CVE-2017-12635 in CouchDB
Summary
by MITRE
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.
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Analysis
by VulDB Data Team • 12/31/2024
The vulnerability described in CVE-2017-12635 represents a critical authorization flaw within Apache CouchDB that stems from inconsistent JSON parsing behavior between its Erlang-based and JavaScript-based parsers. This discrepancy creates a fundamental security weakness that allows authenticated users to manipulate access control mechanisms through carefully crafted user documents. The vulnerability affects versions prior to 1.7.0 for the 1.x series and 2.1.1 for the 2.x series, making it a widespread issue across multiple CouchDB releases. The core problem manifests when users submit _users documents containing duplicate 'roles' keys, which the different parsers handle differently during the document creation and authorization processes.
The technical implementation of this vulnerability exploits a specific parsing inconsistency where the Erlang-based parser processes the first 'roles' key encountered during document creation, while the JavaScript-based parser uses the last 'roles' key for authorization purposes. This creates a scenario where a malicious user can submit a document with two 'roles' fields - the first containing normal user roles, and the second containing administrative privileges including the special '_admin' role. During document write operations, the system uses the second 'roles' key for authorization, allowing the user to appear as an administrator to the system. However, during subsequent authorization checks, the system references the first 'roles' key, which contains the unprivileged roles, creating a mismatch that can be exploited to maintain elevated privileges.
This vulnerability operates under the broader context of privilege escalation and authorization bypass mechanisms, aligning with CWE-284 (Improper Access Control) and CWE-285 (Improper Authorization) classifications within the Common Weakness Enumeration framework. The operational impact extends beyond simple access control manipulation, particularly when combined with CVE-2017-12636 which provides remote code execution capabilities. Security researchers have documented similar patterns in the MITRE ATT&CK framework under techniques such as privilege escalation through application flaws and credential access through manipulation of access control mechanisms. The vulnerability essentially allows any authenticated user to effectively become an administrator by exploiting the parser inconsistency, which represents a severe compromise of the database's security model.
The exploitation of this vulnerability requires minimal privileges and can be executed through standard CouchDB API interactions, making it particularly dangerous in environments where database users have legitimate access to the system. The combination with remote code execution capabilities creates a complete compromise scenario where attackers can execute arbitrary shell commands as the database system user. Organizations should prioritize immediate patching of affected versions, implementing proper input validation for JSON documents, and monitoring for suspicious user creation patterns. Additionally, security teams should consider implementing network segmentation, access control restrictions, and regular vulnerability assessments to prevent exploitation of similar parser-based inconsistencies in other database systems. The vulnerability highlights the critical importance of consistent behavior across different parsing mechanisms in security-sensitive applications and serves as a reminder of the potential for seemingly minor implementation differences to create catastrophic security implications.