How to remediate – ASP.NET ViewState Not Encrypted

1. Introduction

The ASP.NET ViewState Not Encrypted vulnerability occurs when the ViewState parameter in an ASP.NET application is not encrypted. The ViewState stores data between web pages to maintain state, and without encryption, this data can be intercepted and potentially reveal sensitive information about users. This affects applications using the ASP.NET framework where ViewState encryption isn’t enabled. A successful exploit could lead to information disclosure impacting confidentiality.

2. Technical Explanation

The vulnerability stems from a lack of encryption on the ViewState parameter, which is serialized and base64 encoded but not protected against tampering or eavesdropping. An attacker can intercept this data in transit and decode it to potentially retrieve sensitive user information. Exploitation requires network access to the application traffic.

• Exploit mechanism: An attacker intercepts HTTP traffic containing the __viewstate parameter, decodes it from base64, and analyzes the data for sensitive information like user IDs or session tokens.
• Scope: ASP.NET web applications that have not configured ViewState encryption are affected.

3. Detection and Assessment

To confirm vulnerability, check if the __viewstate parameter is present in HTTP responses and whether it appears to be encrypted. A thorough assessment involves examining application configuration files for ViewState encryption settings.

• Quick checks: Inspect network traffic using a browser developer tool or proxy (e.g., Burp Suite) for the presence of the __viewstate parameter in POST requests and responses.
• Scanning: Nessus plugin 10489 can identify missing ViewState encryption, but results should be verified manually.
• Logs and evidence: Examine application logs for any errors related to ViewState decryption or integrity checks.

# No specific command available - check network traffic for unencrypted __viewstate parameters

4. Solution / Remediation Steps

4.1 Preparation

• Change windows are typically not needed for this change unless it’s part of a larger deployment cycle. Approval may be needed depending on your organization’s policies.

4.2 Implementation

1. Step 1: Open your web.config file in a text editor.
2. Step 2: Add the following line inside the section: <pages viewStateEncryptionMode=”Always”>. Alternatively, add <%@Page ViewStateEncryptionMode=’Always’ %> to each page where you want encryption enabled.
3. Step 3: Save the web.config file.
4. Step 4: Restart your application or recycle the application pool in IIS for the changes to take effect.

4.3 Config or Code Example

Before

<system.web>
  ...
</system.web>

After

<system.web>
  <pages viewStateEncryptionMode="Always">
    ...
  </pages>
</system.web>

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue. Least privilege limits the impact of compromised data, while input validation prevents malicious data from being stored in the ViewState. Secure configuration management ensures consistent and secure settings across your application.

• Practice 1: Implement least privilege to minimize the potential damage if an attacker gains access to sensitive information within the ViewState.
• Practice 2: Regularly review and update security configurations, including ViewState encryption settings, to ensure they align with current best practices.

4.5 Automation (Optional)

No specific automation script is provided as this change is typically managed through configuration updates. However, you can use PowerShell or similar scripting tools to automate the modification of web.config files across multiple servers.

# No script available - consider using a config management tool like Ansible or Chef

5. Verification / Validation

Confirm the fix by inspecting network traffic again and verifying that the __viewstate parameter is now encrypted (appears as random characters). Re-test to ensure the issue has been resolved, and perform a smoke test to confirm application functionality remains intact.

• Re-test: Repeat the quick check from step 3, confirming that the __viewstate parameter is encrypted.
• Smoke test: Verify key user actions such as logging in, submitting forms, and navigating between pages to ensure they function correctly.
• Monitoring: Monitor application logs for any errors related to ViewState encryption or decryption failures.

# No specific command available - verify __viewstate parameter is unreadable after base64 decoding

6. Preventive Measures and Monitoring

Update security baselines to include ViewState encryption as a mandatory setting. Incorporate checks in your CI/CD pipelines to ensure that all new deployments have ViewState encryption enabled. Implement a regular patch review cycle to address any potential vulnerabilities related to ASP.NET framework components.

• Baselines: Update security baselines or policies to require ViewState encryption for all ASP.NET applications.
• Pipelines: Add checks in your CI/CD pipeline to verify that the web.config file contains the <pages viewStateEncryptionMode=”Always”> setting.
• Asset and patch process: Review and apply security patches regularly to address any vulnerabilities related to ASP.NET framework components.

7. Risks, Side Effects, and Roll Back

Enabling ViewState encryption may introduce a slight performance overhead due to the additional cryptographic operations. If issues arise, roll back by removing the <pages viewStateEncryptionMode=”Always”> setting from your web.config file and restarting the application.

• Roll back: Remove <pages viewStateEncryptionMode=”Always”> from web.config and restart the application pool in IIS.

8. References and Resources

• Vendor advisory or bulletin: https://devblogs.microsoft.com/aspnet/cryptographic-improvements-in-asp-net-4-5-pt-2/
• NVD or CVE entry: No specific CVE is associated with this configuration issue, but it’s a common security misconfiguration.
• Product or platform documentation relevant to the fix: Updated on October 26, 2025