1.6. Security

In this document, we’ll look at the basic security mechanisms in CouchDB: the Admin Party, Basic Authentication, Cookie Authentication; how CouchDB handles users and protects their credentials.

1.6.1. Authentication

The Admin Party

When you start out fresh, CouchDB allows any request to be made by anyone. Create a database? No problem, here you go. Delete some documents? Same deal. CouchDB calls this the Admin Party. Everybody has privileges to do anything. Neat.

While it is incredibly easy to get started with CouchDB that way, it should be obvious that putting a default installation into the wild is adventurous. Any rogue client could come along and delete a database.

A note of relief: by default, CouchDB will listen only on your loopback network interface (127.0.0.1 or localhost) and thus only you will be able to make requests to CouchDB, nobody else. But when you start to open up your CouchDB to the public (that is, by telling it to bind to your machine’s public IP address), you will want to think about restricting access so that the next bad guy doesn’t ruin your admin party.

In our previous discussions, we dropped some keywords about how things without the Admin Party work. First, there’s admin itself, which implies some sort of super user. Then there are privileges. Let’s explore these terms a little more.

CouchDB has the idea of an admin user (e.g. an administrator, a super user, or root) that is allowed to do anything to a CouchDB installation. By default, everybody is an admin. If you don’t like that, you can create specific admin users with a username and password as their credentials.

CouchDB also defines a set of requests that only admin users are allowed to do. If you have defined one or more specific admin users, CouchDB will ask for identification for certain requests:

Creating New Admin User

Let’s do another walk through the API using curl to see how CouchDB behaves when you add admin users.

> HOST="http://127.0.0.1:5984"
> curl -X PUT $HOST/database
{"ok":true}

When starting out fresh, we can add a database. Nothing unexpected. Now let’s create an admin user. We’ll call her anna, and her password is secret. Note the double quotes in the following code; they are needed to denote a string value for the configuration API:

> curl -X PUT $HOST/_config/admins/anna -d '"secret"'
""

As per the _config API’s behavior, we’re getting the previous value for the config item we just wrote. Since our admin user didn’t exist, we get an empty string.

Hashing Passwords

Seeing the plain-text password is scary, isn’t it? No worries, CouchDB doesn’t show up the plain-text password anywhere. It gets hashed right away. The hash is that big, ugly, long string that starts out with -hashed-. How does that work?

  1. Creates a new 128-bit UUID. This is our salt.
  2. Creates a sha1 hash of the concatenation of the bytes of the plain-text password and the salt (sha1(password + salt)).
  3. Prefixes the result with -hashed- and appends ,salt.

To compare a plain-text password during authentication with the stored hash, the same procedure is run and the resulting hash is compared to the stored hash. The probability of two identical hashes for different passwords is too insignificant to mention (c.f. Bruce Schneier). Should the stored hash fall into the hands of an attacker, it is, by current standards, way too inconvenient (i.e., it’d take a lot of money and time) to find the plain-text password from the hash.

But what’s with the -hashed- prefix? When CouchDB starts up, it reads a set of .ini files with config settings. It loads these settings into an internal data store (not a database). The config API lets you read the current configuration as well as change it and create new entries. CouchDB is writing any changes back to the .ini files.

The .ini files can also be edited by hand when CouchDB is not running. Instead of creating the admin user as we showed previously, you could have stopped CouchDB, opened your local.ini, added anna = secret to the admins, and restarted CouchDB. Upon reading the new line from local.ini, CouchDB would run the hashing algorithm and write back the hash to local.ini, replacing the plain-text password. To make sure CouchDB only hashes plain-text passwords and not an existing hash a second time, it prefixes the hash with -hashed-, to distinguish between plain-text passwords and hashed passwords. This means your plain-text password can’t start with the characters -hashed-, but that’s pretty unlikely to begin with.

Note

Since 1.3.0 release CouchDB uses -pbkdf2- prefix by default to sign about using PBKDF2 hashing algorithm instead of SHA1.

Basic Authentication

Now that we have defined an admin, CouchDB will not allow us to create new databases unless we give the correct admin user credentials. Let’s verify:

> curl -X PUT $HOST/somedatabase
{"error":"unauthorized","reason":"You are not a server admin."}

That looks about right. Now we try again with the correct credentials:

> HOST="http://anna:secret@127.0.0.1:5984"
> curl -X PUT $HOST/somedatabase
{"ok":true}

If you have ever accessed a website or FTP server that was password-protected, the username:password@ URL variant should look familiar.

If you are security conscious, the missing s in http:// will make you nervous. We’re sending our password to CouchDB in plain text. This is a bad thing, right? Yes, but consider our scenario: CouchDB listens on 127.0.0.1 on a development box that we’re the sole user of. Who could possibly sniff our password?

If you are in a production environment, however, you need to reconsider. Will your CouchDB instance communicate over a public network? Even a LAN shared with other collocation customers is public. There are multiple ways to secure communication between you or your application and CouchDB that exceed the scope of this documentation. CouchDB as of version 1.1.0 comes with SSL built in.

