Regilero's blog; Mostly tech things about web stuff.

First part of the 2015 HTTP Smuggling articles. Injecting HTTP in HTTP, the theory.
First part of the 2015 HTTP Smuggling articles. Injecting HTTP in HTTP, the theory.

English version (Version Française disponible sur makina corpus.)

The goal of this serie of articles is to explain clearly what are HTTP smuggling issues and why I think this sort of security issues are critically important and could be used in massive attacks against modern web services.

In the last 6 months I've been struggling with the state of several open source HTTP servers, and helped fixing several issues. The main problem encountered was, usually, to explain the problem.

A first big study on smuggling was done in 2005 and lead to several corrections. This was was done by Chaim Linhart, Amit Klein, Ronen Heled and Steve Orrin, published by Watchfire and still worth a read ten years after. Today even the HTTP/1.1 RFC 7230 contains protections and warnings against Request Smuggling, but an RFC is just a reference, things are really different when you check the implementations. And a lot of people are now starting their own implementations. Seems that a refresh was needed.

Most of the links provided in this article should be easier to understand after reading this first part. This is gonna be a long serie, starting from simple HTTP requests to very strange ones, with details about recently fixed flaws on several tools (and some CVE also). On this first article there is nothing new, just another way of explaining the problems. I hope this will at least help refreshing memories on the problems.

If you use HTTP servers, and especially if you use several HTTP agents (Reverse Proxies, SSL Terminators, Load balancers, etc.), you should be interested by this. If you build an HTTP agent you should master it, best thing would be knowing all this better than me, if you spot any error, do not hesitate to comment or contact me.

HTTP Smuggling: What?

Hiding HTTP queries in HTTP, Injection

That's it, the main idea is to hide HTTP in HTTP.

To hide a message in a protocol you need to find a flaw, an issue, in the way an agent is interpreting (reading) the message.

HTTP Request smuggling is simply an injection of HTTP protocol into the HTTP protocol. As always with security the main problem is injection. If you can inject SQL into SQL, HTML, javascript or css into an HTML response... you have problems.

When injecting javascript in a an HTML page you need to find a flaw in the application outputing some user content. Here the players will tracks flaws in the parsing of HTTP messages.

Some security problems closely related to HTTP smuggling are HPP (HTTP parameters pollution) that you can read on the Stefano di Paola & Luca Carettoni paper in OWASP 09 and HTTP Response splitting.

HPP is a very specific part of HTTP Smuggling, considering only the parameters used on the location and the problems arising from differences in the interpretations of strange parameters (like repetitions of same url parameter).

Response splitting is an attack used on an application (on the final backend) where the backend will send more HTTP responses than expected. It's a tool that could be used in HTTP Smuggling, but flaws are uncommon (they were problems with newlines injections in PHP redirections or in Digest authentication username, but this was a long time ago).

Here I will mostly talk about regular HTTP Smuggling, flaws coming from HTTP syntax mistakes and protocol approximations.


We'll study in details the 3 main kinds of attacks below. But if you can hide HTTP in HTTP you can perform various forms of attack, going from bypassing security checks to hijacking user sessions or defacing content in caches.

This is the story of several HTTP queries. Let's say we have at least 3 different queries, we'll name theses queries for clarity:

  • Suzann: the Smuggler query,
  • Ivan: the Innocent query,
  • Walter: the Wookie, accomplice of the smuggler, usually a Forbidden query. And, yes, it's a wookie. Because usually smugglers are working with wookies.

They will transit from one starting point, the attacker computer, to an HTTP server (your HTTP server). And sometimes they will encounter a middleware server, which is also reading and emitting HTTP. This could be a Load Balancer, an SSL terminator, a reverse proxy cache, as static cache, etc.

Suzann the smuggler is evil, the goal of this query is to attack Ivan the innocent.

