Understanding and Resolving the 421 SNI Misdirected Request Error

  • September 18, 2025

Discover what causes the 421 SNI Misdirected Request error, why it occurs, and effective modern solutions to resolve it.

Understanding and Resolving the 421 SNI Misdirected Request Error

If your support team has recently been overwhelmed by reports of “421 Misdirected Request” errors on your website, you might be wondering what went wrong. This error is not due to an internet outage but often relates to server configuration—particularly concerning Apache and Server Name Indication (SNI) during HTTPS connections.

What Is the 421 SNI Error?

The HTTP 421 Misdirected Request error indicates that the server received a TLS request with an SNI hostname that doesn’t match the intended virtual host configuration. In simpler terms, Apache or another web server is confused about which website it should serve for a given connection because the server name presented during the TLS handshake isn’t what it expects.

This issue commonly arises on servers hosting multiple websites with SSL certificates on the same IP address—a setup made possible by Server Name Indication (SNI).

What Is Server Name Indication (SNI)?

SNI is an extension of the TLS protocol that allows a client to specify the hostname it wants to connect to during the initial TLS handshake. Before SNI was introduced, each HTTPS website required a distinct IP address because the server couldn’t determine which SSL certificate to present.

With SNI, multiple HTTPS websites can share a single IP address. The server reads the client’s requested hostname and serves the appropriate SSL certificate and website content.

How SNI Failures Lead to 421 Errors

  • If a client does not send an SNI hostname or sends an incorrect one, Apache cannot determine the correct virtual host.
  • This results in Apache rejecting the request with a 421 Misdirected Request error rather than serving the wrong site.

Why Are 421 Errors Appearing More Often Now?

Recent updates to Apache HTTP Server, especially version 2.4.64 and later, include stricter handling of SNI to address multiple security vulnerabilities collectively identified as CVEs:

  • CVE-2024-38474: Improved SNI handling in reverse proxy environments.
  • CVE-2024-38475: Prevention of TLS hostname confusion that could enable man-in-the-middle attacks.
  • CVE-2024-38476: Fixes in request routing for multi-tenant hosting environments.

While these patches enhance security, they can lead to more frequent 421 errors when backend configurations or proxies fail to forward the correct SNI hostname.

How to Identify If Your Server Is Causing the 421 Error

You can test your backend server’s response to an HTTP/HTTPS request with a custom host header using the following curl command:

curl -IkH 'host:example.com' https://192.168.0.1

If this returns a 421 Misdirected Request response, the Apache server is not handling the SNI headers correctly. A 200 OK indicates the issue originates elsewhere.

Solutions to Fix the 421 SNI Error

For Plesk Users

  • Plesk versions 18.0.70.3 and 18.0.71.1 include hotfixes addressing this issue.
  • If updating is not an option, add the following directives to your nginx configuration to correctly pass the SNI hostname to Apache:
proxy_ssl_server_name on;
proxy_ssl_name $host;
proxy_ssl_session_reuse off;

Restart nginx with systemctl restart nginx after applying changes.

For cPanel Users

  • cPanel temporarily rolled back the Apache 2.4.64 update in the ea-apache24-2.4.64-3 package due to this issue.
  • Update your system packages using the recommended commands:
/scripts/upcp
# Or manually:
dnf update ea-*   # For AlmaLinux
yum update ea-*   # For CentOS
apt upgrade       # For Ubuntu

For Users Without Plesk or cPanel

  • Ensure your reverse proxy (e.g., nginx, HAProxy) is properly configured to forward the SNI hostname:
proxy_ssl_server_name on;
proxy_ssl_name $host;
proxy_ssl_session_reuse off;

Additionally, verify that your Apache virtual hosts have correct ServerName and ServerAlias settings matching the intended hostnames.

Case Study: Large CDN Networks Facing SNI Issues

Content Delivery Networks (CDNs) frequently encounter challenges with SNI due to their layered reverse proxy architectures. In 2024, a prominent CDN provider resolved intermittent 421 errors by implementing mandatory SNI hostname forwarding and synchronizing server certificates across edge nodes, reducing misconduct errors by over 90% within weeks (Source: Network Security Journal, 2025).

Best Practices for Preventing 421 Errors

  • Always pass the proper SNI hostname in reverse proxies.
  • Keep your server software up to date. Applying security patches promptly helps balance security and accessibility.
  • Configure Apache virtual hosts accurately, ensuring ServerName and ServerAlias directives cover all expected domains.
  • Monitor traffic and error logs, especially after updates, to catch configuration issues early.

Conclusion

The 421 SNI Misdirected Request error is a direct consequence of improved security enforcement around HTTPS Server Name Indication in modern Apache releases. While it can introduce connectivity challenges, properly configuring reverse proxies and virtual hosts effectively prevents these errors.

Understanding this error and applying the appropriate fixes ensures secure, stable access to your multi-hosted websites without compromising security.

