1. Introduction
ASP.NET ViewState is a parameter used by the ASP.NET framework to preserve values and controls between web pages as a user navigates an application. If this viewstate isn’t properly signed, attackers can modify its contents, potentially leading to Remote Code Execution (RCE). This affects websites and applications built using ASP.NET. Successful exploitation could compromise confidentiality, integrity, and availability of the affected system.
2. Technical Explanation
The ViewState stores serialized data in a hidden field on web pages. Without proper MAC validation or encryption, an attacker can tamper with this data. By modifying the viewstate, they might be able to inject malicious code that executes when the page is processed. This vulnerability relies on the lack of protection for the viewstate parameter.
- Root cause: Missing Message Authentication Code (MAC) validation for ViewState parameters.
- Exploit mechanism: An attacker crafts a modified __viewstate parameter and submits it to the server, potentially leading to code execution.
- Scope: ASP.NET applications without viewstate MAC validation enabled.
3. Detection and Assessment
To confirm vulnerability, check if ViewState MAC validation is enabled on your pages. Thorough assessment involves reviewing web application source code for insecure configurations.
- Quick checks: Inspect the HTML source of an ASP.NET page for the presence of `<%@Page EnableViewStateMAC='True' %>‘ or check the `enableViewStateMac` attribute in the web.config file.
- Scanning: Burp Suite and OWASP ZAP can be used to identify potentially vulnerable ViewState parameters during a web application scan. These are examples only, as accurate detection requires understanding of the application logic.
- Logs and evidence: Examine application logs for errors related to viewstate validation failures or unexpected deserialization events.
4. Solution / Remediation Steps
Enable ViewState MAC validation to protect against tampering. Follow these steps carefully to avoid disrupting application functionality.
4.1 Preparation
- A change window may be required depending on your organization’s policies. Approval from a security or application owner might be needed.
4.2 Implementation
- Step 1: Open the web.config file for your ASP.NET application.
- Step 2: Locate the `
` section within the configuration file. If it doesn’t exist, add it. - Step 3: Add or modify the `enableViewStateMac` attribute to be set to ‘true’.
- Step 4: Save the web.config file and restart your website or application service.
4.3 Config or Code Example
Before
<pages enableViewState="true" />After
<pages enableViewState="true" enableViewStateMac="true" />4.4 Security Practices Relevant to This Vulnerability
Several security practices can help prevent this issue and similar vulnerabilities.
- Input validation: Validate all user inputs, including those embedded within ViewState parameters, to prevent malicious data from being processed.
- Secure defaults: Use secure default configurations for ASP.NET applications, enabling features like ViewState MAC validation by default.
4.5 Automation (Optional)
No specific automation script is available for this vulnerability. However, configuration management tools can be used to enforce the `enableViewStateMac=”true”` setting across multiple servers.
5. Verification / Validation
- Post-fix check: Inspect the HTML source of an ASP.NET page again to confirm `<%@Page EnableViewStateMAC='True' %>‘ is present, or verify `enableViewStateMac=”true”` in web.config.
- Re-test: Attempt to modify the __viewstate parameter and submit it to the server. The application should reject the modified request or display an error message indicating a validation failure.
- Monitoring: Monitor application logs for viewstate-related errors or unexpected deserialization events.
6. Preventive Measures and Monitoring
Update security baselines, add checks in CI/CD pipelines, and establish a regular patch review cycle to prevent similar vulnerabilities.
- Baselines: Update your ASP.NET security baseline to include the requirement for ViewState MAC validation enabled by default.
- Pipelines: Integrate SAST tools into your CI/CD pipeline to scan for insecure configurations, such as missing ViewState MAC validation.
- Asset and patch process: Implement a regular review cycle for ASP.NET configuration files to ensure security settings are maintained.
7. Risks, Side Effects, and Roll Back
Enabling ViewState MAC validation may have minor performance impacts. If issues occur, restore the backed-up web.config file.
- Risk or side effect 1: Enabling ViewState MAC validation might introduce a slight performance overhead due to increased processing requirements.
- Roll back: Restore the original web.config file and restart your website or application service.
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 listed for this general vulnerability, but it’s related to deserialization attacks.
- Product or platform documentation relevant to the fix: https://docs.microsoft.com/en-us/dotnet/api/system.web.ui.page.enableviewstatemac?view=netframework-4.8