ESPHome vulnerable to denial-of-service via out-of-bounds check bypass in the API component

Summary

An integer overflow in the API component's protobuf decoder allows denial-of-service attacks when API encryption is not used.

Details

The bounds check ptr + field_length > end in components/api/proto.cpp can overflow when a malicious client sends a large field_length value. This affects all ESPHome device platforms (ESP32, ESP8266, RP2040, LibreTiny). The overflow bypasses the out-of-bounds check, causing the device to read invalid memory and crash.

When using the plaintext API protocol, this attack can be performed without authentication. When noise encryption is enabled, knowledge of the encryption key is required.

Affected Versions

ESPHome 2025.9.0 through 2025.12.6

Mitigation

• Upgrade to ESPHome 2025.12.7 or later (or 2026.1.0b3 or later)
• Enable API encryption with a unique key per device
• Follow the Security Best Practices

Severity

Low - Users following Security Best Practices with API encryption enabled are not affected without knowledge of the encryption key.

Impact

Denial-of-service. An attacker with network access to port 6053 can crash and reboot the device.

Credits

Thanks to @Mat931 for responsibly reporting this vulnerability.