CVE-2017-12635

Due to differences in the Erlang-based JSON parser and JavaScript-based
JSON parser, it is possible in Apache CouchDB before 1.7.0 and 2.x before
2.1.1 to submit _users documents with duplicate keys for ‘roles’ used for
access control within the database, including the special case ‘_admin’
role, that denotes administrative users. In combination with CVE-2017-12636
(Remote Code Execution), this can be used to give non-admin users access to
arbitrary shell commands on the server as the database system user. The
JSON parser differences result in behaviour that if two ‘roles’ keys are
available in the JSON, the second one will be used for authorising the
document write, but the first ‘roles’ key is used for subsequent
authorization for the newly created user. By design, users can not assign
themselves roles. The vulnerability allows non-admin users to give
themselves admin privileges.