ESPHome Vulnerability Allows Unauthorized Access to Smart Devices

A critical authentication bypass flaw in ESPHome’s ESP-IDF web server component allows unauthorized users on the same local network to access and control smart devices without any valid credentials.

Discovered and reported by security researcher jesserockz, the vulnerability (CVE-2025-57808) undermines Basic Authentication by accepting empty or partially correct Authorization headers.

Users of ESPHome version 2025.8.0 are urged to upgrade immediately to 2025.8.1 to mitigate the risk.

ESPHome, a popular open-source framework for building firmware for smart home devices, supports HTTP Basic Authentication through its web_server component.

On the ESP-IDF platform, this authentication mechanism fails to correctly validate client-supplied credentials when the Authorization header’s base64 value is empty or truncated.

By comparing only the initial bytes of the supplied string against the expected value, the server grants access if the provided fragment matches the prefix of the legitimate credential.

Even an empty Authorization value bypasses the check entirely, enabling attackers to reach protected endpoints with no username or password knowledge.

Proof of Concept

In a proof-of-concept demonstration, the following ESPHome configuration was used:

textesp32:
  board: ...
  framework:
    type: esp-idf
web_server:
  auth:
    username: user
    password: somereallylongpass

Accessing the device via a browser with correct credentials (user:somereallylongpass) succeeds, as intended. However, by submitting only a substring of the password—such as somereally, some, or even the single character s—attackers can also gain entry.

The flaw is rooted in the implementation of AsyncWebServerRequest::authenticate, which uses the length of the provided header to truncate the comparison rather than verifying the entire credential string.

A more direct bypass can be executed using a simple curl command:

text$ curl -D- http://example.local/
HTTP/1.1 401 Unauthorized
…

$ curl -D- -H 'Authorization: Basic ' http://example.local/
HTTP/1.1 200 OK
…

By sending an empty base64 token after “Basic”, the server erroneously treats this as valid authentication and responds with HTTP 200 OK.

Impact and Risk

This high-severity vulnerability poses a significant risk to any ESPHome-powered device running on ESP-IDF with the web_server component enabled. Attackers with local network access can bypass authentication to:

• Trigger over-the-air (OTA) firmware updates if OTA is enabled, potentially installing malicious firmware.
• Access device configuration and status pages, revealing sensitive information.
• Manipulate device settings or trigger device actions without user consent.

Given that many smart home installations operate entirely within private LANs, attackers may leverage compromised devices or techniques such as ARP spoofing to exploit this flaw remotely, even without knowledge of legitimate credentials.

Mitigations

ESPHome maintainers have released version 2025.8.1, which corrects the authentication logic by requiring the complete base64-encoded credential string to match exactly. All users of ESPHome 2025.8.0 are strongly advised to upgrade to 2025.8.1 as soon as possible.

Until devices are patched, administrators should consider:

• Disabling the web_server component on all ESP-IDF–based ESPHome devices.
• Restricting network access to trusted hosts only, employing VLANs or firewall rules.
• Disabling OTA updates via the web server and switching to alternative update mechanisms.

Network segmentation and strict access controls can reduce exposure, but the only guaranteed remedy is applying the patch.

• Discovery: Reported by jesserockz on GitHub Security Advisory GHSA-mxh2-ccgj-8635, published 18 hours ago.
• Affected Versions: ESPHome 2025.8.0.
• Patched Versions: ESPHome 2025.8.1.
• CVE ID: CVE-2025-57808.
• CVSS v3.1 Base Score: 7.3 (High) [AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N].

The ESPHome web_server authentication bypass is a stark reminder that subtle implementation mistakes can render security measures ineffective.

Smart home operators must remain vigilant, promptly apply updates, and enforce network security best practices to safeguard against unauthorized access.

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