zulip/docs/production/security-model.md

Security Model

This section attempts to document the Zulip security model. Since this is new documentation, it likely does not cover every issue; if there are details you're curious about, please feel free to ask questions on the Zulip development mailing list (or if you think you've found a security bug, please report it to zulip-security@googlegroups.com so we can do a responsible security announcement).

Secure your Zulip server like your email server

• It's reasonable to think about security for a Zulip server like you do security for a team email server -- only trusted administrators within an organization should have shell access to the server.

  In particular, anyone with root access to a Zulip application server or Zulip database server, or with access to the zulip user on a Zulip application server, has complete control over the Zulip installation and all of its data (so they can read messages, modify history, etc.). It would be difficult or impossible to avoid this, because the server needs access to the data to support features expected of a group chat system like the ability to search the entire message history, and thus someone with control over the server has access to that data as well.

Encryption and Authentication

• Traffic between clients (web, desktop and mobile) and the Zulip is encrypted using HTTPS. By default, all Zulip services talk to each other either via a localhost connection or using an encrypted SSL connection.

• Zulip requires CSRF tokens in all interactions with the web API to prevent CSRF attacks.

• The preferred way to login to Zulip is using an SSO solution like Google Auth, LDAP, or similar, but Zulip also supports password authentication. See the authentication methods documentation for details on Zulip's available authentication methods.

Passwords

Zulip stores user passwords using the standard PBKDF2 algorithm.

When the user is choosing a password, Zulip checks the password's strength using the popular zxcvbn library. Weak passwords are rejected, and strong passwords encouraged. The minimum password strength allowed is controlled by two settings in /etc/zulip/settings.py:

• PASSWORD_MIN_LENGTH: The minimum acceptable length, in characters. Shorter passwords are rejected even if they pass the zxcvbn test controlled by PASSWORD_MIN_GUESSES.

• PASSWORD_MIN_GUESSES: The minimum acceptable strength of the password, in terms of the estimated number of passwords an attacker is likely to guess before trying this one. If the user attempts to set a password that zxcvbn estimates to be guessable in less than PASSWORD_MIN_GUESSES, then Zulip rejects the password.

  By default, PASSWORD_MIN_GUESSES is 10000. This provides significant protection against online attacks, while limiting the burden imposed on users choosing a password.

  Estimating the guessability of a password is a complex problem and impossible to efficiently do perfectly. For background or when considering an alternate value for this setting, the article "Passwords and the Evolution of Imperfect Authentication" is recommended. The 2016 zxcvbn paper adds useful information about the performance of zxcvbn, and a large 2012 study of Yahoo users is informative about the strength of the passwords users choose.

Messages and History

• Zulip message content is rendered using a specialized Markdown parser which escapes content to protect against cross-site scripting attacks.

• Zulip supports both public streams and private ("invite-only") streams. Any Zulip user can join any public stream in the realm, and can view the complete message history of any public stream without joining the stream.

• A private ("invite-only") stream is hidden from users who are not subscribed to the stream.

  • Users who are not members of a private stream cannot read messages on the stream, send messages to the stream, or join the stream, even if they are a Zulip organization administrator.
  • Any member of a private stream can add other users to the stream. This is the only way that one can join a private stream (even organization administrators cannot join a private stream without being added by an existing member).
  • When a new user joins a private stream, they can see future messages sent to the stream, but they do not receive access to the stream's message history.
  • Organization administrators can do some basic management of private streams that they are not subscribed to: Changing the stream name and description, viewing the current subscribers, and removing subscribers.

• Zulip supports editing the content and topics of messages that have already been sent. As a general philosophy, our policies provide hard limits on the ways in which message content can be changed or undone. In contrast, our policies around message topics favor usefulness (e.g. for conversational organization) over faithfulness to the original.

  The message editing policy can be configured on the /#organization page. There are three configurations provided out of the box: (i) users cannot edit messages at all, (ii) users can edit any message they have sent, and (iii) users can edit the content of any message they have sent in the last N minutes, and the topic of any message they have sent. In (ii) and (iii), topic edits can also be propagated to other messages with the same original topic, even if those messages were sent by other users. The default setting is (iii), with N = 10.

  In addition, and regardless of the configuration above, messages with no topic can always be edited to have a topic, by anyone in the organization, and the topic of any message can also always be edited by a realm administrator.

  Also note that while edited messages are synced immediately to open browser windows, editing messages is not a safe way to redact secret content (e.g. a password) shared unintentionally. Other users may have seen and saved the content of the original message, or have an integration (e.g. push notifications) forwarding all messages they receive to another service. Zulip stores the edit history of messages, but it may or may not be available to clients, depending on an organization-level setting.