Suzann will not be a regular HTTP query like this:

GET /suzann.html HTTP/1.1\r\n
Host: www.example.com\r\n
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8\r\n
Accept-Encoding: gzip, deflate\r\n
Cache-Control: max-age=0\r\n
Connection: keep-alive\r\n
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:33.0) Gecko/20100101 Firefox/33.0\r\n

Instead it could be something like that (not all in the same time :-) ):

GET http://www.example.com/suzann.html?foo=%00\tHTTP/11111111111.2\r\n
     HOST: www.evil1.com\r\n
HOST: www.evil2.com\r\n
Content-length: 0\r\n
ContenT-length :\t100\r\n
Connection: keep-alive\n

And your server should reject most of the strange things present in this request example (this one is soo bad that I do not think any server would accept it). If a flaw is found in the server it could be used for smuggling and the attacker could start to think about exploitations of the flaw.

We'll start by a big rollback and give some details about HTTP to understand all this.


Back in the old time they were only a very old version of HTTP available (that we call HTTP/0.9). Smuggling was not available. The only way to send 3 queries was opening 3 time a tcp/ip connection to the server and each time asking for the targeted document:

--> open tcp/ip conn1
GET /Suzann.html\r\n
<-- receive Suzann.html document
<-- conn1 is closed

--> open tcp/ip conn1
GET http://your.server.com/ivan.html\r\n
<-- receive ivan.html document
<-- conn1 is closed

--> open tcp/ip conn1
GET /walter.html\r\n
<-- receive walter.html document
<-- conn1 is closed

HTTP is a quite simple protocol, especially at this time. Note that I'm using \r\n to represent the CR and LF characters end-of-line markers. This will get important later.

Then comes HTTP/1.0, with one important thing added, the headers. You can add them in the request, and the response will always contains headers.

The first query became:

--> open tcp/ip conn1
GET /suzann.html HTTP/1.0\r\n
Host: your.server.com\r\n
User-agent: information gateways navigator 0.8\r\n
Accept: text/html\r\n
Foo Bar\r\n
<-- receive Suzann.html document
HTTP/1.0 400 Bad Request\r\n
Server: Apache\r\n
Date: Fri, 24 Jul 1612 16:24:10 GMT\r\n
Content-Type: text/html\r\n
Content-Length: 138\r\n
Connection: close\r\n
<head><title>400 Bad Request</title></head>\r\n
<body bgcolor="white">\r\n
<center><h1>400 Bad Request</h1></center>\r\n
<-- conn1 is closed

In this response the important thing to note is Content-Length: 138. If you count all characters, starting at <html, including the end-of-line characters, you have exactly 138 characters, with one byte for each ascii7 character, 138 bytes. It seems size matters, and we'll see that size is everything in our problem. Here we also have an error code (400), the server said we made a mistake in our query, this is not a problem, it's a nice thing to have in a protocol (error messages). The error was a missing ':' between Foo and Bar.

Let's go quickly to HTTP/1.1, the real HTTP protocol, still used almost everywhere. The new HTTP/2 protocol is just starting to replace it on some very limited places.

We have several new features on HTTP/1.1 that will allow very bad behaviors for Suzann the smuggler, with some other features, less important for smugglers (like the Host Header which is now required, it was optional before).

  • Keep Alive mode
  • Pipelined queries
  • Chunked queries and responses

Keep Alive

With keep Alive we can open a connection to the server, request Suzann, receive Suzann, then request Ivan and receive it, and at lastly request Walter and receive it, in the same TCP/IP connection.

As an HTTP client you can ask for keepAlive mode, but it's usually enabled by default by the server even if you do not ask for it. You do it by adding this header on the request:

Connection: keepalive

On the server side, the keep alive connection can be cut at any time (and most servers should try to avoid long opened keep alive connections, especially if they keep one dedicated process for each connection and cannot afford more than a few hundred processes like the Apache httpd prefork mpm).

On the response headers you will find:

Connection: close # means we're closing, easy
Connection: keep-alive # means we'll try to maintain this open a few seconds

Usually, after a close the server will close the tcp/ip connection.

