# CVE-2026-42854: arduino-esp32: Stack buffer overflow in WebServer multipart boundary parsing leads to remote crash potential RCE

> Remote attackers can trigger a stack buffer overflow in the WebServer multipart parser, potentially causing crashes or executing arbitrary code on ESP32 devices.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 12 May 2026
- **Updated:** 13 May 2026
- **Also known as:** GHSA-8cmm-3887-r32j

## What is CVE-2026-42854?

CVE-2026-42854 is a critical memory corruption vulnerability in espressif arduino-esp32, published on 12 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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.

Weakness: CWE-121 (Stack-based Buffer Overflow)

## Which versions are affected by CVE-2026-42854?

It affects 1 product.


Products: Espressif Arduino-Esp32.

## How do I fix CVE-2026-42854?

No fix has been published for CVE-2026-42854 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-42854 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-42854 exists.

## Other vulnerabilities in espressif arduino-esp32

- [CVE-2026-41429](https://insights.sbomify.com/CVE-2026-41429/): Improper validation of NBNS name_len in arduino-esp32 NetBIOS leads to memory corruption (high 8.8)
- [CVE-2026-42855](https://insights.sbomify.com/CVE-2026-42855/): arduino-esp32: Digest authentication URI mismatch bypass in WebServer allows cross-resource replay attack (high 7.5)
- [CVE-2019-12586](https://insights.sbomify.com/CVE-2019-12586/): Denial of service in n/a (medium 6.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-42854 and fix it if it is affected.

What it is: CVE-2026-42854 is a critical memory corruption vulnerability in espressif arduino-esp32, published on 12 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-42854 exists.

Affected product: espressif arduino-esp32. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-42854/ (Markdown: https://insights.sbomify.com/CVE-2026-42854/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-42854
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-42854
- OSV: https://osv.dev/vulnerability/CVE-2026-42854

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-42854/
Summaries are generated and may contain mistakes; the official description above is the record.