ASP.NET’s “Validation of viewstate MAC failed” error can interrupt your web applications. In this guide, you’ll learn how to resolve it—and if server-side issues or misconfigurations persist, our expert team at MacRepair (yes, even ASP.NET site support!) can help, typically within 30 minutes. Call 0800 585 222 or Visit Us – MacRepair Store for fast help.
What Causes the ViewState MAC Error?
ASP.NET uses a message authentication code (MAC)—based on your <machineKey>—to protect the hidden __VIEWSTATE. When the returned __VIEWSTATE can’t be validated, you’ll see this error. Common causes include:
- Web farms or clustered servers with inconsistent <machineKey> configurations
- Auto-generated machineKey regenerates on restart, invalidating old MACs
- Disable user profile in IIS, leading to inconsistent HKCU key usage
- ViewStateUserKey mismatch (e.g. multi-tab login), causing MAC validation failure
Step 1: Add a Static <machineKey> in Web.config
In web-farm or IIS environments, set a consistent <machineKey>:
<system.web>
<machineKey
validationKey=”YOUR_GENERATED_VALIDATION_KEY”
decryptionKey=”YOUR_GENERATED_DECRYPTION_KEY”
validation=”HMACSHA256″ />
</system.web>
- Generate keys using IIS GUI, PowerShell, or aspnet_regiis
- Avoid unreliable online key generators
- Deploy the same config to all server instances
This ensures all servers validate ViewState with the same secret.
Step 2: Enable LoadUserProfile in IIS App Pool
If your pool is running with LoadUserProfile=”false”, ViewState MAC may fail due to lack of HKCU registry access. Correct this:
- Open IIS Manager
- Select the relevant Application Pool
- Edit Advanced Settings
- Set LoadUserProfile = True
Then restart the pool
Step 3: Debugging with enableViewStateMac
To confirm if MAC is the cause:
Temporarily set
- <pages enableViewStateMac=”false” />
- Reproduce the error. If it’s gone, MAC validation is the culprit
- Turn enableViewStateMac=”true” afterward. Do not leave it off, as it weakens security
Step 4: Check ViewStateUserKey Usage
If you’re setting Page.ViewStateUserKey, ensure consistency across postbacks:
- Don’t rely on user info that might have changed (e.g., session or login state)
- If your app logs users in a new tab with a different identity, validation may fail
Ensure your logic is consistent before and after postback.
Step 5: Detect Browser or Cache Issues
Sometimes individual browser behavior can trigger the error. Example: Document Mode toggling in IE fixed random MAC failures
Suggest users clear cache, try a different browser, or test using incognito mode.
Step 6: Avoid Removing ViewState Unnecessarily
Some forums suggest workarounds:
- enableViewStateMac=”false”
- viewStateEncryptionMode=”Never”
- Meta tag hacks
These are not recommended. They introduce security vulnerabilities and should only be used briefly for troubleshooting.
Step 7: Alternate Persistence Methods
In rare cases, storing ViewState in Session or a database rather than in a hidden field can avoid truncation issues:
// Example from Microsoft Learn:
protected override object LoadPageStateFromPersistenceMedium()
=> Session[“_ViewState”];
protected override void SavePageStateToPersistenceMedium(object state)
=> Session[“_ViewState”] = state;
This approach reduces ViewState corruption but adds server memory overhead .
Step 8: Thorough Testing and Deployment
After making changes:
- Restart IIS and all servers.
- Test all forms/apps across multiple browsers.
- Test handcrafted negative scenarios.
Using unit tests to verify <machineKey> presence in config can catch errors early .
Why MacRepair? Expert Help in 30 Minutes
If you’re still stuck, our team at MacRepair offers fast, expert support:
- Configuration review—<machineKey>, App Pool profiles, browser tests
- Secure deployments to all servers
- 30-minute service guarantee—or it’s free
- Call us: 1800-585-222
- Visit Us – MacRepair Store for hands-on assistance
Quick Reference Table
| Step | Action | Reason |
| 1 | Add fixed <machineKey> | Ensures same ViewState validation across instances |
| 2 | Enable LoadUserProfile | Allows HKCU-based MAC persistence |
| 3 | Test with enableViewStateMac=false | Confirms MAC is the issue |
| 4 | Check ViewStateUserKey usage | Ensures key consistency |
| 5 | Clear browser cache or change settings | Fixes client-side glitches |
| 6 | Avoid disabling ViewState encryption | Maintains essential security |
| 7 | Use alternate persistence if needed | Prevents ViewState truncation issues |
| 8 | Full reconciliation & testing | Verifies fix across environments |
Final Thoughts
The “Validation of viewstate MAC failed” error is a sign of mismatched cryptographic keys, postback inconsistencies, or environment misconfiguration. Most cases can be resolved by synchronizing <machineKey>, enabling proper IIS settings, and testing thoroughly.
If configuration complexities or persistent errors are blocking production uptime, reach out to MacRepair. Our team is ready to fix your issue—typically within 30 minutes at our store, with expert troubleshooting and guidance.