The goal of this was to retrieve faster all assets coming with an HTML page, by reusing the tcp/ip connection opened for the document, avoiding the slow tcp/ip connection opening.

This keep-alive mode in the protocol is what things like Comet are trying to exploit to maintain pseudo-infinite-time push/pull connections over HTTP. But even without this advanced mode, keep-alive is used a lot in HTTP environments, especially between the end-user and the first part of the middleware. Usage of keep-alive between HTTP servers and proxies (in the middleware or between the middleware and the backend) is less common.

Pipelines \o/

The other big thing in HTTP/1.1 is pipelining. Pipelining is sending several queries before having the responses of theses requests.

Here's a schema of basic pipelining:

    [Client]                  [End Server]
        |                         |
        >-requ. Suzann ---------->|
        >-requ. Ivan ------------>|
        >-requ. Walter----------->|
        |<---------- resp. Suzann-<
        |<------------ resp. Ivan-<
        |<---------- resp. Walter-<

With only Keep Alive the schema was:

    [Client]                  [End Server]
        |                         |
        >-requ. Suzann ---------->|
        |<---------- resp. Suzann-<
        >-requ. Ivan ------------>|
        |<------------ resp. Ivan-<
        >-req. Walter------------>|
        |<---------- resp. Walter-<

With an HTTP proxy in the middle the schema is, usually:

    [Client]             [Middleware]          [End Server]
        |                     |                     |
        >-requ. Suzann ------>|                     |
        >-requ. Ivan -------->|                     |
        >-req. Walter ------->|                     |
        |                     >-requ. Suzann ------>|
        |                     |<------ resp. Suzann-<
        |<------ resp. Suzann-<                     |
        |                     >-requ. Ivan -------->|
        |                     |<-------- resp. Ivan-<
        |<-------- resp. Ivan-<                     |
        |                     >-req. Walter-------->|
        |                     |<------- resp Walter-<
        |<------ resp. Walter-<                     |

You can see that the connection between the middle agent (a reverse proxy cache or an SSL terminator) and the end server is usually not using pipelining (because that's an awfully complex thing to do well with HTTP).

Sometimes the connection between the middleware and the end server is even not using Keep Alive connections. Some other times it's reusing a pool of tcp keep alive connections. But the first protection against HTTP smuggling applied here is breaking your pipeline of requests in several requests, waiting for the end of a first request before handling the next one.

Eventually, the server is never expected to respond to all requests in a pipeline. You can get the first response with a Connection: close header, cutting the Keep Alive connection right after the response.

    [Client]             [Middleware]          [End Server]
        |                     |                     |
        >-requ. Suzann ------>|                     |
        >-requ. Ivan -------->|                     |
        >-req. Walter ------->|                     |
        |                     >-requ. Suzann ------>|
        |                     |<------ resp. Suzann-<
        |<------ resp. Suzann-<                     |
[ client is expected to re-send Ivan & Walter queries]

And this is the second big protection against smuggling. As soon as the middleware detects a bad Suzann query, it should send a 400 bad request response and close the connection (but if you really search you'll find examples of proxies which does not close the keep alive after an error).


Chunked transfer is an alternate way of transmitting long HTTP messages. Instead of a transmission starting with a Content-length header announcing the full message size you can transmit the message by small (or not) chunks, each one annoucing a size (in hexadecimal format).

A special last-chunk empty chunk marks the end of the message.

We'll certainly study in detail chunks in next articles. The important thing with chunks is that's it is another way of manipulating the size of the message.

Chunks can be used on HTTP responses (usually) but also on queries.

For an example you can read the Wikipedia page explaining how chunks can be used to transfer this:

Wikipedia in\r\n\r\nchunks.



So much fun :-)

The key: Size matters

I said it several times. But, yes, size matters. To inject HTTP in HTTP the key is usually to trick the HTTP agent reader about the size of your message.

HTTP queries and responses are mostly a list of strings separated by end of lines. And we saw with pipelines that you could send several queries, one after another.

