# CVE-2026-54571: ESPAsyncWebServer: Integer overflow in multipart boundary parser causes denial of service

> Remote attackers can cause denial of service on ESP32/ESP8266 devices by sending a 256-byte multipart boundary, triggering an infinite loop and watchdog reset.

- **Severity:** High 8.7 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 17 Sep 2026
- **Updated:** 21 Sep 2026
- **Also known as:** GHSA-4phx-fcj6-46r4

## What is CVE-2026-54571?

CVE-2026-54571 is a high-severity vulnerability in ESP32Async ESPAsyncWebServer, published on 17 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

ESPAsyncWebServer is an asynchronous HTTP and WebSocket server library for ESP32, ESP8266, RP2040 and RP2350. Prior to 3.11.1, the multipart/form-data parser in src/WebRequest.cpp stores _boundaryPosition as an 8-bit value while _parseMultipartPostByte processes the boundary. A remote request containing an exactly 256-byte multipart boundary wraps _boundaryPosition from 255 to zero, prevents the boundary parsing loop from terminating, consumes excessive CPU, and triggers a FreeRTOS watchdog reset on affected ESP32 or ESP8266 devices. This issue is fixed in version 3.11.1.

Weakness: CWE-190 (Integer Overflow or Wraparound)

## Which versions are affected by CVE-2026-54571?


## How do I fix CVE-2026-54571?

No fix has been published for CVE-2026-54571 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-54571 being exploited?

There is no known exploitation of CVE-2026-54571 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-54571 and fix it if it is affected.

What it is: CVE-2026-54571 is a high-severity vulnerability in ESP32Async ESPAsyncWebServer, published on 17 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-54571 in the wild and no public exploit code.

Affected product: ESP32Async ESPAsyncWebServer. 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-2026-54571/ (Markdown: https://insights.sbomify.com/CVE-2026-54571/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-54571
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-54571
- OSV: https://osv.dev/vulnerability/CVE-2026-54571

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-54571/
Summaries are generated and may contain mistakes; the official description above is the record.