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

CVSS 3.1

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:L

Severity from

GitHub (reviewed advisory)

Weakness

CWE-290, CWE-345

Also known as

CVE-2026-105741

• github.com/langflow-ai/langflow/security/advisories/GHSA-4f6c-2vvp-gw82
• nvd.nist.gov/vuln/detail/CVE-2026-105741
• github.com/langflow-ai/langflow/pull/13915
• github.com/langflow-ai/langflow/commit/1f39a4b9d62c9dfa1b21fa7f85e23a180c351b72
• github.com/langflow-ai/langflow/commit/94859df33acd70b2a1f816e26d68f5e89a7e5639
• github.com/langflow-ai/langflow
• github.com/langflow-ai/langflow/releases/tag/v1.10.3