What is HTTP Request Smuggling?

Introduction

HTTP Request Smuggling is a critical security vulnerability that can cause significant harm to organizations and their customers if not properly addressed. This type of attack targets the underlying structure of HTTP requests and manipulates it in a way that enables the attacker to bypass security measures and gain unauthorized access to sensitive information. The attack involves crafting multiple HTTP requests in a specific way that confuses the target server and causes it to process the requests unexpectedly.

What is HTTP Request Smuggling?

HTTP Request Smuggling is a security vulnerability that affects web applications and poses a significant threat to organizations and their customers. This attack is carried out by manipulating the underlying structure of HTTP requests, confusing the way that the target server processes the requests. As a result, the attacker can bypass security measures and gain unauthorized access to sensitive information.

One of the key challenges with HTTP Request Smuggling is its difficulty to detect and prevent. Traditional security measures, such as firewalls and intrusion detection systems, may not be equipped to handle this type of attack. Furthermore, the attack takes place at the protocol level, which can make it difficult to detect using conventional security tools. Additionally, the complexity of HTTP Request Smuggling can make it challenging for organizations to identify the attack in real-time, increasing the risk of data breaches and other security incidents.

To minimize the risks associated with HTTP Request Smuggling, organizations must invest in robust security tools and conduct regular security assessments. Proper configuration and maintenance of web applications are also critical in preventing this type of attack. By taking these proactive measures, organizations can better protect their systems and sensitive information from HTTP Request Smuggling and its potential consequences. Organizations need to understand the risks associated with this type of attack and take steps to prevent it, to maintain the security and privacy of their systems and customers.

Types of HTTP Request Smuggling

HTTP Request Smuggling is a security vulnerability that affects web applications and has several types. These types include:

Cloning Attack

In a cloning attack, the attacker creates two identical requests, with one of them being smuggled into the target server. The purpose of this attack is to trick the server into processing two separate requests as one, which can result in the server processing the smuggled request unexpectedly. This can have serious security implications such as session hijacking, where the attacker can gain unauthorized access to a user's session, or bypassing authentication mechanisms, where the attacker can access protected resources without proper authentication.

The cloning attack works by manipulating the way the server processes requests. For example, the attacker may create two requests with different content, but with identical headers and attributes that make them appear to the server as if they are the same request. The attacker then sends these two requests to the server, with the second request being smuggled into the target server by exploiting a vulnerability in the server's request processing logic. The result of this attack is that the server processes both requests as if they were one, potentially allowing the attacker to bypass security controls and gain access to sensitive information or resources.

For example, the attacker may smuggle a request with a malicious payload into the target server, tricking it into processing the request in an unintended way, such as bypassing authentication mechanisms or session hijacking.

Transfer-Encoding Attack

In a transfer-encoding attack, the attacker manipulates the transfer encoding of a request to confuse the way that the target server processes the request. The transfer encoding of a request is responsible for specifying how the data contained within the request should be encoded and transmitted. By manipulating the transfer encoding, the attacker can trick the server into processing the request unexpectedly, potentially leading to a security vulnerability such as cache poisoning, where the attacker can inject malicious data into the server's cache, or bypass security controls, where the attacker can access protected resources without proper authentication.

The transfer-encoding attack works by exploiting a vulnerability in the way the server processes transfer-encoding headers. For example, the attacker may create a request with a transfer-encoding header that contains a malformed value, causing the server to process the request unexpectedly. The attacker may also create a request with a transfer-encoding header that is not properly handled by the server, potentially leading to the server processing the smuggled request in an unintended way.

For example, the attacker may alter the transfer encoding of a request to cause the server to misinterpret the length of the request, leading to cache poisoning or bypassing security controls.

Host Header Attack

In a host header attack, the attacker manipulates the host header of a request to confuse it in the way that the target server processes the request. The host header of a request is responsible for specifying the target host that the request is intended for. By manipulating the host header, the attacker can trick the server into processing the request unexpectedly, potentially leading to a security vulnerability such as server-side request forgery, where the attacker can make the server issue unauthorized requests on behalf of the attacker, or bypass access controls, where the attacker can access protected resources without proper authentication.

The host header attack works by exploiting a vulnerability in the way the server processes host headers. For example, the attacker may create a request with a host header that contains a malformed value, causing the server to process the request unexpectedly. The attacker may also create a request with a host header that points to a different target host, potentially leading to the server processing the smuggled request in an unintended way.

For example, the attacker may alter the host header of a request to cause the server to send the request to the wrong website, potentially leading to server-side request forgery or bypassing access controls.

Content-Length Attack

In a content-length attack, the attacker manipulates the content length of a request to cause confusion in the way that the target server processes the request. The content length of a request is responsible for specifying the size of the data contained within the request. By manipulating the content length, the attacker can trick the server into processing the request unexpectedly, potentially leading to a security vulnerability such as HTTP response splitting, where the attacker can inject malicious data into the server's response, or bypass input validation controls, where the attacker can submit malicious data to the server without being detected.

The content length attack works by exploiting a vulnerability in the way the server processes content length headers. For example, the attacker may create a request with a content-length header that contains a malformed value, causing the server to process the request unexpectedly. The attacker may also create a request with a content-length header that is not properly handled by the server, potentially leading to the server processing the smuggled request in an unintended way.

For example, the attacker may alter the content length of a request to cause the server to misinterpret the length of the request, potentially leading to HTTP response splitting or bypassing input validation controls.

How does HTTP Request Smuggling Work?

The process of HTTP Request Smuggling can be broken down into several steps, each of which plays a critical role in the exploitation of the vulnerability.