Users and Bots

• There are three types of users in a Zulip realm: Administrators, normal users, and bots. Administrators have the ability to deactivate and reactivate other human and bot users, delete streams, add/remove administrator privileges, as well as change configuration for the overall realm (e.g. whether an invitation is required to join the realm). Being a Zulip administrator does not provide the ability to interact with other users' private messages or the messages sent to private streams to which the administrator is not subscribed. However, a Zulip administrator subscribed to a stream can toggle whether that stream is public or private. Also, Zulip realm administrators have administrative access to the API keys of all bots in the realm, so a Zulip administrator may be able to access messages sent to private streams that have bots subscribed, by using the bot's credentials.

  In the future, Zulip's security model may change to allow realm administrators to access private messages (e.g. to support auditing functionality).

• Every Zulip user has an API key, available on the settings page. This API key can be used to do essentially everything the user can do; for that reason, users should keep their API key safe. Users can rotate their own API key if it is accidentally compromised.

• To properly remove a user's access to a Zulip team, it does not suffice to change their password or deactivate their account in the SSO system, since neither of those prevents authenticating with the user's API key or those of bots the user has created. Instead, you should deactivate the user's account in the "Organization settings" interface (/#organization); this will automatically also deactivate any bots the user had created.

• The Zulip mobile apps authenticate to the server by sending the user's password and retrieving the user's API key; the apps then use the API key to authenticate all future interactions with the site. Thus, if a user's phone is lost, in addition to changing passwords, you should rotate the user's Zulip API key.

• Zulip bots are used for integrations. A Zulip bot can do everything a normal user in the realm can do including reading other, with a few exceptions (e.g. a bot cannot login to the web application or create other bots). In particular, with the API key for a Zulip bot, one can read any message sent to a public stream in that bot's realm. A likely future feature for Zulip is limited bots that can only send messages.

• Certain Zulip bots can be marked as "API super users"; these special bots have the ability to send messages that appear to have been sent by another user (an important feature for implementing integrations like the Jabber, IRC, and Zephyr mirrors). They also have the ability to see the names of all streams (including private streams).

  They can only be created on the command line (with manage.py knight --permission=api_super_user).

User-uploaded content

• Zulip supports user-uploaded files. Ideally they should be hosted from a separate domain from the main Zulip server to protect against various same-domain attacks (e.g. zulip-user-content.example.com).

  We support two ways of hosting them: the basic LOCAL_UPLOADS_DIR file storage backend, where they are stored in a directory on the Zulip server's filesystem, and the S3 backend, where the files are stored in Amazon S3. It would not be difficult to add additional supported backends should there be a need; see zerver/lib/upload.py for the full interface.

  For both backends, the URLs used to access uploaded files are long, random strings, providing one layer of security against unauthorized users accessing files uploaded in Zulip (an authorized user would need to share the URL with an unauthorized user in order for the file to be accessed by the unauthorized user; and of course, any such authorized user could have just downloaded and sent the file instead of the URL, so this is arguably the best protection possible). However, to help protect against consequences accidental sharing of URLs to restricted files (e.g. by forwarding a missed-message email or leaks involving the Referer header), we provide additional layers of protection in both backends as well.

  In the Zulip S3 backend, the random URLs to access files that are presented to users don't actually host the content. Instead, the S3 backend verifies that the user has a valid Zulip session in the relevant realm (and that has access to a Zulip message linking to the file), and if so, then redirects the browser to a temporary S3 URL for the file that expires a short time later. In this way, possessing a URL to a secret file in Zulip does not provide unauthorized users with access to that file.

  We have a similar protection for the LOCAL_UPLOADS_DIR backend, that is currently only available in Ubuntu Xenial (this is the one place in Zulip where behavior is currently different between Ubuntu Trusty and Ubuntu Xenial). On Ubuntu Xenial, every access to an uploaded file has access control verified verified (confirming that the browser is logged into a Zulip account that has received the uploaded file in question).

  On Ubuntu Trusty, because the older version of nginx available there doesn't have proper Unicode support for the X-Accel-Redirect feature, the LOCAL_UPLOADS_DIR backend only has the single layer of security described at the beginning of this section (long, randomly generated secret URLs). This could be fixed with further engineering, but given the upcoming end-of-life of Ubuntu Trusty, we have no plans to do that further work.

• Zulip supports using the Camo image proxy to proxy content like inline image previews that can be inserted into the Zulip message feed by other users over HTTPS.

• By default, Zulip will provide image previews inline in the body of messages when a message contains a link to an image. You can control this using the INLINE_IMAGE_PREVIEW setting.

Final notes and security response

If you find some aspect of Zulip that seems inconsistent with this security model, please report it to zulip-security@googlegroups.com so that we can investigate and coordinate an appropriate security release if needed.

Zulip security announcements will be sent to zulip-announce@googlegroups.com, so you should subscribe if you are running Zulip in production.