GHSA-cc5h-2pwp-pvcc

### 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):** - `upload_attachment`'s `file_path` flows unsanitized into `open(file_path, 'rb')` — Confluence `src/mcp_atlassian/confluence/attachments.py:477` (from `servers/confluence.py:1294-1369`); Jira `src/mcp_atlassian/jira/attachments.py:386` (from `servers/jira.py:1609-1673`). No `validate_safe_path` / allowlist (contrast the download flow, hardened after CVE-2026-27825).

### Proof of Concept ``` # unauthenticated (no Authorization header) against a default streamable-http deployment: tools/call upload_attachment { "file_path": "/proc/self/environ", "page_id": "<attacker-chosen>" } # then read it back: tools/call download_attachment { ... } # returns the bytes (base64) -> operator's ATLASSIAN token + .env secrets ```

### Impact Unauthenticated arbitrary local file read on the MCP server host — including the operator's Atlassian API token and `.env` secrets — a boundary the Atlassian-scoped tool must not cross, plus unauthenticated use of every read/write Atlassian tool as the operator's (often admin) principal. Distinct from GHSA-xjgw-4wvw-rgm4 (file *write* via download_path) and GHSA-7r34-79r5-rcc9 (URL-header SSRF).

### Suggested fix Require authentication on the streamable-http transport by default (do not fall back to operator global credentials for unauthenticated requests); apply `validate_safe_path`/allowlist to `file_path` in `upload_attachment` as the download flow already does.

### Affected `mcp-atlassian` <= 0.21.1.