Host Header Injection | How to Attack the Header of a Request

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In this blog, we will learn about the Host Header Injection Attack, by which we can get the password of a website, so it is important to prevent this attack. We will see how to prevent it and some practicalities related to it.

Table of Contents

Before learning about Host Header Injection or Attack, we must first learn about the Host Header.

Introduction to Host Header

The Host header is an HTTP (Hypertext Transfer Protocol) header that is used to specify the target host or domain name in a request to a server. It plays a crucial role in virtual hosting, where multiple websites or services are hosted on a single server with a single IP address.

When a client makes an HTTP request to a server, it includes the Host header to indicate which specific domain or website it wants to communicate with. This is particularly important in scenarios where a single physical server hosts multiple websites or services, each with its own unique domain name.

The Host header allows the server to distinguish between the different websites or services being requested and route the request to the appropriate virtual host. This enables a server to serve content for multiple domains or websites using a single IP address.

For example, let’s say there is a server with the IP address hosting two websites: and When a client wants to access, it sends an HTTP request with the Host header set to “” Similarly, when accessing, the client sets the Host header to “” The server then uses the information provided in the Host header to determine which web site’s content to serve.

The Host header is part of the HTTP 1.1 specification and is mandatory for all HTTP/1.1 requests. It allows servers to support multiple domains on a single IP address, facilitating efficient resource utilization and reducing the need for dedicated IP addresses for each website.

It’s worth noting that the Host header can be modified by clients, which can lead to security risks such as HTTP host header injection attacks. Server administrators need to validate and sanitize the Host header value to ensure proper handling and prevent potential vulnerabilities.

Overall, the Host header is a fundamental component of the HTTP protocol, enabling virtual hosting and efficient resource allocation on servers with multiple websites or services.

Introduction to Host Header Injection

Host header injection is a security vulnerability that occurs when an attacker can manipulate or inject malicious content into the Host header of an HTTP request. This vulnerability can have serious consequences, allowing attackers to perform various types of attacks, such as server-side request forgery (SSRF), cache poisoning, session hijacking, and cross-site scripting (XSS).

The Host header is an essential part of the HTTP protocol, used to specify the target domain or website in a request. It helps servers distinguish between different virtual hosts hosted on the same server. However, if the server fails to properly validate or sanitize the Host header, an attacker can exploit this vulnerability.

For Example:

They might turn to collecting the domain from the Host header to create an absolute URL that is contained in an email:

<a href="https://_SERVER['HOST']/support">Contact support</a>

The header value may also be used in a variety of interactions between different systems of the website’s infrastructure.

As the Host header is in fact user controllable, this practice can lead to a number of issues. If the input is not properly escaped or validated, the Host header is a potential vector for exploiting a range of other vulnerabilities, most notably:

  • Web cache poisoning
  • Business logic flaws in specific functionality
  • Routing-based SSRF
  • Classic server-side vulnerabilities, such as SQL injection

How do HTTP Host header vulnerabilities arise?

  1. The attacker sends a manipulated HTTP request to a vulnerable web application.
  2. In the Host header, the attacker inserts a malicious domain or an IP address controlled by them, instead of the legitimate domain.
  3. The server, without proper validation, accepts the manipulated Host header and processes the request.
  4. The server may then establish a connection with the malicious domain or IP address specified in the Host header.
  5. The attacker can now launch various attacks leveraging the established connection, such as SSRF to access internal resources, cache poisoning to manipulate the server’s caching mechanism, session hijacking to impersonate users, or even XSS to inject malicious scripts into responses.

Host header injection attacks can have severe consequences, including unauthorized access to sensitive information, data leakage, compromised user sessions, and the potential for further exploitation of the affected system.

How to prevent HTTP Host Header attacks

To prevent HTTP Host header attacks and enhance the security of your web application or server, consider implementing the following preventive measures:

A) Validate and sanitize input:

Implement strict validation and sanitization checks on all user-supplied input, including the Host header. Ensure that it conforms to expected formats and doesn’t contain any unexpected or malicious characters.

B) Whitelist validation:

Maintain a whitelist of allowed domain names or IP addresses that your server should accept in the Host header. Reject any requests that contain unrecognized or unauthorized values.

C) Normalize the Host header:

Normalize the Host header value to prevent ambiguities and inconsistencies. Remove leading/trailing spaces, lowercase the domain names, and ensure consistent formatting.

D) Use canonicalized URLs:

When constructing URLs within your application, use canonicalized URLs that explicitly specify the protocol (HTTP or HTTPS) and the fully qualified domain name (FQDN). Avoid relying solely on user-supplied Host headers to generate URLs.

E) Implement server-side checks:

On the server side, perform additional checks to verify that the requested resource matches the value in the Host header. Cross-validate the Host header value with the actual server configuration or application context.

F) HTTP Strict Transport Security (HSTS):

Enforce HSTS on your web server to ensure that all communication with your website is conducted over a secure, encrypted connection (HTTPS). HSTS helps mitigate the risk of SSL-stripping attacks.

G) Content Security Policy (CSP):

Implement a strong CSP to control the sources of content that your web application can load. This helps prevent the execution of malicious scripts injected through the Host header or other vectors.

H) Web Application Firewall (WAF):

Utilize a WAF that includes rules specifically designed to detect and block Host header injection attacks. WAFs can provide an additional layer of protection by analyzing traffic and applying security rules.

I) Regular security updates:

Keep your web server, application framework, and any associated libraries up to date with the latest security patches. Vulnerabilities in these components could be exploited to bypass Host header security measures.

J) Security testing and code reviews:

Perform regular security testing, including vulnerability assessments and penetration testing, to identify and address any weaknesses in your application’s handling of the Host header. Conduct code reviews to ensure secure coding practices.

By implementing these preventive measures, you can significantly reduce the risk of Host header attacks and enhance the overall security of your web application or server.

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