The HTTP agent reading the queries or parsing the responses MUST know where this list of strings ends. this way, the agent can check if what's coming after is another query (or response if it's a backend stream).

The tools used by the HTTP reader is either the chunks mecanism or the Content-Length header.

And if something goes wrong here you can start hiding some queries or responses. One of the composants will parse the stream and will not understand the incoming characters as new requests but as the previous request body, or will not understand the stream as the first request response but as a new one.

That's the key of HTTP Smuggling.

GET /suzann.html HTTP/1.1\r\n
Host: example.com\r\n
Content-Length: 0\r\n
Content-Length: 46\r\n
GET /walter.html HTTP/1.1\r\n
Host: example.com\r\n

Here if you accept the first Content-length header you have 2 requests. If you take the second one as the right one instead, then you have one GET query, with a body containing some bytes that you do not care about -- even if it looks like a query-- (a GET query with a body is something strange, POST query have bodies and usually GET queries have only parameters, but it is allowed).

Ok, so it's one OR two queries, no problem, but if you are a proxy seing one query and transmitting this unique query to a backend which then sends you 2 responses you'd better know what to do with this second response. Or maybe it would have been better to detect a bad HTTP request and avoid this problem.

HTTP Smuggling: the basics

HTTP smuggling may be used in 3 sort attacks (mainly).

Attack 1 : Bypass security filters

The first type of attack is bypassing security filters on Walter forbidden query. In this type of attack the Walter query is forbidden (Wookies is a forbidden species), but Suzann is hiding Walter from the middleware filters (storm troopers filtering the docks).
Eventually Walter is executed on the target, behind the filters (was hidden in the cargo).

    [Attacker]              [Middleware]             [End Server]
        |                       |                        |
        >-req. Suzann(+Walter)->|                        |
        |                       >-requ. Suzann(+Walter)->|
        |                       |             \-Suzann-->|
        |                       |<--------- resp. Suzann-<
        |<-------- resp. Suzann-<                        |
        |                       |             \-Walter-->| [*]
        |                       |<---X----- resp. Walter-<

The problem occurs at [*]

Note here the <--X--- arrow, the middleware may not be really aware that a Walter query was emitted, and can reject the response (and close it). But the query has already been emitted, and this enough could be a problem.

You have an example of such issue in my previous blog post with Nginx as end server, Varnish as middleware, and very huge queries. In this variant the Middleware receive a response while it still thinks the 1st query is not even completly transmitted (just to say that between theory and real exploits things can get quite complex).

To avoid loosing the response from Walter the attacker can sometimes try to pipeline some other queries. But the attacker goal is maybe just to run the Walter query without being filtered (like accessing known security exploit on a CMS where an HTTP filter prevents regular access to the backoffice).

Attack 2 : Replacement of regular response

The second type is defacement of Ivan. On a successful attack by Suzann, anyone requesting Ivan would get a Walter response. This can be used to prevent regular use of Ivan (Deny of Service), but could also be worst, Walter the wookie could contain some very dangerous content (like javascript code). Just imagine Ivan is a regular javascript library on a CDN used by several thousands of people daily, if the CDN sends the Walter javascript in place of this one...

Queries have to be pipelined by the attacker for this sort of attack.

    [Attacker]              [Middleware]             [End Server]
        |                        |                        |
        >-req. Suzann(+Walter) ->|                        |
        |-req. Ivan ---#2------->|                        |
        |                        >--req. Suzann(+Walter)->| [*1*]
        |                        |         \_req. Suzann->|
        |                        |<-------- resp. Suzann -<
        |<----#1--- resp. Suzann <                        |
        |                        |         \_req. Walter->| t1
        |                        >--req. Ivan ----------->| t2
        |                  [*2*] |<-------- resp. Walter -< t3
        |<----#2--- resp. Walter |                        |
        |                        |<---X-------- resp Ivan |

Suzann(+Walter) means that for the Middleware this is a simple Suzann request but for the backend this is two pipelined queries.
The middleware see a pipeline of 2 queries (Suzann/Ivan) but the backend receive a pipeline of two queries (Suzann/Walter) and a third query (Ivan).

