Cloudflare captcha [split] prox-config-sidki_2019-01-26b1

[JJoe]

So... We need the Proxomitron to share a browser's fingerprint.

blog.cloudflare.com/monsters-in-the-middleboxes/ Wrote:Introducing MITMEngine: Cloudflare’s HTTPS Interception Detector

Many TLS client implementations can be uniquely identified by features of the Client Hello message such as the supported version, cipher suites, extensions, elliptic curves, point formats, compression, and signature algorithms. The technique introduced by “The Security Impact of HTTPS Interception” is to construct TLS Client Hello signatures for common browser and middlebox implementations. Then, to identify HTTPS requests that have been intercepted, a server can look up the signature corresponding to the request’s HTTP User Agent, and check if the request’s Client Hello message matches the signature. A mismatch indicates either a spoofed User Agent or an intercepted HTTPS connection. The server can also compare the request’s Client Hello to those of known HTTPS interception tools to understand which interceptors are responsible for intercepting the traffic....
...
...To accomplish these goals, the Cryptography team at Cloudflare developed MITMEngine, an open-source HTTPS interception detector. MITMEngine is a Golang library that ingests User Agents and TLS Client Hello fingerprints, then returns the likelihood of HTTPS interception and the factors used to identify interception. To learn how to use MITMEngine, check out the project on GitHub.

MITMEngine works by comparing the values in an observed TLS Client Hello to a set of known browser Client Hellos. The fields compared include:

TLS version,
Cipher suites,
Extensions and their values,
Supported elliptic curve groups, and
Elliptic curve point formats.

When given a pair of User Agent and observed TLS Client Hello, MITMEngine detects differences between the given Client Hello and the one expected for the presented User Agent. For example, consider the following User Agent:

Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko)
Chrome/47.0.2526.111 Safari/537.36

This User Agent corresponds to Chrome 47 running on Windows 7. The paired TLS Client Hello includes the following cipher suites, displayed below as a hex dump:

0000 c0 2b c0 2f 00 9e c0 0a c0 14 00 39 c0 09 c0 13 .+./.... ...9....
0010 00 33 00 9c 00 35 00 2f 00 0a .3...5./ ..

These cipher suites translate to the following list (and order) of 13 ciphers:

TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 (0xc02b)
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (0xc02f)
TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 (0x009e)
TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA (0xc00a)
TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA (0xc014)
TLS_DHE_RSA_WITH_AES_256_CBC_SHA (0x0039)
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA (0xc009)
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA (0xc013)
TLS_DHE_RSA_WITH_AES_128_CBC_SHA (0x0033)
TLS_RSA_WITH_AES_128_GCM_SHA256 (0x009c)
TLS_RSA_WITH_AES_256_CBC_SHA (0x0035)
TLS_RSA_WITH_AES_128_CBC_SHA (0x002f)
TLS_RSA_WITH_3DES_EDE_CBC_SHA (0x000a)

The reference TLS Client Hello cipher suites for Chrome 47 are the following:

0000 c0 2b c0 2f 00 9e cc 14 cc 13 c0 0a c0 14 00 39 .+./.... .......9
0010 c0 09 c0 13 00 33 00 9c 00 35 00 2f 00 0a .....3.. .5./..

Looking closely, we see that the cipher suite list for the observed traffic is shorter than we expect for Chrome 47; two cipher suites have been removed, though the remaining cipher suites remain in the same order. The two missing cipher suites are

TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256 (0xcc14)
TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256 (0xcc13)

Chrome prioritizes these two ChaCha ciphers above AES-CBC ciphers--a good choice, given that CBC (cipher block chaining) mode is vulnerable to padding oracle attacks. It looks like the traffic we received underwent HTTPS interception, and the interceptor potentially didn't support ChaCha ciphers.

Using contextual clues like the used cipher suites, as well as additional User Agent text, we can also detect which software was used to intercept the HTTPS connection. In this case, MITMEngine recognizes that the fingerprint observed actually matches a fingerprint collected from Sophos antivirus software, and indicates that this software is likely the cause of this interception.

We welcome contributions to MITMEngine. We are particularly interested in collecting more fingerprints of MITM software and browser TLS Client Hellos, because MITMEngine depends on these reference fingerprints to detect HTTPS interception. Contributing these fingerprints is as simple as opening Wireshark, capturing a pcap file with a TLS Client Hello, and submitting the pcap file in a PR. More instructions on how to contribute can be found in the MITMEngine documentation.