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templates: properly escape inline JavaScript values
TLDR: Kallithea has issues with escaping values for use in inline JS.
Despite judicious poking of the code, no actual security vulnerabilities
have been found, just lots of corner-case bugs. This patch fixes those,
and hardens the code against actual security issues.
The long version:
To embed a Python value (typically a 'unicode' plain-text value) in a
larger file, it must be escaped in a context specific manner. Example:
>>> s = u'<script>alert("It\'s a trap!");</script>'
1) Escaped for insertion into HTML element context
>>> print cgi.escape(s)
<script>alert("It's a trap!");</script>
2) Escaped for insertion into HTML element or attribute context
>>> print h.escape(s)
<script>alert("It's a trap!");</script>
This is the default Mako escaping, as usually used by Kallithea.
3) Encoded as JSON
>>> print json.dumps(s)
"<script>alert(\"It's a trap!\");</script>"
4) Escaped for insertion into a JavaScript file
>>> print '(' + json.dumps(s) + ')'
("<script>alert(\"It's a trap!\");</script>")
The parentheses are not actually required for strings, but may be needed
to avoid syntax errors if the value is a number or dict (object).
5) Escaped for insertion into a HTML inline <script> element
>>> print h.js(s)
("\x3cscript\x3ealert(\"It's a trap!\");\x3c/script\x3e")
Here, we need to combine JS and HTML escaping, further complicated by
the fact that "<script>" tag contents can either be parsed in XHTML mode
(in which case '<', '>' and '&' must additionally be XML escaped) or
HTML mode (in which case '</script>' must be escaped, but not using HTML
escaping, which is not available in HTML "<script>" tags). Therefore,
the XML special characters (which can only occur in string literals) are
escaped using JavaScript string literal escape sequences.
(This, incidentally, is why modern web security best practices ban all
use of inline JavaScript...)
Unsurprisingly, Kallithea does not do (5) correctly. In most cases,
Kallithea might slap a pair of single quotes around the HTML escaped
Python value. A typical benign example:
$('#child_link').html('${_('No revisions')}');
This works in English, but if a localized version of the string contains
an apostrophe, the result will be broken JavaScript. In the more severe
cases, where the text is user controllable, it leaves the door open to
injections. In this example, the script inserts the string as HTML, so
Mako's implicit HTML escaping makes sense; but in many other cases, HTML
escaping is actually an error, because the value is not used by the
script in an HTML context.
The good news is that the HTML escaping thwarts attempts at XSS, since
it's impossible to inject syntactically valid JavaScript of any useful
complexity. It does allow JavaScript errors and gibberish to appear on
the page, though.
In these cases, the escaping has been fixed to use either the new 'h.js'
helper, which does JavaScript escaping (but not HTML escaping), OR the
new 'h.jshtml' helper (which does both), in those cases where it was
unclear if the value might be used (by the script) in an HTML context.
Some of these can probably be "relaxed" from h.jshtml to h.js later, but
for now, using h.jshtml fixes escaping and doesn't introduce new errors.
In a few places, Kallithea JSON encodes values in the controller, then
inserts the JSON (without any further escaping) into <script> tags. This
is also wrong, and carries actual risk of XSS vulnerabilities. However,
in all cases, security vulnerabilities were narrowly avoided due to other
filtering in Kallithea. (E.g. many special characters are banned from
appearing in usernames.) In these cases, the escaping has been fixed
and moved to the template, making it immediately visible that proper
escaping has been performed.
Mini-FAQ (frequently anticipated questions):
Q: Why do everything in one big, hard to review patch?
Q: Why add escaping in specific case FOO, it doesn't seem needed?
Because the goal here is to have "escape everywhere" as the default
policy, rather than identifying individual bugs and fixing them one
by one by adding escaping where needed. As such, this patch surely
introduces a lot of needless escaping. This is no different from
how Mako/Pylons HTML escape everything by default, even when not
needed: it's errs on the side of needless work, to prevent erring
on the side of skipping required (and security critical) work.
As for reviewability, the most important thing to notice is not where
escaping has been introduced, but any places where it might have been
missed (or where h.jshtml is needed, but h.js is used).
Q: The added escaping is kinda verbose/ugly.
That is not a question, but yes, I agree. Hopefully it'll encourage us
to move away from inline JavaScript altogether. That's a significantly
larger job, though; with luck this patch will keep us safe and secure
until such a time as we can implement the real fix.