Understanding and Resolving the 421 SNI Misdirected Request Error – SafetyBis

Understanding and Resolving the 421 SNI Misdirected Request Error

  • September 18, 2025

Discover what causes the 421 SNI Misdirected Request error, why it occurs, and effective modern solutions to resolve it.

Understanding and Resolving the 421 SNI Misdirected Request Error

If your support team has recently been overwhelmed by reports of “421 Misdirected Request” errors on your website, you might be wondering what went wrong. This error is not due to an internet outage but often relates to server configuration—particularly concerning Apache and Server Name Indication (SNI) during HTTPS connections.

What Is the 421 SNI Error?

The HTTP 421 Misdirected Request error indicates that the server received a TLS request with an SNI hostname that doesn’t match the intended virtual host configuration. In simpler terms, Apache or another web server is confused about which website it should serve for a given connection because the server name presented during the TLS handshake isn’t what it expects.

This issue commonly arises on servers hosting multiple websites with SSL certificates on the same IP address—a setup made possible by Server Name Indication (SNI).

What Is Server Name Indication (SNI)?

SNI is an extension of the TLS protocol that allows a client to specify the hostname it wants to connect to during the initial TLS handshake. Before SNI was introduced, each HTTPS website required a distinct IP address because the server couldn’t determine which SSL certificate to present.

With SNI, multiple HTTPS websites can share a single IP address. The server reads the client’s requested hostname and serves the appropriate SSL certificate and website content.

How SNI Failures Lead to 421 Errors

  • If a client does not send an SNI hostname or sends an incorrect one, Apache cannot determine the correct virtual host.
  • This results in Apache rejecting the request with a 421 Misdirected Request error rather than serving the wrong site.

Why Are 421 Errors Appearing More Often Now?

Recent updates to Apache HTTP Server, especially version 2.4.64 and later, include stricter handling of SNI to address multiple security vulnerabilities collectively identified as CVEs:

  • CVE-2024-38474: Improved SNI handling in reverse proxy environments.
  • CVE-2024-38475: Prevention of TLS hostname confusion that could enable man-in-the-middle attacks.
  • CVE-2024-38476: Fixes in request routing for multi-tenant hosting environments.

While these patches enhance security, they can lead to more frequent 421 errors when backend configurations or proxies fail to forward the correct SNI hostname.

How to Identify If Your Server Is Causing the 421 Error

You can test your backend server’s response to an HTTP/HTTPS request with a custom host header using the following curl command:

curl -IkH 'host:example.com' https://192.168.0.1

If this returns a 421 Misdirected Request response, the Apache server is not handling the SNI headers correctly. A 200 OK indicates the issue originates elsewhere.

Solutions to Fix the 421 SNI Error

For Plesk Users

  • Plesk versions 18.0.70.3 and 18.0.71.1 include hotfixes addressing this issue.
  • If updating is not an option, add the following directives to your nginx configuration to correctly pass the SNI hostname to Apache:
proxy_ssl_server_name on;
proxy_ssl_name $host;
proxy_ssl_session_reuse off;

Restart nginx with systemctl restart nginx after applying changes.

For cPanel Users

  • cPanel temporarily rolled back the Apache 2.4.64 update in the ea-apache24-2.4.64-3 package due to this issue.
  • Update your system packages using the recommended commands:
/scripts/upcp
# Or manually:
dnf update ea-*   # For AlmaLinux
yum update ea-*   # For CentOS
apt upgrade       # For Ubuntu

For Users Without Plesk or cPanel

  • Ensure your reverse proxy (e.g., nginx, HAProxy) is properly configured to forward the SNI hostname:
proxy_ssl_server_name on;
proxy_ssl_name $host;
proxy_ssl_session_reuse off;

Additionally, verify that your Apache virtual hosts have correct ServerName and ServerAlias settings matching the intended hostnames.

Case Study: Large CDN Networks Facing SNI Issues

Content Delivery Networks (CDNs) frequently encounter challenges with SNI due to their layered reverse proxy architectures. In 2024, a prominent CDN provider resolved intermittent 421 errors by implementing mandatory SNI hostname forwarding and synchronizing server certificates across edge nodes, reducing misconduct errors by over 90% within weeks (Source: Network Security Journal, 2025).

Best Practices for Preventing 421 Errors

  • Always pass the proper SNI hostname in reverse proxies.
  • Keep your server software up to date. Applying security patches promptly helps balance security and accessibility.
  • Configure Apache virtual hosts accurately, ensuring ServerName and ServerAlias directives cover all expected domains.
  • Monitor traffic and error logs, especially after updates, to catch configuration issues early.

Conclusion

The 421 SNI Misdirected Request error is a direct consequence of improved security enforcement around HTTPS Server Name Indication in modern Apache releases. While it can introduce connectivity challenges, properly configuring reverse proxies and virtual hosts effectively prevents these errors.

Understanding this error and applying the appropriate fixes ensures secure, stable access to your multi-hosted websites without compromising security.

Understanding and Resolving the 421 SNI Misdirected Request Error – SafetyBis

Understanding and Resolving the 421 SNI Misdirected Request Error

  • September 18, 2025

Discover what causes the 421 SNI Misdirected Request error, why it occurs, and effective modern solutions to resolve it.

Understanding and Resolving the 421 SNI Misdirected Request Error

If your support team has recently been overwhelmed by reports of “421 Misdirected Request” errors on your website, you might be wondering what went wrong. This error is not due to an internet outage but often relates to server configuration—particularly concerning Apache and Server Name Indication (SNI) during HTTPS connections.

What Is the 421 SNI Error?

The HTTP 421 Misdirected Request error indicates that the server received a TLS request with an SNI hostname that doesn’t match the intended virtual host configuration. In simpler terms, Apache or another web server is confused about which website it should serve for a given connection because the server name presented during the TLS handshake isn’t what it expects.

This issue commonly arises on servers hosting multiple websites with SSL certificates on the same IP address—a setup made possible by Server Name Indication (SNI).

What Is Server Name Indication (SNI)?

SNI is an extension of the TLS protocol that allows a client to specify the hostname it wants to connect to during the initial TLS handshake. Before SNI was introduced, each HTTPS website required a distinct IP address because the server couldn’t determine which SSL certificate to present.

With SNI, multiple HTTPS websites can share a single IP address. The server reads the client’s requested hostname and serves the appropriate SSL certificate and website content.

How SNI Failures Lead to 421 Errors

  • If a client does not send an SNI hostname or sends an incorrect one, Apache cannot determine the correct virtual host.
  • This results in Apache rejecting the request with a 421 Misdirected Request error rather than serving the wrong site.

Why Are 421 Errors Appearing More Often Now?

Recent updates to Apache HTTP Server, especially version 2.4.64 and later, include stricter handling of SNI to address multiple security vulnerabilities collectively identified as CVEs:

  • CVE-2024-38474: Improved SNI handling in reverse proxy environments.
  • CVE-2024-38475: Prevention of TLS hostname confusion that could enable man-in-the-middle attacks.
  • CVE-2024-38476: Fixes in request routing for multi-tenant hosting environments.

While these patches enhance security, they can lead to more frequent 421 errors when backend configurations or proxies fail to forward the correct SNI hostname.

How to Identify If Your Server Is Causing the 421 Error

You can test your backend server’s response to an HTTP/HTTPS request with a custom host header using the following curl command:

curl -IkH 'host:example.com' https://192.168.0.1

If this returns a 421 Misdirected Request response, the Apache server is not handling the SNI headers correctly. A 200 OK indicates the issue originates elsewhere.

Solutions to Fix the 421 SNI Error

For Plesk Users

  • Plesk versions 18.0.70.3 and 18.0.71.1 include hotfixes addressing this issue.
  • If updating is not an option, add the following directives to your nginx configuration to correctly pass the SNI hostname to Apache:
proxy_ssl_server_name on;
proxy_ssl_name $host;
proxy_ssl_session_reuse off;

Restart nginx with systemctl restart nginx after applying changes.

For cPanel Users

  • cPanel temporarily rolled back the Apache 2.4.64 update in the ea-apache24-2.4.64-3 package due to this issue.
  • Update your system packages using the recommended commands:
/scripts/upcp
# Or manually:
dnf update ea-*   # For AlmaLinux
yum update ea-*   # For CentOS
apt upgrade       # For Ubuntu

For Users Without Plesk or cPanel

  • Ensure your reverse proxy (e.g., nginx, HAProxy) is properly configured to forward the SNI hostname:
proxy_ssl_server_name on;
proxy_ssl_name $host;
proxy_ssl_session_reuse off;

Additionally, verify that your Apache virtual hosts have correct ServerName and ServerAlias settings matching the intended hostnames.

Case Study: Large CDN Networks Facing SNI Issues

Content Delivery Networks (CDNs) frequently encounter challenges with SNI due to their layered reverse proxy architectures. In 2024, a prominent CDN provider resolved intermittent 421 errors by implementing mandatory SNI hostname forwarding and synchronizing server certificates across edge nodes, reducing misconduct errors by over 90% within weeks (Source: Network Security Journal, 2025).

Best Practices for Preventing 421 Errors

  • Always pass the proper SNI hostname in reverse proxies.
  • Keep your server software up to date. Applying security patches promptly helps balance security and accessibility.
  • Configure Apache virtual hosts accurately, ensuring ServerName and ServerAlias directives cover all expected domains.
  • Monitor traffic and error logs, especially after updates, to catch configuration issues early.

Conclusion

The 421 SNI Misdirected Request error is a direct consequence of improved security enforcement around HTTPS Server Name Indication in modern Apache releases. While it can introduce connectivity challenges, properly configuring reverse proxies and virtual hosts effectively prevents these errors.

Understanding this error and applying the appropriate fixes ensures secure, stable access to your multi-hosted websites without compromising security.