Zulip attempts to validate that the regular expressions that admins
enter for linkifiers are well-formatted, and only contain a specific
subset of regex grammar. The process of checking these
properties (via a regex!) can cause denial-of-service via
backtracking.
Furthermore, this validation itself does not prevent the creation of
linkifiers which themselves cause denial-of-service when they are
executed. As the validator accepts literally anything inside of a
`(?P<word>...)` block, any quadratic backtracking expression can be
hidden therein.
Switch user-provided linkifier patterns to be matched in the Markdown
processor by the `re2` library, which is guaranteed constant-time.
This somewhat limits the possible features of the regular
expression (notably, look-head and -behind, and back-references);
however, these features had never been advertised as working in the
context of linkifiers.
A migration removes any existing linkifiers which would not function
under re2, after printing them for posterity during the upgrade; they
are unlikely to be common, and are impossible to fix automatically.
The denial-of-service in the linkifier validator was discovered by
@erik-krogh and @yoff, as GHSL-2021-118.
This commit adds an API to `zproject/urls.py` to edit/update
the realm linkifier. Its helper function to update the
database is added in `zerver/lib/actions.py`.
`zulip.yaml` is documented accordingly as well, clearly
stating that this API updates one linkifier at a time.
The tests are added for the API and helper function which
updates the realm linkifier.
Fixes#10830.
Use backend-endpoint function instead of helper function in
`test_realm_linkifiers.py` so that tests are more end-to-end.
The removed helper function: `do_add_linkifier` is tested in
`zerver/tests/test_events.py`.
Linkifier error message: `Filter not found` is
updated to `Linkifier not found.`.
Similarly, `filter_id` description is updated to:
`The ID of the linkifier that you want to remove.`,
renamed the term `filter` with `linkifier`, in `zulip.yaml`.
* This introduces a new event type `realm_linkifiers` and
a new key for the initial data fetch of the same name.
Newer clients will be expected to use these.
* Backwards compatibility is ensured by changing neither
the current event nor the /register key. The data which
these hold is the same as before, but internally, it is
generated by processing the `realm_linkifiers` data.
We send both the old and the new event types to clients
whenever the linkifiers are changed.
Older clients will simply ignore the new event type, and
vice versa.
* The `realm/filters:GET` endpoint (which returns tuples)
is currently used by none of the official Zulip clients.
This commit replaces it with `realm/linkifiers:GET` which
returns data in the new dictionary format.
TODO: Update the `get_realm_filters` method in the API
bindings, to hit this new URL instead of the old one.
* This also updates the webapp frontend to use the newer
events and keys.
This is a prep change to eventually completely
replace the term "filter" with "linkifier" in
the codebase.
This only renames files. Code changes will be
done in further commits.