1.6.2. Authentication Database

You may already note that CouchDB administrators are defined within the config file and are wondering if regular users are also stored there. No, they are not. CouchDB has a special authentication database, named _users by default, that stores all registered users as JSON documents.

This special database is a system database, this means that while it shares the common database API, there are some special security-related constraints applied. Below is listed how the authentication database is different from the other databases.

These draconian rules are necessary since CouchDB cares about its users’ personal information and takes not to disclose it to just anyone. Often, user documents contain system information like login, password hash and roles, apart from sensitive personal information like: real name, email, phone, special internal identifications and more. This is not information that you want to share with the World.

Users Documents

Each CouchDB user is stored in document format. These documents contain several mandatory fields, that CouchDB needs for authentication:

  • _id (string): Document ID. Contains user’s login with special prefix Why org.couchdb.user: prefix?
  • derived_key (string): PBKDF2 key
  • name (string): User’s name aka login. Immutable e.g. you cannot rename existed user - you have to create new one
  • roles (array of string): List of user roles. CouchDB doesn’t provides any builtin roles, so you’re free to define your own depending on your needs. However, you cannot set system roles like _admin there. Also, only administrators may assign roles to users - by default all users have no roles
  • password_sha (string): Hashed password with salt. Used for simple password_scheme
  • password_scheme (string): Password hashing scheme. May be simple or pbkdf2
  • salt (string): Hash salt. Used for simple password_scheme
  • type (string): Document type. Constantly have value user

Additionally, you may specify any custom fields that relate to the target user. This is a good place to store user’s private information because only the target user and CouchDB administrators may browse it.

Why org.couchdb.user: prefix?

The reason there is a special prefix before a user’s login name is to have namespaces that users belong to. This prefix is designed to prevent replication conflicts when you try merging two _user databases or more.

For current CouchDB releases, all users belong to the same org.couchdb.user namespace and this cannot be changed. This may be changed in future releases.

Creating New User

Creating a new user is a very trivial operation. You just need to do a PUT request with user’s data to CouchDB. Let’s create a user with login jan and password apple:

curl -X PUT http://localhost:5984/_users/org.couchdb.user:jan \
     -H "Accept: application/json" \
     -H "Content-Type: application/json" \
     -d '{"name": "jan", "password": "apple", "roles": [], "type": "user"}'

This curl command will produce the following HTTP request:

PUT /_users/org.couchdb.user:jan HTTP/1.1
Accept: application/json
Content-Length: 62
Content-Type: application/json
Host: localhost:5984
User-Agent: curl/7.31.0

And CouchDB responds with:

HTTP/1.1 201 Created
Cache-Control: must-revalidate
Content-Length: 83
Content-Type: application/json
Date: Fri, 27 Sep 2013 07:33:28 GMT
ETag: "1-e0ebfb84005b920488fc7a8cc5470cc0"
Location: http://localhost:5984/_users/org.couchdb.user:jan
Server: CouchDB (Erlang OTP)

{"ok":true,"id":"org.couchdb.user:jan","rev":"1-e0ebfb84005b920488fc7a8cc5470cc0"}

The document was successfully created! The user jan should now exist in our database. Let’s check if this is true:

curl -X POST http://localhost:5984/_session -d 'name=jan&password=apple'

CouchDB should respond with:

{"ok":true,"name":"jan","roles":[]}

This means that the username was recognized and the password’s hash matches with the stored one. If we specify an incorrect login and/or password, CouchDB will notify us with the following error message:

{"error":"unauthorized","reason":"Name or password is incorrect."}

Password Changing

Let’s define what is password changing from the point of view of CouchDB and the authentication database. Since “users” are “documents”, this operation is just updating the document with a special field password which contains the plain text password. Scared? No need to be, the authentication database has a special internal hook on document update which looks for this field and replaces it with the secured hash depending on the chosen password_scheme.

Summarizing the above process - we need to get the document’s content, add the password field with the new password in plain text and then store the JSON result to the authentication database.

curl -X GET http://localhost:5984/_users/org.couchdb.user:jan
{
  "_id": "org.couchdb.user:jan",
  "_rev": "1-e0ebfb84005b920488fc7a8cc5470cc0",
  "derived_key": "e579375db0e0c6a6fc79cd9e36a36859f71575c3",
  "iterations": 10,
  "name": "jan",
  "password_scheme": "pbkdf2",
  "roles": [],
  "salt": "1112283cf988a34f124200a050d308a1",
  "type": "user"
}

Here is our user’s document. We may strip hashes from the stored document to reduce the amount of posted data:

curl -X PUT http://localhost:5984/_users/org.couchdb.user:jan \
     -H "Accept: application/json" \
     -H "Content-Type: application/json" \
     -H "If-Match: 1-e0ebfb84005b920488fc7a8cc5470cc0" \
     -d '{"name":"jan", "roles":[], "type":"user", "password":"orange"}'
{"ok":true,"id":"org.couchdb.user:jan","rev":"2-ed293d3a0ae09f0c624f10538ef33c6f"}

