MCP Atlassian: Unauthenticated arbitrary local file read via upload_attachment file_path, chained with missing auth on streamable-http transport

Summary

In the documented multi-user HTTP deployment (--transport streamable-http with global operator Atlassian credentials), sooperset/mcp-atlassian exposes all tools to unauthenticated network clients, and the upload_attachment tool reads an attacker-supplied file_path with no path validation. Chained, an unauthenticated network attacker reads arbitrary files on the MCP server host (e.g. /proc/self/environ → the operator's Atlassian API token + .env secrets, ~/.ssh/id_rsa, /etc/passwd) by uploading them to an attacker-chosen page/issue and reading them back.

Details

Missing authentication (transport):

• streamable-http binds 0.0.0.0 by default (src/mcp_atlassian/__init__.py:151).
• The OAuth-proxy auth provider is opt-in (ATLASSIAN_OAUTH_PROXY_ENABLE, default false), so main_mcp is built with auth=None (src/mcp_atlassian/servers/main.py:724-731, 813-817).
• UserTokenMiddleware._parse_auth_header sets auth_validation_error only for a malformed Authorization header; a request with no Authorization header passes through (main.py:416-446, 582-595).
• _get_fetcher then falls through to the global credential fallback using the operator's .env Atlassian token (src/mcp_atlassian/servers/dependencies.py:644-676). check_write_access gates only on read-only mode, not auth → read and write tools reachable.

Arbitrary file read (sink):