Description
Due to differences in CouchDB's Erlang-based JSON parser and JavaScript-based JSON parser, it is possible 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.
Remediation
Upgrade to the latest version of CouchDB
References
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