Updated! Now let’s check that the password was really changed:

curl -X POST http://localhost:5984/_session -d 'name=jan&password=apple'

CouchDB should respond with:

{"error":"unauthorized","reason":"Name or password is incorrect."}

Looks like the password apple is wrong, what about orange?

curl -X POST http://localhost:5984/_session -d 'name=jan&password=orange'

CouchDB should respond with:

{"ok":true,"name":"jan","roles":[]}

Hooray! You may wonder why this was so complex - we need to retrieve user’s document, add a special field to it, post it back - where is that one big button that changes the password without worrying about the document’s content? Actually, Futon has one such thing at the bottom right corner if are logged in. Using that will hide all the implementation details described above and keep it real simple for you.

Note

There is no password confirmation for API request: you should implement it on your application layer like Futon does.

Users Public Information

New in version 1.4.

Sometimes users want to share some information with the world. For instance, their contact email to let other users get in touch with them. To solve this problem, but still keep sensitive and private information secured, there is a special configuration option public_fields. In this option you may define a comma-separated lis of users document fields that will be publicly available.

Normally, if you request a user document and you’re not an administrator or document’s owner, CouchDB will respond with 404 Not Found:

curl http://localhost:5984/_users/org.couchdb.user:robert
{"error":"not_found","reason":"missing"}

This response is constant for both cases when user exists or doesn’t exist for security reasons.

Now let’s share the field name. First, setup the public_fields configuration option. Remember, that this action requires administrator privileges. The next command will prompt you for user admin‘s password:

curl -X PUT http://localhost:5984/_config/couch_http_auth/public_fields
-H “Content-Type: application/json” -d ‘“name”’ -u admin

What has changed? Let’s check Robert’s document once again:

curl http://localhost:5984/_users/org.couchdb.user:robert
{"_id":"org.couchdb.user:robert","_rev":"6-869e2d3cbd8b081f9419f190438ecbe7","name":"robert"}

Good news! Now, we may read the field name in every user document without needing to be an administrator. Keep in mind though not to publish sensitive information, especially without user’s consent!

1.6.3. Authorization

Now that you have a few users who can log in, you probably want to set up some restrictions on what actions they can perform based on their identity and their roles. Each database on a CouchDB server can contain its own set of authorization rules that specify which users are allowed to read and write documents, create design documents, and change certain database configuration parameters. The authorization rules are set up by a server admin and can be modified at any time.

Database authorization rules assign a user into one of two classes:

  • members, who are allowed to read all documents and create and modify any document except for design documents.
  • admins, who can read and write all types of documents, modify which users are members or admins, and set certain per-database configuration options.

Note that a database admin is not the same as a server admin – the actions of a database admin are restricted to a specific database.

When a database is first created, there are no members or admins. HTTP requests that have no authentication credentials or have credentials for a normal user are treated as members, and those with server admin credentials are treated as database admins. To change the default permissions, you must create a _security document in the database:

> curl -X PUT http://localhost:5984/mydatabase/_security \
     -u anna:secret \
     -H "Content-Type: application/json" \
     -d '{"admins": { "names": [], "roles": [] }, "members": { "names": ["jan"], "roles": [] } }'

The HTTP request to create the _security document must contain the credentials of a server admin. CouchDB will respond with:

{"ok":true}

The database is now secured against anonymous reads and writes:

> curl http://localhost:5984/mydatabase/
{"error":"unauthorized","reason":"You are not authorized to access this db."}

You declared user “jan” as a member in this database, so he is able to read and write normal documents:

> curl -u jan:apple http://localhost:5984/mydatabase/
{"db_name":"mydatabase","doc_count":1,"doc_del_count":0,"update_seq":3,"purge_seq":0,
"compact_running":false,"disk_size":12376,"data_size":272,"instance_start_time":"1397672867731570",
"disk_format_version":6,"committed_update_seq":3}

If Jan attempted to create a design doc, however, CouchDB would return a 401 Unauthorized error because the username “jan” is not in the list of admin names and the /_users/org.couchdb.user:jan document doesn’t contain a role that matches any of the declared admin roles. If you want to promote Jan to an admin, you can update the security document to add “jan” to the names array under admin. Keeping track of individual database admin usernames is tedious, though, so you would likely prefer to create a database admin role and assign that role to the org.couchdb.user:jan user document:

> curl -X PUT http://localhost:5984/mydatabase/_security \
     -u anna:secret \
     -H "Content-Type: application/json" \
     -d '{"admins": { "names": [], "roles": ["mydatabase_admin"] }, "members": { "names": [], "roles": [] } }'

See the _security document reference page for additional details about specifying database members and admins.

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