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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

6.8 · CVSS:4.0/AV:A/AC:L/AT:N/PR:H/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-23833: 0.30% estimated exploitation activity in the next 30 days; percentile 22.92 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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…

Published by the source

21 Jan 2026, 01:02 UTC

Last modified by the source

7 Jul 2026, 17:56 UTC

Source last checked

26 Oct 2026, 13:36 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-4h3h-63v6-88qx

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-4h3h-63v6-88qx

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# Patch Next advisory brief: CVE-2026-23833

Advisory: ESPHome vulnerable to denial-of-service via out-of-bounds check bypass in the API component
IDs: CVE-2026-23833, GHSA-4h3h-63v6-88qx
Record: https://patchnext.com/advisories/cve-2026-23833 (revision 3, source checked 26 Oct 2026, 13:36 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: esphome 2025.12.7. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI esphome: affected >= 2025.9.0, < 2025.12.7; fixed in 2025.12.7

## Suggested inspection (read-only)
- `python -m pip show esphome`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on esphome.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-4h3h-63v6-88qx (OSV record GHSA-4h3h-63v6-88qx lists affected ranges: PyPI esphome >=2025.9.0 <2025.12.7 (fixed))
- Verified: https://github.com/advisories/GHSA-4h3h-63v6-88qx (GitHub-reviewed advisory GHSA-4h3h-63v6-88qx lists affected ranges: PyPI esphome >=2025.9.0 <2025.12.7)

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