zulip/docs/production/security-model.md

Security Model

This section attempts to document the Zulip security model. It likely does not cover every issue; if there are details you're curious about, please feel free to ask questions in #production help on the Zulip community server (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. See password strength for an extended discussion on how we chose this value.

  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 streams.

  • Any non-guest user can join any public stream in the organization, and can view the complete message history of any public stream without joining the stream. Guests can only access streams that another user adds them to.

  • Organization admins can see and modify most aspects of a private stream, including the membership and estimated traffic. Admins generally cannot see messages sent to private streams or do things that would indirectly give them access to those messages, like adding members or changing the stream privacy settings.

  • Non-admins cannot easily see which private streams exist, or interact with them in any way until they are added. Given a stream name, they can figure out whether a stream with that name exists, but cannot see any other details about the stream.

  • See Stream permissions for more details.

• 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. In all configurations:

  • Message content can only ever be modified by the original author.

  • Any message visible to an organization administrator can be deleted at any time by that administrator.

  • See Configuring message editing and deletion for more details.

Users and Bots

• There are four types of users in a Zulip organization: Organization Administrators, Members (normal users), Guests, 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 organization.

  Being an organization administrator does not generally provide the ability to read other users' private messages or messages sent to private streams to which the administrator is not subscribed. There are two exceptions:

  • Administrators may get access to private messages via some types of data export.

  • Administrators can change the ownership of a bot. If a bot is subscribed to a private stream, then an administrator can indirectly get access to stream messages by taking control of the bot, though the access will be limited to what the bot can do. (E.g. incoming webhook bots cannot read messages.)

• 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 a 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 via Zulip's "Organization settings" interface.

• 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.

• Guest users are like Members, but they do not have automatic access to public streams.

• Zulip supports several kinds of bots with different capabilities.

  • Incoming webhook bots can only send messages into Zulip.

  • Outgoing webhook bots and Generic bots can essentially do anything a non-administrator user can, with a few exceptions (e.g. a bot cannot login to the web application, register for mobile push notifications, or create other bots).

  • API super user bots can send messages that appear to have been sent by another user. They also have the ability to see the names of all streams, including private streams. This is important for implementing integrations like the Jabber, IRC, and Zephyr mirrors.

    API super user bots cannot be created by Zulip users, including organization administrators. They can only be created on the command line (via 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. Of course, any such authorized user could have just downloaded and sent the file instead of the URL, so this is arguably pretty good protection.) However, to help protect against 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 organization (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. Every access to an uploaded file has access control verified (confirming that the browser is logged into a Zulip account that has received the uploaded file in question).

• 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.