Q: Why not use Mako filter syntax ("${val|h.js}")?
Because of long-standing Mako bug #140, preventing use of 'h' in
filters.
Q: Why not work around bug #140, or even use straight "${val|js}"?
Because Mako still applies the default h.escape filter before the
explicitly specified filters.
Q: Where do we go from here?
Longer term, we should stop doing variable expansions in script blocks,
and instead pass data to JS via e.g. data attributes, or asynchronously
using AJAX calls. Once we've done that, we can remove inline JavaScript
altogether in favor of separate script files, and set a strict Content
Security Policy explicitly blocking inline scripting, and thus also the
most common kind of cross-site scripting attack.
| author | Søren Løvborg <sorenl@unity3d.com> |
|---|---|
| date | Tue, 28 Feb 2017 17:19:00 +0100 |
| parents | 9c6f717823e1 |
| children | 2c3d30095d5e |
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.. _contributing: ========================= Contributing to Kallithea ========================= Kallithea is developed and maintained by its users. Please join us and scratch your own itch. Infrastructure -------------- The main repository is hosted on Our Own Kallithea (aka OOK) at https://kallithea-scm.org/repos/kallithea/, our self-hosted instance of Kallithea. For now, we use Bitbucket_ for `pull requests`_ and `issue tracking`_. The issue tracker is for tracking bugs, not for support, discussion, or ideas -- please use the `mailing list`_ or :ref:`IRC <readme>` to reach the community. We use Weblate_ to translate the user interface messages into languages other than English. Join our project on `Hosted Weblate`_ to help us. To register, you can use your Bitbucket or GitHub account. See :ref:`translations` for more details. Getting started --------------- To get started with development:: hg clone https://kallithea-scm.org/repos/kallithea cd kallithea virtualenv ../kallithea-venv source ../kallithea-venv/bin/activate pip install --upgrade pip setuptools pip install -e . paster make-config Kallithea my.ini paster setup-db my.ini --user=user --email=user@example.com --password=password --repos=/tmp paster serve my.ini --reload & firefox http://127.0.0.1:5000/ You can also start out by forking https://bitbucket.org/conservancy/kallithea on Bitbucket_ and create a local clone of your own fork. Running tests ------------- After finishing your changes make sure all tests pass cleanly. Install the test dependencies, then run the testsuite by invoking ``py.test`` from the project root:: pip install -r dev_requirements.txt py.test Note that testing on Python 2.6 also requires ``unittest2``. You can also use ``tox`` to run the tests with all supported Python versions (currently Python 2.6--2.7). When running tests, Kallithea uses `kallithea/tests/test.ini` and populates the SQLite database specified there. It is possible to avoid recreating the full test database on each invocation of the tests, thus eliminating the initial delay. To achieve this, run the tests as:: paster serve kallithea/tests/test.ini --pid-file=test.pid --daemon KALLITHEA_WHOOSH_TEST_DISABLE=1 KALLITHEA_NO_TMP_PATH=1 py.test kill -9 $(cat test.pid) In these commands, the following variables are used:: KALLITHEA_WHOOSH_TEST_DISABLE=1 - skip whoosh index building and tests KALLITHEA_NO_TMP_PATH=1 - disable new temp path for tests, used mostly for testing_vcs_operations You can run individual tests by specifying their path as argument to py.test. py.test also has many more options, see `py.test -h`. Some useful options are:: -k EXPRESSION only run tests which match the given substring expression. An expression is a python evaluable expression where all names are substring-matched against test names and their parent classes. Example: -x, --exitfirst exit instantly on first error or failed test. --lf rerun only the tests that failed at the last run (or all if none failed) --ff run all tests but run the last failures first. This may re-order tests and thus lead to repeated fixture setup/teardown --pdb start the interactive Python debugger on errors. -s, --capture=no don't capture stdout (any stdout output will be printed immediately) Contribution guidelines ----------------------- Kallithea is GPLv3 and we assume all contributions are made by the committer/contributor and under GPLv3 unless explicitly stated. We do care a lot about preservation of copyright and license information for existing code that is brought into the project. Contributions will be accepted in most formats -- such as pull requests on Bitbucket, something hosted on your own Kallithea instance, or patches sent by email to the `kallithea-general`_ mailing list. When contributing via Bitbucket, please make your fork of https://bitbucket.org/conservancy/kallithea/ `non-publishing`_ -- it is one of the settings on "Repository details" page. This ensures your commits are in "draft" phase and makes it easier for you to address feedback and for project maintainers to integrate your changes. .. _non-publishing: https://www.mercurial-scm.org/wiki/Phases#Publishing_Repository Make sure to test your changes both manually and with the automatic tests before posting. We care about quality and review and keeping a clean repository history. We might give feedback that requests polishing contributions until they are "perfect". We might also rebase and collapse and make minor adjustments to your changes when we apply them. We try to make sure we have consensus on the direction the project is taking. Everything non-sensitive should be discussed in public -- preferably on the mailing list. We aim at having all non-trivial changes reviewed by at least one other core developer before pushing. Obvious non-controversial changes will be handled more casually. For now we just have one official branch ("default") and will keep it so stable that it can be (and is) used in production. Experimental changes should live elsewhere (for example in a pull request) until they are ready. Coding guidelines ----------------- We don't have a formal coding/formatting standard. We are currently using a mix of Mercurial's (https://www.mercurial-scm.org/wiki/CodingStyle), pep8, and consistency with existing code. Run ``scripts/run-all-cleanup`` before committing to ensure some basic code formatting consistency. We support both Python 2.6.x and 2.7.x and nothing else. For now we don't care about Python 3 compatibility. We try to support the most common modern web browsers. IE9 is still supported to the extent it is feasible, IE8 is not. We primarily support Linux and OS X on the server side but Windows should also work. HTML templates should use 2 spaces for indentation ... but be pragmatic. We should use templates cleverly and avoid duplication. We should use reasonable semantic markup with element classes and IDs that can be used for styling and testing. We should only use inline styles in places where it really is semantic (such as ``display: none``). JavaScript must use ``;`` between/after statements. Indentation 4 spaces. Inline multiline functions should be indented two levels -- one for the ``()`` and one for ``{}``. Variables holding jQuery objects should be named with a leading ``$``. Commit messages should have a leading short line summarizing the changes. For bug fixes, put ``(Issue #123)`` at the end of this line. Use American English grammar and spelling overall. Use `English title case`_ for page titles, button labels, headers, and 'labels' for fields in forms. .. _English title case: https://en.wikipedia.org/wiki/Capitalization#Title_case Template helpers (that is, everything in ``kallithea.lib.helpers``) should only be referenced from templates. If you need to call a helper from the Python code, consider moving the function somewhere else (e.g. to the model). Notes on the SQLAlchemy session ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Each HTTP request runs inside an independent SQLAlchemy session (as well as in an independent database transaction). Database model objects (almost) always belong to a particular SQLAlchemy session, which means that SQLAlchemy will ensure that they're kept in sync with the database (but also means that they cannot be shared across requests). Objects can be added to the session using ``Session().add``, but this is rarely needed: * When creating a database object by calling the constructor directly, it must explicitly be added to the session. * When creating an object using a factory function (like ``create_repo``), the returned object has already (by convention) been added to the session, and should not be added again. * When getting an object from the session (via ``Session().query`` or any of the utility functions that look up objects in the database), it's already part of the session, and should not be added again. SQLAlchemy monitors attribute modifications automatically for all objects it knows about and syncs them to the database. SQLAlchemy also flushes changes to the database automatically; manually calling ``Session().flush`` is usually only necessary when the Python code needs the database to assign an "auto-increment" primary key ID to a freshly created model object (before flushing, the ID attribute will be ``None``). "Roadmap" --------- We do not have a road map but are waiting for your contributions. Refer to the wiki_ for some ideas of places we might want to go -- contributions in these areas are very welcome. Thank you for your contribution! -------------------------------- .. _Weblate: http://weblate.org/ .. _issue tracking: https://bitbucket.org/conservancy/kallithea/issues?status=new&status=open .. _pull requests: https://bitbucket.org/conservancy/kallithea/pull-requests .. _bitbucket: http://bitbucket.org/ .. _mailing list: http://lists.sfconservancy.org/mailman/listinfo/kallithea-general .. _kallithea-general: http://lists.sfconservancy.org/mailman/listinfo/kallithea-general .. _Hosted Weblate: https://hosted.weblate.org/projects/kallithea/kallithea/ .. _wiki: https://bitbucket.org/conservancy/kallithea/wiki/Home