How to remediate – Microsoft SharePoint Exposed Interfaces

1. Introduction

Microsoft SharePoint Exposed Interfaces refers to a misconfiguration in Microsoft SharePoint Server that allows anonymous users access to the SharePoint interface page. This can lead to information disclosure and potential further exploitation of the system. Affected systems are typically those running on-premises deployments of Microsoft SharePoint Server where anonymous access has not been properly restricted. A successful exploit could compromise confidentiality, integrity, and availability of data within the SharePoint environment.

2. Technical Explanation

The vulnerability occurs because of incorrect configuration settings that permit unauthenticated access to SharePoint resources. An attacker does not need credentials to view parts of the interface. The Common Weakness Enumeration (CWE) identifier for this issue is 200, Improper Input Validation. A simple example involves directly accessing a SharePoint URL without authentication and receiving a valid response. Affected versions include those with default or poorly configured anonymous access settings.

• Root cause: Incorrectly configured permissions allowing anonymous access to the SharePoint interface.
• Exploit mechanism: An attacker navigates to a publicly accessible SharePoint URL, bypassing typical authentication checks.
• Scope: On-premises deployments of Microsoft SharePoint Server with misconfigured anonymous access settings.

3. Detection and Assessment

To confirm vulnerability, check the current configuration for anonymous access. A thorough assessment involves reviewing all SharePoint web application permissions.

• Quick checks: Use Central Administration to verify if anonymous access is enabled on any site collections.
• Scanning: Nessus plugin 16279 can identify exposed SharePoint interfaces, but results should be verified manually.
• Logs and evidence: Review IIS logs for requests accessing SharePoint resources without authentication. Look for HTTP status codes 200 (OK) from anonymous IP addresses.

Get-SPWebApplication | Where {$_.AllowsAnonymousAccess -eq $true}

4. Solution / Remediation Steps

The solution involves restricting anonymous access to the SharePoint configuration. This should be done carefully to avoid disrupting legitimate users.

4.1 Preparation

• No services need to be stopped, but plan for potential downtime during testing. A roll back plan involves restoring from backup or reverting configuration changes.
• Changes should be approved by the IT security team and implemented during a scheduled maintenance window.

4.2 Implementation

1. Step 1: Open SharePoint Central Administration.
2. Step 2: Navigate to Application Management > Web Applications.
3. Step 3: Select the web application you want to configure.
4. Step 4: In the ribbon, click Authentication Providers.
5. Step 5: Click Zone and then edit each zone.
6. Step 6: Ensure that “Allow Anonymous Access” is set to ‘No’ for all zones.
7. Step 7: Repeat steps 3-6 for any other affected web applications.

4.3 Config or Code Example

Before

AllowsAnonymousAccess : True

After

AllowsAnonymousAccess : False

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue. Least privilege is key, as it limits the impact of a compromised account. Secure configuration management ensures settings are correct and consistent. Regular patch cadence helps address known vulnerabilities.

• Practice 1: Implement least privilege principles to restrict access only to authorized users.
• Practice 2: Regularly review SharePoint configurations for secure defaults and proper permissions.

4.5 Automation (Optional)

PowerShell can be used to automate the process of disabling anonymous access across multiple web applications. Use with caution, as incorrect scripting could disrupt service.

# Requires SharePoint Online Management Shell module
# Get all web applications and disable anonymous access
Get-SPWebApplication | Where {$_.AllowsAnonymousAccess -eq $true} | Set-SPWebApplication -AllowsAnonymousAccess $false

5. Verification / Validation

Confirm the fix by checking that anonymous access is no longer permitted. Re-test by attempting to access SharePoint resources without authentication. A simple service smoke test involves logging in with a valid user account and accessing key features.

• Post-fix check: Run `Get-SPWebApplication | Where {$_.AllowsAnonymousAccess -eq $true}`. The output should be empty.
• Re-test: Attempt to access the SharePoint URL without credentials; you should receive an authentication prompt.
• Smoke test: Verify that users can log in and access their normal SharePoint resources.
• Monitoring: Monitor IIS logs for failed login attempts from anonymous IP addresses, indicating blocked access.

Get-SPWebApplication | Where {$_.AllowsAnonymousAccess -eq $true}

6. Preventive Measures and Monitoring

Updating security baselines to include restrictions on anonymous access can prevent future occurrences. Incorporate configuration checks into CI/CD pipelines. A regular patch cycle ensures timely application of security updates.

• Baselines: Update your SharePoint security baseline to enforce the restriction of anonymous access.
• Pipelines: Add a check in deployment pipelines to verify that anonymous access is disabled on all web applications.
• Asset and patch process: Implement a quarterly review cycle for SharePoint configurations and apply security patches promptly.

7. Risks, Side Effects, and Roll Back

• Roll back: Step 1: Open SharePoint Central Administration, navigate to Web Applications, select the affected application, and re-enable anonymous access in Authentication Providers.

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 specific CVE is listed in the provided context.
• Product or platform documentation relevant to the fix: https://learn.microsoft.com/en-us/sharepoint/administration/security/manage-anonymous-access