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Arduino Esp32Application · Espressif

CVE-2026-42854

CRITICAL · 9.8 CVSS v3.1 Published 2026-05-12
Fix available
A fix is available. Upgrade to 3.3.8 or later.
See remediation →
100/100
Remediation priority · Urgent
Public exploit Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
arduino-esp32 is an Arduino core for the ESP32, ESP32-S2, ESP32-S3, ESP32-C3, ESP32-C6 and ESP32-H2 microcontrollers. Prior to 3.3.8, the WebServer multipart form parser in arduino-esp32 allocates a Variable Length Array (VLA) on the stack whose size is derived from an attacker-controlled HTTP header field (Content-Type: multipart/form-data; boundary=...) without enforcing any length limit. Sending a boundary string longer than ~8000 characters overflows the 8192-byte task stack of the loopTask, causing a crash and potential remote code execution. This vulnerability is fixed in 3.3.8.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

The arduino-esp32 WebServer component contains a stack-based buffer overflow vulnerability where the multipart form parser allocates a Variable Length Array (VLA) using size derived from the attacker-controllable boundary parameter in the Content-Type HTTP header. No length validation is performed, allowing an attacker to trigger stack overflow by sending a boundary string exceeding ~8000 characters, crashing the device or achieving RCE.

MitigationUpdate arduino-esp32 core to version 3.3.8 or later, which enforces length limits on the boundary parameter. Consider additional input validation and stack canaries as defense-in-depth.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Arduino Esp32Application
Affected:< 3.3.8

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify arduino-esp32 WebServer usage
    Search your project source code for '#include <WebServer.h>' or '#include <AsyncTCP.h>' with WebServer instantiation. This indicates the WebServer component is included in your sketch.
    Affected if The project includes WebServer.h and handles HTTP requests.
  2. Check arduino-esp32 core version
    In PlatformIO: run 'pio pkg list' and check the 'espressif32' package version. In Arduino IDE: Tools > Board > Board Manager, search for 'ESP32' and check the installed version number.
    Affected if The installed espressif32 core version is lower than 3.3.8.
  3. Identify multipart form handling code
    Search your project source code for file upload handling patterns: 'server.on' with 'HTTP_POST', or functions named 'upload', 'handleFileUpload', or multipart form handlers. Also search for 'multipart/form-data' in comments or string literals.
    Affected if The code explicitly handles multipart form submissions via WebServer upload callbacks.
  4. Verify WebServer is exposed to network
    Check your device's network configuration. In your sketch, locate the WiFi or Ethernet server start calls (WiFiServer, EthernetServer) and confirm the device IP is accessible from untrusted networks.
    Affected if The ESP32 WebServer is accessible from network interfaces that are not restricted by firewall rules.

You are affected if your device runs arduino-esp32 core version below 3.3.8 AND implements WebServer with multipart form handling exposed to network attackers.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 3.3.8 or later
Fixed in 3.3.8
Interim mitigation

Update arduino-esp32 core to version 3.3.8 or later, which enforces length limits on the boundary parameter. Consider additional input validation and stack canaries as defense-in-depth.

Recommended fix High confidence

3.3.8 or later

  1. 1. Identify the current arduino-esp32 version installed in your Arduino IDE or platform (check via Boards Manager or platformio.ini if using PlatformIO)
  2. 2. Open Boards Manager, search for 'ESP32', and select version 3.3.8 or later for upgrade
  3. 3. If using PlatformIO, update the platformio.ini board version to a release >=3.3.8
  4. 4. After upgrade, rebuild all sketches that use the WebServer class
  5. 5. Redeploy firmware to all affected ESP32 devices

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Arduino Esp32 Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing6.0 h
  • Review / QA2.0 h
14.0 hours of engineering $2,380
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-42854 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data