GHSA-3wqj-33cg-xc48

Описание

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

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

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

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

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

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:

You should see output like:

Step 2: Send the Exploit Request

Open a second terminal and run this Python script:

Or use curl directly:

Step 3: Verify in Server Logs

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

Understanding the Results

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:

Option 2: Validate model_path Against Allowlist

If custom models must be supported via HTTP:

Option 3: Document Security Considerations

At minimum, add security warnings to the documentation:

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