In the timeline you also have 3 times, t1, t2 and t3.
The attack will fail if t2 occurs after t3 (for this the first Suzann may sometimes be choosen to be slow enough to avoid sending the Walter response too early).

The regular Ivan response may be rejected by the middleware (which already have 2 responses), that's just a side effect.

Here the biggest issue is at [*2*], the middleware receive a Walter response which is assigned to ivan request (request #2).

Suzann has performed some kind of Jedi trick on the empire dock's guards and they feed the next ship requesting something from the docks with one or more wookies instead of the regular cargo.

Wait, how do other requests/people get impacted in the second type of attack?

That's a very important question. In the first type of attack the goal was to bypass a security, so the role of the attacker was obvious.

On the second defacement type of attack the attacker seems the only guy impacted by the defacement. But you need to get a large view of the picture.

First usage is to get a response on the Walter query (if Walter was a forbidden query like in type 1 attack).

Second usage, if the middleware is a cache server the goal of the attack is cache poisoning, where the faked response is stored on the wrong cache entry. A successful attack will deface the responses for everybody, not only for the attacker. This is the obvious attack, very dramatic (Ivan was replaced by Walter in the cache and this will be for the cache lifetime duration).

But even without a caching problem an attacker can make a proxy server becomes crazy. On some successful attacks the proxy will mix queries from several clients. The attacker queries will be mixed with some other innocent queries from innocent clients, even without a cache, remember this point. Most middlewares have to trust the backend responses timeline and when backend responses goes wild strange things happen. This mix of communications between different users is also in the last attack type (type 3, credential hijacking). But we'll see in a coming article how user mix can happen without going to type 3.

Attack 3 : Credentials Hijacking

This third type of attack was referenced in the 2005 Watchfire study. Most proxy are now engineered well enough to prevent this from hapenning. It is now very hard to have a proxy sending a query to a backend, reusing the same connection as the one used on a previous unclosed communication (or I did not try hard enough, maybe).. I did miss some tests here, having backend connection pooling is not rare at all. But usually it is done by HTTP agents which are robust enough to avoid being transmitters of splitting attacks (@see next part for transmitters and splitters).

The trick was to inject a partial query in the stream, and wait for the regular user query, coming in the same backend connection and added to the partial request. It means the proxy is able to add some data in [+] to a tcp/ip connection with the backend that was unfinished in [-]. But the proxy does not know two queries were send, for the proxy there was only one query and the response is already received.

           [Attacker]               [Middleware]          [End Server]
                |                         |                     |
                >-req. Suzann[+Walter] -->|                     |
                |                         >-requ. Suzann ------>|
                |                         |<----- resp. Suzann  |
                |<---------- resp. Suzann |                     |
                |                         | [*]                 |
 [Innocent]     |                         | \-requ. Walter ---->|
     |          |                         | (unterminated)      | [-]
     >--------------------- req. Ivan --->|                     |
     |                                    >-req. Ivan --------->| [+]
     |                                    |<----- resp. Walter -<
     |<-------------------- resp. Walter -<                     |

This was a complexe scheme, but for example the Ivan request could contain a valid session that Walter did not have (cookies, HTTP Authentication). Also this valid session was needed for Walter query to succeed. Credentials used in Ivan query are stolen (hijacked) for a Walter query.

Damages of such issues are very high (you can make user perform unwanted POST actions, using his own credentials and rights). Keep alive and pipelines are not used in most proxies while communicating with backends.
Implementing shared backends connections or pools is a dangerous thing, unless you are very carefull of not being a splitting attack transmitter.

Transmitters and Splitters

In smuggling attacks you will need mainly two types of actors behaving differently on some HTTP protocol issues.

On the first part you need transmitters.

