CVE-2026-40086: Rembg Path Traversal Vulnerability

CVE-2026-40086 Overview

CVE-2026-40086 is a path traversal vulnerability affecting Rembg, a popular tool used to remove image backgrounds. Prior to version 2.0.75, the Rembg HTTP server is vulnerable to unauthenticated remote attacks that allow arbitrary file reading 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.

Critical Impact

Unauthenticated remote attackers can read arbitrary files from servers running vulnerable versions of Rembg, potentially exposing sensitive configuration files, credentials, and application data.

Affected Products

• Rembg versions prior to 2.0.75
• Systems running the Rembg HTTP server component

Discovery Timeline

• April 10, 2026 - CVE-2026-40086 published to NVD
• April 13, 2026 - Last updated in NVD database

Technical Details for CVE-2026-40086

Vulnerability Analysis

This path traversal vulnerability (CWE-22) exists in the Rembg HTTP server's handling of the model_path parameter. The server fails to properly validate and sanitize user-supplied input for this parameter before using it in file system operations. This allows attackers to navigate outside the intended directory structure using path traversal sequences such as ../ to access arbitrary files on the host system.

The vulnerability is particularly concerning because it can be exploited remotely without any authentication. When the server attempts to load the specified file as an ONNX model, error messages may reveal sensitive information about the file system, including whether files exist, their permissions, and potentially portions of their contents.

Root Cause

The root cause of CVE-2026-40086 is insufficient input validation on the model_path parameter in the Rembg HTTP server. The application fails to implement proper path canonicalization and does not restrict file access to a designated safe directory. This allows user-controlled input containing directory traversal characters to escape the intended file system boundary.

Attack Vector

The attack vector for this vulnerability is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Sending an HTTP request to the vulnerable Rembg server
2. Including a specially crafted model_path parameter containing path traversal sequences (e.g., ../../etc/passwd)
3. The server attempts to load the targeted file as an ONNX model
4. Error messages returned by the server may disclose file existence, permissions, or contents

The vulnerability enables reconnaissance of the server's file system structure and can be used to exfiltrate sensitive data such as configuration files, application secrets, or system credentials.

Detection Methods for CVE-2026-40086

Indicators of Compromise

• HTTP requests to the Rembg server containing path traversal patterns (../, ..%2f, ..%5c) in the model_path parameter
• Unusual error messages in server logs indicating failed attempts to load system files as ONNX models
• Access attempts to sensitive files like /etc/passwd, /etc/shadow, or application configuration files
• Network traffic showing responses containing file system error details or unexpected file contents

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block path traversal sequences in request parameters
• Configure intrusion detection systems (IDS) to alert on patterns matching directory traversal attempts
• Monitor Rembg server logs for unusual model_path values or repeated ONNX loading errors
• Deploy file integrity monitoring on sensitive system files to detect unauthorized access attempts

Monitoring Recommendations

• Enable verbose logging on the Rembg HTTP server to capture all incoming requests and parameters
• Set up alerts for requests containing directory traversal indicators in any parameter
• Monitor for abnormal patterns of 4xx or 5xx errors that may indicate exploitation attempts
• Review server access logs regularly for requests targeting common sensitive file paths

How to Mitigate CVE-2026-40086

Immediate Actions Required

• Upgrade Rembg to version 2.0.75 or later immediately, as this version contains the security fix
• If immediate upgrade is not possible, disable or restrict access to the Rembg HTTP server
• Implement network-level access controls to limit who can reach the Rembg server
• Review server logs for evidence of prior exploitation attempts

Patch Information

The vulnerability has been fixed in Rembg version 2.0.75. The security patch is available through the official GitHub repository:

• GitHub Release v2.0.75 - Contains the security fix for this vulnerability
• GitHub Commit - The specific commit addressing the path traversal issue
• GitHub Security Advisory GHSA-3wqj-33cg-xc48 - Official security advisory with additional details

Workarounds

• Place the Rembg HTTP server behind a reverse proxy that filters requests containing path traversal patterns
• Implement IP allowlisting to restrict access to the server to trusted clients only
• Run the Rembg server in a containerized environment with limited file system access
• Use network segmentation to isolate the server from sensitive internal resources