![]() For more information, see Advanced request smuggling. This process is known as HTTP downgrading. This means that the front-end effectively has to translate the requests it receives into HTTP/1. However, many websites have an HTTP/2-speaking front-end server, but deploy this in front of back-end infrastructure that only supports HTTP/1. As the HTTP/2 specification introduces a single, robust mechanism for specifying the length of a request, there is no way for an attacker to introduce the required ambiguity. Websites that use HTTP/2 end-to-end are inherently immune to request smuggling attacks. Header, then they might disagree about the boundaries between successive requests, leading to request smuggling vulnerabilities. If the front-end and back-end servers behave differently in relation to the (possibly obfuscated) Transfer-Encoding Some servers that do support the Transfer-Encoding header can be induced not to process it if the Some servers do not support the Transfer-Encoding header in requests. In this situation, problems can arise for two When only a single server is in play, but not when two or more servers are chained together. This might be sufficient to avoid ambiguity Present, then the Content-Length header should be ignored. Stating that if both the Content-Length and Transfer-Encoding headers are The specification attempts to prevent this problem by Message to use both methods at once, such that they conflict with each other. Many security testers are unaware that chunked encoding can be used in HTTP requests, for two reasons:īurp Suite automatically unpacks chunked encoding to make messages easier to view and edit.īrowsers do not normally use chunked encoding in requests, and it is normally seen only in server responses.Īs the HTTP/1 specification provides two different methods for specifying the length of HTTP messages, it is possible for a single The message is terminated with a chunk of size zero. Hexadecimal), followed by a newline, followed by the chunk contents. Each chunk consists of the chunk size in bytes (expressed in Means that the message body contains one or more chunks of data. The Transfer-Encoding header can be used to specify that the message body uses chunked encoding. ForĬontent-Type: application/x-www-form-urlencoded The Content-Length header is straightforward: it specifies the length of the message body in bytes. Most HTTP request smuggling vulnerabilities arise because the HTTP/1 specification provides two different ways to specify where a requestĮnds: the Content-Length header and the Transfer-Encoding header. How do HTTP request smuggling vulnerabilities arise? Smuggling attack, and it can have devastating results. It isĮffectively prepended to the next request, and so can interfere with the way the application processes that request. 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. ![]() Otherwise, anĪttacker might be able to send an ambiguous request that gets interpreted differently by the front-end and back-end systems: In this situation, it is crucial that the front-end and back-end systems agree about the boundaries between requests. The receiving server has to determine where one request ends and the next one begins: The protocol is very simple HTTP requests are sent one after another, and When the front-end server forwards HTTP requests to a back-end server, it typically sends several requests over the same back-end networkĬonnection, because this is much more efficient and performant. This type of architecture is increasingly common, and in some cases unavoidable, in modern cloud-based applications. ![]() Users send requests toĪ front-end server (sometimes called a load balancer or reverse proxy) and this server forwards requests to one or more back-end servers. Today's web applications frequently employ chains of HTTP servers between users and the ultimate application logic. What happens in an HTTP request smuggling attack? Browser-powered desync attacks: A new frontier in HTTP request smuggling.HTTP desync attacks: Request smuggling reborn.For details, check out the following whitepapers: HTTP request smuggling was first documented in 2005, and repopularized by PortSwigger's extensive research on the topic. Testing for pause-based CL.0 vulnerabilities.Pivoting attacks against internal infrastructure.Building a proof of concept in a browser.Non-blind request tunnelling using HEAD requests.Understanding the aftermath of request smuggling. ![]() Turning root-relative redirects into open redirects.Turning an on-site redirect into an open redirect.
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