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Some applications receive client-submitted data, embed it on the server-side into an XML document, and then parse the document. However, you might be able to use XInclude instead. You can place an XInclude attack within any data value in an XML document, so the attack can be performed in situations where you only control a single item of data that is placed into a server-side XML document.
To perform an XInclude attack, you need to reference the XInclude namespace and provide the path to the file that you wish to include. For example:. Some applications allow users to upload files which are then processed server-side. For example, an application might allow users to upload images, and process or validate these on the server after they are uploaded.
Some web sites expect to receive requests in this format but will tolerate other content types, including XML. The vast majority of XXE vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. Virtually all XXE vulnerabilities arise because the application's XML parsing library supports potentially dangerous XML features that the application does not need or intend to use. The easiest and most effective way to prevent XXE attacks is to disable those features.
Generally, it is sufficient to disable resolution of external entities and disable support for XInclude. This can usually be done via configuration options or by programmatically overriding default behavior. Consult the documentation for your XML parsing library or API for details about how to disable unnecessary capabilities.
Want to track your progress and have a more personalized learning experience? It's free! Labs If you're already familiar with the basic concepts behind XXE vulnerabilities and just want to practice exploiting them on some realistic, deliberately vulnerable targets, you can access all of the labs in this topic from the link below. View all XXE labs. Note With real-world XXE vulnerabilities, there will often be a large number of data values within the submitted XML, any one of which might be used within the application's response.
Read more Finding and exploiting blind XXE vulnerabilities. Sign up Login. Register for free to track your learning progress. Practise exploiting vulnerabilities on realistic targets. Record your progression from Apprentice to Expert. See where you rank in our Hall of Fame. Request smuggling vulnerabilities are often critical in nature, allowing an attacker to bypass security controls, gain unauthorized access to sensitive data, and directly compromise other application users.
HTTP request smuggling was first documented in , and recently repopularized by PortSwigger's research on the topic. Today's web applications frequently employ chains of HTTP servers between users and the ultimate application logic.
Users send requests to a front-end server sometimes called a load balancer or reverse proxy and this server forwards requests to one or more back-end servers. This type of architecture is increasingly common, and in some cases unavoidable, in modern cloud-based applications.
When the front-end server forwards HTTP requests to a back-end server, it typically sends several requests over the same back-end network connection, because this is much more efficient and performant. The protocol is very simple: HTTP requests are sent one after another, and the receiving server parses the HTTP request headers to determine where one request ends and the next one begins:. In this situation, it is crucial that the front-end and back-end systems agree about the boundaries between requests.
Otherwise, an attacker might be able to send an ambiguous request that gets interpreted differently by the front-end and back-end systems:.
Here, the attacker causes part of their front-end request to be interpreted by the back-end server as the start of the next request. It is effectively prepended to the next request, and so can interfere with the way the application processes that request.
This is a request smuggling attack, and it can have devastating results. The Content-Length header is straightforward: it specifies the length of the message body in bytes. For example:. The Transfer-Encoding header can be used to specify that the message body uses chunked encoding. This means that the message body contains one or more chunks of data. Each chunk consists of the chunk size in bytes expressed in hexadecimal , followed by a newline, followed by the chunk contents.
The message is terminated with a chunk of size zero. Many security testers are unaware that chunked encoding can be used in HTTP requests, for two reasons:.
Since the HTTP specification provides two different methods for specifying the length of HTTP messages, it is possible for a single message to use both methods at once, such that they conflict with each other. The HTTP specification attempts to prevent this problem by stating that if both the Content-Length and Transfer-Encoding headers are present, then the Content-Length header should be ignored. This might be sufficient to avoid ambiguity when only a single server is in play, but not when two or more servers are chained together.
In this situation, problems can arise for two reasons:. If the front-end and back-end servers behave differently in relation to the possibly obfuscated Transfer-Encoding header, then they might disagree about the boundaries between successive requests, leading to request smuggling vulnerabilities. Request smuggling attacks involve placing both the Content-Length header and the Transfer-Encoding header into a single HTTP request and manipulating these so that the front-end and back-end servers process the request differently.
The exact way in which this is done depends on the behavior of the two servers:. Here, the front-end server uses the Content-Length header and the back-end server uses the Transfer-Encoding header. We can perform a simple HTTP request smuggling attack as follows:. This request is forwarded on to the back-end server.
The back-end server processes the Transfer-Encoding header, and so treats the message body as using chunked encoding. It processes the first chunk, which is stated to be zero length, and so is treated as terminating the request. Here, the front-end server uses the Transfer-Encoding header and the back-end server uses the Content-Length header. To send this request using Burp Repeater, you will first need to go to the Repeater menu and ensure that the "Update Content-Length" option is unchecked.
The front-end server processes the Transfer-Encoding header, and so treats the message body as using chunked encoding. It processes the second chunk, which is stated to be zero length, and so is treated as terminating the request.
