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HTTP Host Header Poisoning

Description

The HTTP Host specifies the domain name the HTTP Client would like to access. It is mandatory as part of the HTTP/1.1 standard.

For instance, to access the domain www.ostorlab.co, the HTTP client would send the following request with the Host header:

http request GET / HTTP/1.1 Host: www.ostorlab.co

The Host header is important to enabling routing traffic to virtual hosts.

Applications that handle the Host header insecurely are vulnerable to multiple classes of vulnerabilities, like:

  • Server-side request forgery
  • Web Cache poisoning
  • Insecure redirects

Host header poisoning can materialize in different ways:

  • Arbitrary Host header reflection
  • Duplicate Host headers injection
  • Absolute URL injection and ignoring the Host header value
  • Header injection by adding a line wrapper
  • Injection of common Host override-headers, like X-Host, X-Forwarded-Server, X-HTTP-Host-Override

Recommendation

Protection against Host header attacks will require multiple checks that depend on the application target architecture, like support for a virtual host, use of a reverse proxy, and presence in certain cloud environments, the support extra routing headers.

The recommendations to protect against these attacks are:

  • Avoid using the Host header value in application logic.
  • Implement a whitelist check of accepted values; most web frameworks commonly support this.
  • Disable host override headers; this depends on the intermediary components deployed in your architecture. Common places to check are reverse-proxies and Kubernetes ingress controllers.

Standards

  • PCI_STANDARDS:
    • REQ_6_2
    • REQ_6_3
    • REQ_6_4
    • REQ_11_3
  • HIPAA_CONTROLS:
    • SECURITY221
    • SECURITY212
    • SECURITY213