Spoofing JARM signatures. I am the Cobalt Strike server now!

TL;DR: JARM is very useful fingerprinting tool, but can be deceived by replaying server hello’s from other services.

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The JARM scanner created by @SalesforceEng is quite an effective tool for system fingerprinting. It uses the Server Hello responses from a TLS handshake to generate a signature. These can then be used to find similar software or services. Ideal for finding C2 or other malicious servers that implement TLS. So, It doesn’t come as a surprise that Shodan.io uses this fingerprinting mechanism in their scanners. Read the Salesforce post for more information about the JARM library, scanner and its uses.

The question, then, arises: Is it possible to spoof these JARM signatures? Let’s find out! Salesforce stated in their post that scanning a Cobalt Strike server would result in the following signature 07d14d16d21d21d07c42d41d00041d24a458a375eef0c576d23a7bab9a9fb1

That this signature isn’t Cobalt Strike specific, was revealed in the Cobalt Strike blog. Let’s still use it as a starting point anyway.

First I used the list of addresses published by Salesforce to find a server with a matching hash. I scanned it using jarmscan and created a packet capture of the response. The ssl handshake (filter: ssl.handshake.type == 1) filter in Wireshark will display all TLS client hello’s sent by the scanner.

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Wireshark capture of 10 TLS Client Hello’s

And in turn the “Cobalt Strike“ server will return its Server Hello’s. These are used by jarmscan to generate a unique signature (filter: ssl.handshake.type == 2).

Wireshark capture of 10 TLS Server Hello’s

These Server Hello’s are the packets we want to replay. This can easily be done by setting up a TCP server listening for the specific Client Hello’s, then replaying their corresponding Server Hello’s captured from the alleged Cobalt Strike server. A rather lazy, but effective approach.

I scanned the server on three separate occasions and found the duplicate bytes for every request. I used these bytes to identify each specific Client Hello.

Luckily Wireshark has an option to display packets as C Arrays. This made it pretty easy to get the Server Hello’s working in my Golang spoofing application.

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By replaying these responses, slowly but steadily the fingerprint can be rebuilt.

if bytes.Contains(request, [] byte {
0x00, 0x8c, 0x1a, 0x1a, 0x00, 0x16, 0x00, 0x33, 0x00,
0x67, 0xc0, 0x9e, 0xc0, 0xa2, 0x00, 0x9e, 0x00, 0x39,
0x00, 0x6b, 0xc0, 0x9f, 0xc0, 0xa3, 0x00, 0x9f, 0x00,
0x45, 0x00, 0xbe, 0x00, 0x88, 0x00, 0xc4, 0x00, 0x9a,
... ...
}) {
fmt.Println("replaying: tls12Forward")
conn.Write([] byte {
0x16, 0x03, 0x03, 0x00, 0x5a, 0x02, 0x00, 0x00,
0x56, 0x03, 0x03, 0x17, 0xa6, 0xa3, 0x84, 0x80,
0x0b, 0xda, 0xbb, 0x3d, 0xe9, 0x3e, 0x92, 0x65,
0x9a, 0x68, 0x7d, 0x70, 0xda, 0x00, 0xe9, 0x7c,
... ...

A full signature can be faked after implementing a reply for all ten different requests.

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(Mis)usage of spoofed signatures

You’re probably thinking: So what? What is the use of spoofed TLS fingerprints? They could be used by malicious actors to hide their applications when tools like JARM scanners are deployed to identify services in a network or on the internet. It can also be used for good. A honeypot replaying the fingerprint of a specific service can be used to setup a digital smokescreen for attackers.


  • jarmscan (jarm-go) is not a product of Salesforce. They’ve published JARM a Python based JARM scanner implementation. Jarmscan (the scanner used here) is a Golang based implementation by @RumbleDiscovery

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