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# Security in IPython notebooks

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 problem

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.

Unlike other programs, an IPython notebook document includes output. Unlike other documents, that output exists in a context that can execute code (via Javascript).

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.

## Our security model

- Untrusted HTML is always sanitized
- Untrusted Javascript is never executed
- HTML and Javascript in Markdown cells are never trusted
- Outputs generated by the user are trusted
- Any other HTML or Javascript (in Markdown cells, output generated by others) is never trusted
- The central question of trust is “Did the current user do this?”

## The details of trust

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:

    ~/.ipython/profile_default/security/notebook_secret

When a notebook is opened by a user, the server computes a signature with the user’s key, and compares it with the signature stored in the notebook’s metadata. If the signature matches, HTML and Javascript output in the notebook will be trusted at load, otherwise it will be untrusted.

Any output generated during an interactive session is trusted.

### Updating trust

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.

### Explicit trust

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.

## Reporting security issues

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 security@ipython.org.

If you prefer to encrypt your security reports, you can use `this PGP public key`.

## Affected use cases

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.

### Javascript and CSS in Markdown cells

While never officially supported, it had become common practice to put hidden Javascript or CSS styling in Markdown cells, so that they would not be visible on the page. Since Markdown cells are now sanitized (by Google Caja), all Javascript (including click event handlers, etc.) and CSS will be stripped.

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.

### Collaboration

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:

- re-run notebooks when you get them (not always viable)
- explicitly trust notebooks via `ipython trust` or the notebook menu (annoying, but easy)
- share a notebook secret, and use an IPython profile dedicated to the collaboration while working on the project.

### Multiple profiles or machines

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.

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