step 3: Select the affected web application.
4. Step 4: In the ribbon, click Authentication Providers.
5. Step 5: Click Zone settings and ensure Anonymous access is disabled for all zones where user enumeration is a concern.
6. Step 6: Save your changes.

4.3 Config or Code Example

Before

Anonymous access enabled for default zone.

After

Anonymous access disabled for default zone.

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue. Least privilege reduces the impact of successful exploitation. Secure defaults ensure systems are configured with minimal permissions by default. Input validation prevents attackers from sending malicious requests. A regular patch cadence ensures timely application of security updates.

• Practice 1: Implement least privilege to limit access to sensitive resources, reducing potential damage if an account is compromised.
• Practice 2: Enforce secure defaults during SharePoint Server installation and configuration.

4.5 Automation (Optional)

Automation scripts are not provided as they depend on the specific SharePoint environment and PowerShell modules used. However, you can use PowerShell to modify web application settings programmatically. Exercise caution when using automation in production environments.

# Example only - adapt for your environment
# Get-SPWebApplication | Where {$_.Name -eq "YourWebApp"} | Set-SPWebConfigProperty -PropertyName AnonymousAccess -Value $false

5. Verification / Validation

• Post-fix check: Attempt to browse the SharePoint site without logging in. You should receive an error message indicating access is denied or be redirected to a login page.
• Re-test: Repeat the initial detection method (trial-and-error requests) and confirm that user information is no longer accessible anonymously.
• Smoke test: Verify users can still log in, create documents, and collaborate on existing files.
• Monitoring: Monitor SharePoint logs for failed access attempts from anonymous IP addresses targeting user enumeration endpoints as an example alert.

curl -k https://yoursharepointsite/_layouts/15/people.aspx # Should return a 403 Forbidden error

6. Preventive Measures and Monitoring

Update security baselines to include restrictions on anonymous access in SharePoint Server configurations for example, using CIS controls or GPO settings. Implement checks in CI/CD pipelines to prevent deployment of misconfigured instances. Establish a regular patch review cycle to ensure timely application of security updates.

• Baselines: Incorporate the restriction of anonymous access into your SharePoint Server security baseline.
• Pipelines: Add configuration validation steps to your CI/CD pipeline to prevent deployment of instances with anonymous access enabled.
• Asset and patch process: Review and apply security patches for SharePoint Server on a regular schedule (e.g., monthly).

7. Risks, Side Effects, and Roll Back

• Roll back: Restore the SharePoint Server configuration from the backup taken prior to making changes.

8. References and Resources

• Vendor advisory or bulletin: https://mindedsecurity.com/wp-content/uploads/2020/10/Fedon_Athcon_June11.pdf
• NVD or CVE entry: No CVE identified in the provided context.
• Product or platform documentation relevant to the fix: