HTTP/0.9 Security Risks

TL;DR

Yes, accepting HTTP/0.9 requests can introduce significant cyber security risks due to its simplicity and lack of modern security features. It’s strongly recommended to disable support for it unless absolutely necessary, and if you must support it, implement strict input validation and sanitisation.

What is HTTP/0.9?

HTTP/0.9 was the very first version of the Hypertext Transfer Protocol. It’s incredibly basic: a single line request followed by an empty line, then the response body. Crucially, it lacks headers, methods beyond GET, and any built-in security features like content negotiation or persistent connections.

Why is HTTP/0.9 insecure?

The simplicity of HTTP/0.9 opens up several attack vectors:

1. Request Smuggling: Because there’s no header parsing, attackers can potentially craft malicious requests that are misinterpreted by the server or proxy servers.
2. Lack of Method Support: Only GET is supported. This limits functionality but also means it’s harder to detect unusual activity compared to a full HTTP/1.1 implementation.
3. No Content Length: The absence of a Content-Length header makes it difficult for the server to determine where the request body ends, leading to potential buffer overflows or denial-of-service attacks if an attacker sends a very long request without a clear termination signal.
4. Cache Poisoning: Without proper caching controls (which HTTP/0.9 doesn’t have), attackers might be able to poison caches with malicious content.
5. Cross-Site Scripting (XSS): While not unique to 0.9, the lack of input validation makes XSS attacks easier to execute if the server blindly outputs request data in responses.

How to Mitigate Risks

Here’s a step-by-step guide to reducing the risks associated with accepting HTTP/0.9 requests:

1. Disable Support: This is the best option. Most modern web servers can be configured to reject HTTP/0.9 requests entirely.
• Apache: In your server configuration file (e.g., httpd.conf or apache2.conf), ensure you don’t have any configurations explicitly enabling HTTP/0.9 support. If using a module that handles older protocols, disable it.
• Nginx: Nginx generally doesn’t support HTTP/0.9 by default and requires no specific configuration to block it. However, verify your proxy settings aren’t inadvertently forwarding 0.9 requests.
• IIS: In IIS Manager, check the protocol bindings for your website. Ensure only HTTP/1.0 or higher are enabled.
2. Strict Input Validation (If you *must* support it): If disabling isn’t possible, implement rigorous input validation on all data received in the request line.
• Character Restrictions: Allow only a limited set of characters in the URL. Disallow control characters and potentially dangerous symbols.
• URL Length Limits: Enforce a maximum length for the requested URL to prevent excessively long requests.
• Whitespace Handling: Carefully handle whitespace within the request line. Unexpected whitespace could be exploited.
3. Sanitise Output: Always sanitise any data received from the HTTP/0.9 request before including it in responses to prevent XSS attacks. Use appropriate encoding functions for your chosen output format (e.g., HTML escaping).
4. Logging and Monitoring: Log all HTTP/0.9 requests with detailed information, including the client IP address, requested URL, and timestamp. Monitor these logs for suspicious activity.
5. Web Application Firewall (WAF): Deploy a WAF to detect and block malicious HTTP/0.9 requests based on predefined rules or custom signatures.

Example Input Validation (Python)

This is a simplified example; real-world validation should be more comprehensive.

import re

def validate_http09_request(request_line):
  # Allow only alphanumeric characters, slashes, and dots in the URL.
  pattern = r'^[a-zA-Z0-9/.]+$'
  if not re.match(pattern, request_line):
    return False
  if len(request_line) > 256: # Limit URL length
    return False
  return True

# Example usage:
request = "/index.html"
if validate_http09_request(request):
  print("Valid request")
else:
  print("Invalid request")

Conclusion

Supporting HTTP/0.9 introduces unnecessary cyber security risks in most modern environments. Disabling support is the recommended approach. If you absolutely must support it, implement robust input validation and sanitisation measures to protect your application from potential attacks.