A transmitter is an HTTP agent, a proxy, which receive an altered HTTP query and transmits the alteration to an HTTP backend. When testing HTTP proxies you will encounter a lot of proxies which are cleaning up strange queries (like you were using tabulations as space separators, but the proxy is replacing tabulations with spaces when talking to the backend). Here the transmitters, by definition, is not cleaning up the strange part of the request. The transmitters, also, see the altered HTTP request as an unique query.

The second actor of the attack is a splitter, an involuntary accomplice. This agent receive the evil request from the transmitter. For the splitter this transmitted request is not unique, it's a multiple request (a pipeline) and this actor will emit several HTTP responses. This agent is splitting the request.
Sending 2 responses for one query is enough, but it may also be one hundred.
The splitter made a parsing error and detects a pipeline of queries (or the transmitters made this error, not detecting it was really a pipeline).

We'll use a typology for the next schemas:

>----qA-----> : HTTP query for request A
<-----rAqA--< : HTTP response A matched with request A
<-X---rAqA--< : HTTP response A rejected (connection close for example)
<----*rAqB*-< : HTTP response A matched with request B (very Bad thing)
>--qA+(qB)--> : HTTP query for request A, Hiding a query B
       [*CP*] : Cache poisoning
       [*RS*] : Response Splitting

The basic schema is:

    [Origin]            [transmitter]         [Splitter]
        |                    |                    |
        >-----qA+(qB)------->|                    |
        |                    >-----qA+(qB)------->| [*RS*]
        |                    |<-----------rAqA----<
        |<-----------rAqA----<                    |
        |                    |<-----------rBqB----<

Here we have a security issue in [*RS*] where a request splitting occurs.

If the Splitting attack can be issued by an application issue you do not need a transmitter communicating an altered HTTP query.

In terms of responsability the HTTP splitter is having a real security issue. Or at least that's what we could assume, when talking with project maintainers it could be quite hard to have HTTP splitting issues considered as security issues (let's hope this attitude will change in the future).
The transmitter could detect the smuggling tentative and should clean up the query before transmitting, but that's usually not considered a security issue, unless every other actor implementing the HTTP RFC would see 2 queries when the transmitter is only seing one (something like an inverted splitter).

Using this sort of schema attack of type 1 (filters bypass) is already achieved with the simple case.

Attack of type 2 (defacement) needs a pipeline of queries. It also need a third actor, a target. The target is something like a cache which will be the final victim.
The target is usually also the transmitter.

Here is a type2 issue:

    [Origin]        [transmitter-target]         [Splitter]
        |                    |                    |
        >-----qA+(qB)------->|                    |
        >-----qC------------>|                    |
        |                    >-----qA+(qB)------->| [*RS*]
        |                    |<-----------rAqA----<
        |<-----------rAqA----<                    |
        |                    >-----qC------------>|
        |             [*CP*] |<-----------rBqB----<
        |<-----------*rBqC*--<                    |
        |                    |<-X---------rCqC----<

This sort of behavior can also go wrong without caching (no [*CP*]) if you can make the <--*rBqC*--< response redirected to another user than the original attacker (obviously this should never happen...).

If the Splitting attack can be issued by an application issue you still do not need a transmitter but you need this actor to be a target.

Encapsulating and Fingerprinting

In real life, the attacker may need to navigate through several transmitters. Like hiting an HAProxy first, then an Apache mod_proxy, then a Varnish and finally an Nginx (yes, that happens).

The first job of the attacker is fingerprinting the middleware. To identify the layers present in the middleware you have some Headers in the responses (like the Server header or somes variations on X-Cache). But you also have the ability of checking the behaviors of the agents for each HTTP protocol error.

Every agent can have is own list of rejected syntax errors. For example Nginx will always reject an HTTP request using CR as line terminator instead of CRLF. Apache would not. Varnish3 will understand CR end of lines, Varnish 4 would not, etc.

You can build a fingerprinting test to identify who is rejecting you (and sometimes you'll get lucky and have the server signature in the error page).