1. The attacker sends two or more requests to the target server, in a specific sequence. This sequence is designed to confuse the server and trick it into processing the requests unexpectedly. The attacker may use a variety of techniques to manipulate the requests, including altering the content length, transfer-encoding, or host header fields.
2. The server processes the first request in the usual way, without any indication that the second request has been manipulated. This is because the server is not designed to detect or prevent HTTP Request Smuggling attacks.
3. The second request is manipulated in such a way that it confuses the way that the server processes the request. For example, the attacker may alter the content-length field to indicate that the request is smaller than it is or manipulates the transfer-encoding field to cause the server to interpret the request unexpectedly.
4. The attacker exploits this confusion to trick the server into processing the second request in an unintended way, potentially leading to a security vulnerability. The exact form that the vulnerability takes will depend on the specifics of the attack and the target server's configuration.
5. The security vulnerability can take various forms, including session hijacking, bypassing authentication mechanisms, cache poisoning, server-side request forgery, and bypassing input validation controls, among others. In some cases, the vulnerability may allow the attacker to access sensitive information or execute malicious code on the target server.

Impact of HTTP Request Smuggling

HTTP Request Smuggling can have serious consequences for the security and integrity of a web application and its users. Some of the most significant impacts of this vulnerability include:

1. Data Theft : HTTP Request Smuggling can be used to bypass authentication mechanisms and steal sensitive information, such as passwords, financial data, and personal information. This can result in financial losses, identity theft, and other types of data breaches.
2. Session Hijacking : Attackers can use HTTP Request Smuggling to hijack a user's session and take control of their account, potentially accessing sensitive information and carrying out unauthorized actions.
3. Server-Side Request Forgery : This vulnerability can allow attackers to send requests to a server that appear to come from a trusted source, potentially bypassing access controls and exposing sensitive information or carrying out malicious actions.
4. Cache Poisoning : Attackers can use HTTP Request Smuggling to manipulate the cache of a web application, potentially causing the server to serve malicious content or misdirect users to malicious websites.
5. Bypassing Security Controls : HTTP Request Smuggling can be used to bypass security controls, such as firewalls, intrusion detection systems, and input validation mechanisms, potentially exposing the web application to other types of attacks.

These impacts can have serious consequences for the security and stability of a web application, potentially exposing sensitive information, compromising user accounts, and disrupting the normal operation of the application. It is important for web application developers and security professionals to be aware of the risks posed by HTTP Request Smuggling and to take steps to prevent and mitigate these attacks.

How to Perform HTTP Request Smuggling?

HTTP request smuggling attacks take advantage of a mismatch in how the front-end and back-end servers process incoming requests. The attack involves including both the Content-Length header and Transfer-Encoding header in a single request and manipulating them in a way that causes the two servers to process the request differently. The exact method of manipulation depends on the behaviour of the front-end and back-end servers, which can be classified into three types: CL.TE, TE.CL, and TE.TE.

In the CL.TE scenario, the front-end server uses the Content-Length header and the back-end server uses the Transfer-Encoding header. In the TE.CL scenario, the front-end server uses the Transfer-Encoding header and the back-end server uses the Content-Length header. In the TE.TE scenario, both the front-end and back-end servers support the Transfer-Encoding header, but the attacker can exploit a vulnerability in one of the servers by obfuscating the header in a specific way.

CL.TE Vulnerability

The process of exploiting a CL.TE vulnerability involves the front-end server using the Content-Length header while the back-end server uses the Transfer-Encoding header. To perform an HTTP request smuggling attack, the following steps are taken:

1. The front-end server reads the Content-Length header and determines that the request body is 13 bytes long, up to the end of the word "SMUGGLED". This request is then forwarded to the backend server.
2. The back-end server then reads the Transfer-Encoding header, treating the message body as using chunked encoding. It processes the first chunk, which is stated to be of zero length, which is then interpreted as the end of the request. The following bytes, "SMUGGLED", are not processed and the back-end server will consider these as the start of the next request in the sequence.

TE.CL Vulnerability

TE.CL vulnerabilities refer to a situation where the front-end server utilizes the Transfer-Encoding header and the back-end server uses the Content-Length header. To carry out an HTTP Request Smuggling attack in this scenario, the following steps can be taken:

1. The front-end server processes the Transfer-Encoding header and recognizes that the message body is in a chunked encoding format. It then processes the first chunk which is stated to have a length of 8 bytes, up until the line that starts with "SMUGGLED". The front-end server then processes the second chunk which is stated to have a length of zero bytes, signifying the end of the request. The processed request is then forwarded to the back-end server.
2. The back-end server, on the other hand, processes the Content-Length header and determines that the request body is only 3 bytes long, up until the start of the line following "8". As a result, the remaining bytes starting with "SMUGGLED" are left unprocessed and treated by the back-end server as the start of a new request.

TE.TE Vulnerability

Obfuscating the Transfer-Encoding Header Both the front-end and back-end servers in this scenario support the Transfer-Encoding header, but one of the servers can be manipulated to ignore it by obscuring the header in a specific manner. There are numerous methods of obscuring the Transfer-Encoding header, and each technique entails a slight deviation from the HTTP specification. For example :

In practice, it's common for real-world code that implements a protocol specification to not strictly follow it, and different implementations may tolerate variations from the specification. To identify a TE.TE vulnerability, it's necessary to find a variation of the Transfer-Encoding header that one of the front-end or back-end servers processes, while the other server disregards it.

Depending on which front-end or back-end servers can be convinced to ignore the obscured Transfer-Encoding header, the attack will take the same form as the previously described CL.TE or TE.CL vulnerabilities.