CVE-2026-23833: 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 + fieldlength > end in components/api/proto.cpp can overflow when a malicious client sends a large fieldlength 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.
Other sources
ESPHome is a system to control microcontrollers remotely through Home Automation systems. In versions 2025.9.0 through 2025.12.6, an integer overflow in the API component's protobuf decoder allows denial-of-service attacks when API encryption is not used. The bounds check ptr + fieldlength > end in components/api/proto.cpp can overflow when a malicious client sends a large fieldlength 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. Users should upgrade to ESPHome 2025.12.7 or later to receive a patch, enable API encryption with a unique key per device, and follow the Security Best Practices.
— MITRE
Affected Software
Remediation
Patch Available
Event History
Frequently Asked Questions
What is the severity of CVE-2026-23833?
CVE-2026-23833 is classified as a high severity vulnerability due to its potential to cause denial-of-service attacks.
How do I fix CVE-2026-23833?
To fix CVE-2026-23833, upgrade to ESPHome version 2025.12.7 or later.
What software is affected by CVE-2026-23833?
CVE-2026-23833 affects ESPHome versions from 2025.9.0 to 2025.12.6.
Can CVE-2026-23833 be exploited without API encryption?
Yes, CVE-2026-23833 can be exploited when API encryption is not used.
What type of vulnerability is CVE-2026-23833?
CVE-2026-23833 is a denial-of-service vulnerability caused by an integer overflow in the API component's protobuf decoder.