As IPython notebooks become more popular for sharing and collaboration, the potential for malicious people to attempt to exploit the notebook for their nefarious purposes increases. IPython 2.0 introduces a security model to prevent execution of untrusted code without explicit user input.
The whole point of IPython is arbitrary code execution. We have no desire to limit what can be done with a notebook, which would negatively impact its utility.
The security problem we need to solve is that no code should execute just because a user has opened a notebook that they did not write. Like any other program, once a user decides to execute code in a notebook, it is considered trusted, and should be allowed to do anything.
IPython notebooks store a signature in metadata, which is used to answer the question “Did the current user do this?”
This signature is a digest of the notebooks contents plus a secret key, known only to the user. The secret key is a user-only readable file in the IPython profile’s security directory. By default, this is:
Any output generated during an interactive session is trusted.
A notebook’s trust is updated when the notebook is saved. If there are any untrusted outputs still in the notebook, the notebook will not be trusted, and no signature will be stored. If all untrusted outputs have been removed (either via Clear Output or re-execution), then the notebook will become trusted.
While trust is updated per output, this is only for the duration of a single session. A notebook file on disk is either trusted or not in its entirety.
Sometimes re-executing a notebook to generate trusted output is not an option, either because dependencies are unavailable, or it would take a long time. Users can explicitly trust a notebook in two ways:
At the command-line, with:
ipython trust /path/to/notebook.ipynb
After loading the untrusted notebook, with File / Trust Notebook
These two methods simply load the notebook, compute a new signature with the user’s key, and then store the newly signed notebook.
If you find a security vulnerability in IPython, either a failure of the code to properly implement the model described here, or a failure of the model itself, please report it to firstname.lastname@example.org.
If you prefer to encrypt your security reports, you can use this PGP public key.
Some use cases that work in IPython 1.0 will become less convenient in 2.0 as a result of the security changes. We do our best to minimize these annoyance, but security is always at odds with convenience.
We plan to provide a mechanism for notebook themes, but in the meantime styling the notebook can only be done via either custom.css or CSS in HTML output. The latter only have an effect if the notebook is trusted, because otherwise the output will be sanitized just like Markdown.
When collaborating on a notebook, people probably want to see the outputs produced by their colleagues’ most recent executions. Since each collaborator’s key will differ, this will result in each share starting in an untrusted state. There are three basic approaches to this:
Since the notebook secret is stored in a profile directory by default, opening a notebook with a different profile or on a different machine will result in a different key, and thus be untrusted. The only current way to address this is by sharing the notebook secret. This can be facilitated by setting the configurable:
c.NotebookApp.secret_file = "/path/to/notebook_secret"
in each profile, and only sharing the secret once per machine.