CVE-2026-77246: MCP Atlassian: MCP HTTP Client Server-Local File Exfiltration via Unvalidated Attachment Upload Path

The mcp-atlassian server exposes an MCP tool (confluence_upload_attachment and the Jira attachment variant) that accepts an arbitrary server-side file path and opens it for upload without any path validation. When the server is deployed in HTTP transport mode (streamable-http or sse), a remote, unauthenticated attacker can supply attacker-controlled Atlassian service headers (X-Atlassian-Confluence-Url / X-Atlassian-Confluence-Personal-Token) to redirect the upload to an attacker-controlled endpoint, then pass an arbitrary file_path (e.g. /etc/passwd, ~/.env, SSH private keys, cloud credentials) to exfiltrate any file readable by the server process. No prior account, session token, or Authorization header is required. The vulnerability was confirmed through both static code analysis (Phase 1) and a live Docker-based proof-of-concept (Phase 2).