And you can also use encapsulation to target the fingerprinting test at a precise level.

Encapsulation, is the ability to hide your HTTP query in several layers of HTTP smuggling issues. Usually the first layers are applying some strict rules on the HTTP headers or location, but if you find a transmitter issue in the layer you can carry in the request body another type of smuggling issue (one that would be detected if used directly on this layer). The encapsulation is available because usually the Proxy will not filter the request body (and against a filter trying to decode the request body, you could use several layers of Content-Transfer encoding).

In this example Middleware1 is a transmitter to a first type of smuggling noted "()" but would prevent any smuggling of a second type "{}". We could say for example that the "()" smuggling issue is using a chunked encoding issue and that the "{}" one is based on doubling Content-Length headers.
Middleware2 is a transmitter of the second type "{}" of smuggling (double Content-Length headers).
The End server is very sensible to the "{}" issue and is splitting the query.

Goal of the attack is cache poisonning in Middleware2 with a W query response on a I request. W is forbidden on Middleware 1 and also on Middleware 2.

[Attacker]         [Middleware1]     [Middleware2]   [End Server]
    |            [transmitter ()]  [transmitter {}]  [ Splitter ]
    |                   |                |               |
    >-qS(+qA{+qW})----->|                |               |
    >-qB -------------->|                |               |
    >-qI--------------->|                |               |
    |                   >-qS(+qA{+qW})-->| [*RS*]        |
    |                   |                >----qS-------->|
    |                   |                |<-----rSqS-----<
    |                   |<----rSqS-------<               |
    |<---------rSqS-----<                |               |
    |                   >-qB------------>|               |
    |                   |                >--qA{+qW}----->| [*RS*]
    |                   |         [*CP*] <---------rAqA--<
    |                   |<---*rAqB*------<               |
    |<--------*rAqB*----<                |               |
    |                   >-qI------------>|               |
    |                   |         [*CP*] <---------rWqW--<
    |                   |<---*rWqI*------<               |
    |<--------*rWqI*----<                |               |
    |                   |                >--qB-->(...)

Just to add one step of complexity you can also imagine a system where an HTTP response is forged (via a flaw in an application or via a stored attack), and this HTTP response could contain a response splitting attack, something like <--*rAqA(+rWqX)*--<. Securities are always stronger in request filters than in response filters on proxies, and most project would reject theses issues as securty problems ("we have to trust the backend responses, you see, that's a backend issue").

If the attackers can guess the servers and versions at each step of the middleware, and if a lot of smuggling issues exists in the ecosystem, a complex and very targeted attack can be made.

If you understood the previous paragraphs you are ready to test it (or to read the 2005 Watchfire study).

SSL/HTTPS as a protection?

Well, no, not really.

SSL is sometimes mentionned as one way of preventing HTTP Smuggling. It's not.

Having your HTTP message transmission encoded in an SSL tunnel is not preventing bad interpretation of the message by the HTTP agent. HTTP Smuggling occurs after the transmission. Maybe having SSL tunnelled through a proxy which is not trying to understand the content of the message (a pipe) could prevent a Smuggling issue on this proxy, but that's all. Mmmh, yes it could also make the simple tests more complex to perform (as it's hard to communicate in SSL in a telnet session) but that's not a real defense for this subject (not that you should not use https for other reasons).

Testing HTTP

Usually an HTTP request is made by a browser. You have some really nice tools to alter HTTP request on the browser for testing purpose. If you already try to attack yourself (of course yourself) with XSS or HTML injections you certainly already altered parameters on the queries with tools like Live HTTP Headers (and others), or maybe extracted curl queries from live queries.

But for HTTP smuggling you will usually need to test HTTP without a browser, because you will need to make very bad queries, and browsers never does bad queries. Well, in the past they was an issue end-of-line injections on Digest Authentication names or with bad separators on Ajax queries. But finding a flaw in a browser that allows HTTP smuggling requests coming from regular browsers is an exception. Smuggling does not usually imply that Suzann is an innocent smuggler using a regular browser.

