CVE-2026-105741: Langflow: IP Spoofing Bypass via `X-Forwarded-For` Allowing Remote Configuration Write

**Description**: ### Summary An IP spoofing vulnerability in the Model Context Protocol (MCP) configuration installation endpoint (`POST /api/v1/mcp/project/{project_id}/install`) allowed authenticated remote attackers to bypass the "local-only" access restriction. By sending a spoofed `X-Forwarded-For: 127.0.0.1` header, an attacker could make the server treat the request as originating from localhost, letting them write/overwrite an MCP client configuration file on the server's filesystem. ### Details The vulnerability lived in the `get_client_ip` helper, used to enforce the local-only gate for `install_mcp_config`. It trusted the leftmost (fully client-controlled) entry of `X-Forwarded-For` unconditionally, with no check for whether the request had actually passed through a trusted proxy. **Vulnerable code (introduced by commit `d3d06be8e5`, first released in v1.5.0):** `src/backend/base/langflow/api/v1/mcp_projects.py` ```python def get_client_ip(request: Request) -> str: # Check for X-Forwarded-For header (common when behind proxies) forwarded_for = request.headers.get("X-Forwarded-For") if forwarded_for: # The client IP is the first one in the list return forwarded_for.split(",")[0].strip() if request.client: return request.client.host return "255.255.255.255" @router.post("/{project_id}/install") async def install_mcp_config( project_id: UUID, body: MCPInstallRequest, # {client: str, transport: "sse" | "streamablehttp" | None} request: Request, current_user: CurrentActiveMCPUser, ): client_ip = get_client_ip(request) if not is_local_ip(client_ip): raise HTTPException(status_code=500, detail="MCP configuration can only be installed from a local connection") ... ``` **Correction vs. the original report:** the request body accepted by this endpoint is `MCPInstallRequest {client: str, transport: str | None}` (`src/backend/base/langflow/api/v1/schemas/__init__.py`). There is no `mcp_path` field, and the destination path is never attacker-supplied. `install_mcp_config` resolves the write target itself, via `get_config_path(body.client)`, to one of a fixed, small set of well-known per-OS developer-tool config paths under the server process's home directory: `~/.cursor/mcp.json` (Cursor), `~/.codeium/windsurf/mcp_config.json` (Windsurf), or the Claude Desktop config (`~/Library/Application Support/Claude/claude_desktop_config.json` on macOS, `%APPDATA%\Claude\claude_desktop_config.json` on Windows/WSL). The impact is therefore "attacker-influenced content written into one of these fixed files," not an arbitrary-path write. ### PoC 1. Authenticate to obtain a valid access token. 2. Identify a `project_id` the attacker has access to. 3. Send: ```bash curl -X POST "http://<server-ip>:7860/api/v1/mcp/project/<project_id>/install" \ -H "Authorization: Bearer <token>" \ -H "X-Forwarded-For: 127.0.0.1" \ -H "Content-Type: application/json" \ -d '{"client": "cursor"}' ``` 4. The server returns `200 OK` and writes/overwrites `~/.cursor/mcp.json` on the host with an attacker-influenced MCP server entry, despite the request originating from a remote, non-local address. ### Impact Authenticated Remote Configuration Write to one of a fixed set of IDE/MCP client config files on the host. Could be leveraged to: - Inject a malicious MCP server definition into Cursor/Windsurf/Claude Desktop config, so a local developer who later opens that IDE on the host connects to an attacker-controlled MCP server. - Disrupt or corrupt the existing MCP configuration for those tools. - Bypass an intended network-boundary control ("local-only"). ### Status: already fixed This exact bypass (single-line, comma-separated `X-Forwarded-For` spoofing, default configuration) is fixed as of: - **Fix PR:** langflow-ai/langflow#13915 — "fix(security): stop trusting X-Forwarded-For for the MCP install locality check", landed as part of the broader hardening effort in langflow-ai/langflow#13530. - **Fix:** `get_client_ip` now uses the real TCP peer (`request.client.host`) by default and ignores `X-Forwarded-For` entirely unless the operator has explicitly opted in via the `rate_limit_trust_proxy` setting (default `False`); when opted in, it takes the **rightmost** entry, mirroring `langflow.services.rate_limit.service.get_client_ip`. - **Released in:** v1.11.0, and backported to v1.10.3 (langflow-ai/langflow#14071). - **Related follow-up:** a narrower, related bypass — reachable only when an operator has explicitly set `rate_limit_trust_proxy=true` behind a proxy that emits `X-Forwarded-For` as repeated header lines rather than a single comma-separated line (e.g. HAProxy's `option forwardfor`) — was separately closed by langflow-ai/langflow#14425, released in v1.11.3. This does not affect default deployments (`rate_limit_trust_proxy` defaults to `False`). - This report is a near-duplicate of GHSA-qvvj-g573-9638, which describes the same root cause and is fixed by the same PR. ### Affected versions - The vulnerable endpoint/helper was introduced in **v1.5.0** (langflow-ai/langflow#8271, "add one click install to mcp servers on specific clients", 2025-07-08). Versions prior to v1.5.0 do not contain this endpoint and are not affected by this issue. - **Vulnerable:** >= 1.5.0, < 1.10.3 (and < 1.11.0 on mainline). - **Fixed:** v1.10.3 (backport) and v1.11.0 onward.