The back-end server processes the Content-Length header and determines that the request body is 3 bytes long, up to the start of the line following 8.
The following bytes, starting with SMUGGLED , are left unprocessed, and the back-end server will treat these as being the start of the next request in the sequence.
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Some applications receive client-submitted data, embed it on the server-side into an XML document, and then parse the document. However, you might be able to use XInclude instead.
You can place an XInclude attack within any data value in an XML document, so the attack can be performed in situations where you only control a single item of data that is placed into a server-side XML document. To perform an XInclude attack, you need to reference the XInclude namespace and provide the path to the file that you wish to include.
For example:. Some applications allow users to upload files which are then processed server-side. For example, an application might allow users to upload images, and process or validate these on the server after they are uploaded. Some web sites expect to receive requests in this format but will tolerate other content types, including XML. The vast majority of XXE vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner.
Virtually all XXE vulnerabilities arise because the application's XML parsing library supports potentially dangerous XML features that the application does not need or intend to use. The easiest and most effective way to prevent XXE attacks is to disable those features. Generally, it is sufficient to disable resolution of external entities and disable support for XInclude. This can usually be done via configuration options or by programmatically overriding default behavior.
Consult the documentation for your XML parsing library or API for details about how to disable unnecessary capabilities. Want to track your progress and have a more personalized learning experience? It's free! Labs If you're already familiar with the basic concepts behind XXE vulnerabilities and just want to practice exploiting them on some realistic, deliberately vulnerable targets, you can access all of the labs in this topic from the link below.
View all XXE labs. Note With real-world XXE vulnerabilities, there will often be a large number of data values within the submitted XML, any one of which might be used within the application's response. Read more Finding and exploiting blind XXE vulnerabilities. Sign up Login. Register for free to track your learning progress. Practise exploiting vulnerabilities on realistic targets. Record your progression from Apprentice to Expert.
See where you rank in our Hall of Fame. It processes the first chunk, which is stated to be zero length, and so is treated as terminating the request. Here, the front-end server uses the Transfer-Encoding header and the back-end server uses the Content-Length header. To send this request using Burp Repeater, you will first need to go to the Repeater menu and ensure that the "Update Content-Length" option is unchecked.
The front-end server processes the Transfer-Encoding header, and so treats the message body as using chunked encoding. It processes the second chunk, which is stated to be zero length, and so is treated as terminating the request. The back-end server processes the Content-Length header and determines that the request body is 3 bytes long, up to the start of the line following 8.
The following bytes, starting with SMUGGLED , are left unprocessed, and the back-end server will treat these as being the start of the next request in the sequence. Here, the front-end and back-end servers both support the Transfer-Encoding header, but one of the servers can be induced not to process it by obfuscating the header in some way.
There are potentially endless ways to obfuscate the Transfer-Encoding header. Each of these techniques involves a subtle departure from the HTTP specification. Real-world code that implements a protocol specification rarely adheres to it with absolute precision, and it is common for different implementations to tolerate different variations from the specification.
To uncover a TE. TE vulnerability, it is necessary to find some variation of the Transfer-Encoding header such that only one of the front-end or back-end servers processes it, while the other server ignores it. Depending on whether it is the front-end or the back-end server that can be induced not to process the obfuscated Transfer-Encoding header, the remainder of the attack will take the same form as for the CL.
TE or TE. CL vulnerabilities already described. Check out the following section for some tips on how to identify HTTP request smuggling vulnerabilities for yourself. We've also provided some interactive LABS , so you can see how this works in practice. Now that you're familiar with the basic concepts, let's take a look at how HTTP request smuggling can be used to craft a number of high-severity attacks. As usual, there are plenty of fully interactive LABS , so you can try your hand at attacking realistic targets.
If you've already completed the rest of our request smuggling labs, you're ready to learn some more advanced techniques. HTTP request smuggling vulnerabilities arise in situations where the front-end server and back-end server use different mechanisms for determining the boundaries between requests. To prevent HTTP request smuggling vulnerabilities, we recommend taking the following general steps:.
For example, reject requests that contain newlines in the headers, colons in header names, and spaces in the request method. Make the front-end server normalize ambiguous requests and make the back-end server reject any that are still ambiguous, closing the TCP connection in the process. As we've demonstrated in the learning materials, disabling reuse of back-end connections will help to mitigate certain kinds of attack, but this still doesn't protect you from request tunnelling attacks.
Want to track your progress and have a more personalized learning experience? It's free! Labs If you're already familiar with HTTP request smuggling and just want to practice on a series of deliberately vulnerable sites, check out the link below for an overview of all labs in this topic. View all HTTP request smuggling labs. Note HTTP request smuggling was first documented in , and recently repopularized by PortSwigger's research on the topic.
