So if you’re like me and use emacs, there’s a chance you’ll also use emacs protocol to capture bookmarks. I’ve recently discovered how this function can be leveraged to perform code execution.

It’s fairly convoluted, and requires user interaction, but you can go from webpage to code running locally on the target with 2 user interactions.

Org protocol

The root of the issue, and the first step is that org protocol does not filter orgmode special characters from the input. This wouldn’t be much of an issue if it wasn’t for org-babel.

Org babel

Babel allows code blocks to be embedded into an orgmode document and executed when needed by the user. It can also write code to files on the file system. This is where the heart of the problem lies. Org-babel assumes that the data in an orgmode file is entirely user controlled, but this is not always the case.

Babels ability to write code to a file is called “Tangling” and has some caveats. Specifically that you cannot “tangle” one single code block, you have to tangle every code block in a file. This means that if one of those code blocks is malicious, and tangles to ~/.bashrc, anytime you tangle that file the malicious code will be written to a file.

Example

Say a user uses the org-capture chrome extension to capture web-pages for later reading. This means that data from the webpage is being sent to emacs to be handled. This data can be controlled by an attacker, in the title HTML tag.

The title is passed into an orgmode capture template and written to an orgmode file. If said title contains a babel codeblock with a tangle pointing towards ~/.bashrc, that would be written to the file intact.

If you at any point tangle that file, or refile the entry to another file that you will tangle, you will have the attackers code inside the babel codeblock written to ~/.bashrc

Thus with only two user interactions, and a willing attacker, one can get malicious code running on the target machine.

PoC

Usually I’d format this in a orgmode codeblock, but as it’s a nested codeblock it breaks pretty much everything. So here’s it in plaintext

emacsclient “org-protocol:/capture:/google.com/]

#+BEGIN_SRC bash :tangle test.sh

echo ‘test’

#+END_SRC

/test”

Run this command, tangle the file that the capture template you select writes to. and you should see a test.sh file containing “echo ‘test’”, this can easily be made to write to a location that will autoexecute the script.

Potential mitigations

Perhaps org-capture should sanitise it’s input