ESPHome Web Server Authentication Bypass Vulnerability Exposes Smart Devices

Cybersecurity

A critical security vulnerability identified in the ESPHome web server component has exposed numerous smart home devices to unauthorized access, effectively bypassing basic authentication protections on ESP-IDF platform implementations.

Technical Specifications

The vulnerability, designated CVE-2025-57808 with a CVSS score of 8.1, affects ESPHome version 2025.8.0. This flaw allows attackers to bypass authentication mechanisms without requiring knowledge of legitimate credentials.

The issue originates from a fundamental logic error in the HTTP basic authentication check within the web_server_idf component of ESPHome.

Attack Mechanism and Technical Exploitation

The vulnerability’s technical foundation lies in improper string comparison logic that processes base64-encoded credentials. The AsyncWebServerRequest::authenticate function only compares bytes up to the length of the client-supplied authorization value, rather than validating the complete credential string.

This flaw creates two primary attack vectors:

• An attacker can bypass authentication by sending an empty authorization header, gaining full access with Authorization: Basic followed by an empty string.
• The system accepts partial password matches, allowing an attacker who discovers a substring of the correct password to authenticate successfully.

Practical exploitation requires minimal technical sophistication. For example, the following command can be used to bypass authentication:

curl -D- -H 'Authorization: Basic ' http://target.local/

This command returns HTTP 200 responses instead of the expected 401 Unauthorized status.

Resolution

ESPHome addressed this critical flaw in version 2025.8.1 by implementing proper credential validation that compares complete authorization strings rather than partial matches. This update is essential for maintaining the security of devices, especially when Over-The-Air (OTA) update functionality is enabled, as it prevents attackers from gaining control over device firmware and configuration settings.