Note Many security testers are unaware that chunked encoding can be used in HTTP requests, for two reasons: Burp Suite automatically unpacks chunked encoding to make messages easier to view and edit. Browsers do not normally use chunked encoding in requests, and it is normally seen only in server responses. TE vulnerability. Note To send this request using Burp Repeater, you will first need to go to the Repeater menu and ensure that the "Update Content-Length" option is unchecked. CL vulnerability.
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In this section, we'll explain what XML external entity injection is, describe some common examples, explain how to find and exploit various kinds of XXE injection, and summarize how to prevent XXE injection attacks. XML external entity injection also known as XXE is a web security vulnerability that allows an attacker to interfere with an application's processing of XML data.
It often allows an attacker to view files on the application server filesystem, and to interact with any back-end or external systems that the application itself can access. In some situations, an attacker can escalate an XXE attack to compromise the underlying server or other back-end infrastructure, by leveraging the XXE vulnerability to perform server-side request forgery SSRF attacks.
If you're already familiar with the basic concepts behind XXE vulnerabilities and just want to practice exploiting them on some realistic, deliberately vulnerable targets, you can access all of the labs in this topic from the link below. Some applications use the XML format to transmit data between the browser and the server. Applications that do this virtually always use a standard library or platform API to process the XML data on the server. XXE vulnerabilities arise because the XML specification contains various potentially dangerous features, and standard parsers support these features even if they are not normally used by the application.
External entities are particularly interesting from a security perspective because they allow an entity to be defined based on the contents of a file path or URL. To perform an XXE injection attack that retrieves an arbitrary file from the server's filesystem, you need to modify the submitted XML in two ways:. For example, suppose a shopping application checks for the stock level of a product by submitting the following XML to the server:.
This causes the application's response to include the contents of the file:. With real-world XXE vulnerabilities, there will often be a large number of data values within the submitted XML, any one of which might be used within the application's response. To test systematically for XXE vulnerabilities, you will generally need to test each data node in the XML individually, by making use of your defined entity and seeing whether it appears within the response.
Aside from retrieval of sensitive data, the other main impact of XXE attacks is that they can be used to perform server-side request forgery SSRF. This is a potentially serious vulnerability in which the server-side application can be induced to make HTTP requests to any URL that the server can access.
If you can use the defined entity within a data value that is returned in the application's response, then you will be able to view the response from the URL within the application's response, and so gain two-way interaction with the back-end system. If not, then you will only be able to perform blind SSRF attacks which can still have critical consequences.
In the following XXE example, the external entity will cause the server to make a back-end HTTP request to an internal system within the organization's infrastructure:. Many instances of XXE vulnerabilities are blind. This means that the application does not return the values of any defined external entities in its responses, and so direct retrieval of server-side files is not possible. Blind XXE vulnerabilities can still be detected and exploited, but more advanced techniques are required.
You can sometimes use out-of-band techniques to find vulnerabilities and exploit them to exfiltrate data. And you can sometimes trigger XML parsing errors that lead to disclosure of sensitive data within error messages. In other cases, the attack surface is less visible. Some applications receive client-submitted data, embed it on the server-side into an XML document, and then parse the document. However, you might be able to use XInclude instead. You can place an XInclude attack within any data value in an XML document, so the attack can be performed in situations where you only control a single item of data that is placed into a server-side XML document.
To perform an XInclude attack, you need to reference the XInclude namespace and provide the path to the file that you wish to include. For example:. Some applications allow users to upload files which are then processed server-side. For example, an application might allow users to upload images, and process or validate these on the server after they are uploaded.
Some web sites expect to receive requests in this format but will tolerate other content types, including XML. The vast majority of XXE vulnerabilities can be found quickly and reliably using Burp Suite's web vulnerability scanner. Virtually all XXE vulnerabilities arise because the application's XML parsing library supports potentially dangerous XML features that the application does not need or intend to use.
The easiest and most effective way to prevent XXE attacks is to disable those features. Generally, it is sufficient to disable resolution of external entities and disable support for XInclude. This can usually be done via configuration options or by programmatically overriding default behavior. Consult the documentation for your XML parsing library or API for details about how to disable unnecessary capabilities. Want to track your progress and have a more personalized learning experience?
It's free! Labs If you're already familiar with the basic concepts behind XXE vulnerabilities and just want to practice exploiting them on some realistic, deliberately vulnerable targets, you can access all of the labs in this topic from the link below.
View all XXE labs. Note With real-world XXE vulnerabilities, there will often be a large number of data values within the submitted XML, any one of which might be used within the application's response. Read more Finding and exploiting blind XXE vulnerabilities. Sign up Login. Register for free to track your learning progress. Practise exploiting vulnerabilities on realistic targets. Record your progression from Apprentice to Expert.
See where you rank in our Hall of Fame.
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