No, what you will need is the full control of all characters in a query. For example you will need to control what characters are used as space separators or end-of-line.

Hopefully HTTP is a text based protocol, and is quite simple. If you never tried it you can always try an HTTP session with a telnet on port 80 of your server.

$ telnet 80
Connected to
Escape character is '^]'.
GET / HTTP/1.1                <---start typing here... fast!
Host: foobar                  <---required header in HTTP/1.1
                              <--- last enter for end of request
HTTP/1.1 301 Moved Permanently  <-- and now the server response
Server: nginx
Date: Sat, 25 Jul 2015 16:02:21 GMT
Content-Type: text/html
Content-Length: 178
Connection: keep-alive
Keep-Alive: timeout=15
Location: http://foobar.example.com/
X-Frame-Options: SAMEORIGIN

<head><title>301 Moved Permanently</title></head>
<body bgcolor="white">
<center><h1>301 Moved Permanently</h1></center>

But you need to type fast, and do not have a nice control on characters. The other method for fast testing is using printf to print a query on screen:

$ printf 'GET / HTTP/1.1\r\nHost:\tfoobar\r\n\r\n'
GET / HTTP/1.1
Host:   foobar

Or directly to the server (here I use netcat instead of telnet for that):

$ printf 'GET / HTTP/1.1\r\nHost:\tfoobar\r\n\r\n' | nc 80
HTTP/1.1 301 Moved Permanently
Server: nginx
Date: Sat, 25 Jul 2015 16:02:21 GMT
Content-Type: text/html
Content-Length: 178
Connection: keep-alive
Keep-Alive: timeout=15
Location: http://foobar.example.com/
X-Frame-Options: SAMEORIGIN

<head><title>301 Moved Permanently</title></head>
<body bgcolor="white">
<center><h1>301 Moved Permanently</h1></center>

With this sort of queries you can test easily the response of the server to a degraded HTTP query, let's for example try to replace all CR-LF (\r\n) end of lines with just CR (\r).

$ printf 'GET / HTTP/1.1\rHost:\tfoobar\r\r' | nc 80
<head><title>400 Bad Request</title></head>
<body bgcolor="white">
<center><h1>400 Bad Request</h1></center>

And you can do advanced queries with printf alone. Like a simple pipeline of queries:

$ printf 'GET /Suzann.html HTTP/1.1\r\nHost: example.com\r\n\nGET /ivan.html HTTP/1.1\nHost:example.com\r\n\r\n' | nc 80

Which becomes hard to read (and here we have only the strict minimum headers).

Next step is to build your own tool. For my extensive tests I've build my tools with python, using the socket library you have a very nice low level HTTP client where all the strange things are allowed, and you have an high level language to compute sizes (hiding queries in chunks, counting bytes, etc), or to add SSL support.

If you really want to study smuggling you will have to use tcpdump or wireshark to study the transmission of the signal between the actors, who is cleaning up the messages, what is not cleaned up, how does timers and size thresholds alter the behaviors, etc.

The final tool, the best one, is the code, do not be afraid of reading the code (when available). Learn the protocol and check implementations, that's the reason of open source code, open source needs critical eyes studying the code. That's the reason of superior robustness for open source code, but you will certainly discover that a lot of code still need to be fixed.

First final words

I'll end this article here, next things to come soon, with real world issues. But while waiting for new contents you can already try to read some posts and test your tools.

If you are using HTTP (and who isn't?), and usually use reverse proxies, SSL terminators, reverse proxy caches, my first advice is to check that you have recent versions. Smuggling issues are real, some have been fixed in 2015, avoid keeping old versions in production.

But I know this can be a hard task. So my second advice is to add an HTTP cleaner in front of your infrastructure. Something like HAProxy. This tool is very strong to protect against smugglers (but take recent versions, of course). Simply reading the configuration documentation of this product you can find an excellent introduction to the HTTP protocol, with common pitfalls documented.

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