High · CVSS 8.7CVE-2026-54571

ESPAsyncWebServer: Integer overflow in multipart boundary parser causes denial of serviceCVE-2026-54571

Answer in brief

CVE-2026-54571 records a High severity (CVSS 8.7) vulnerability in ESPAsyncWebServer: Integer overflow in multipart boundary parser causes denial of service. The current sources do not mark it as known exploited. The current feed maps ESP32Async/ESPAsyncWebServer (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Sep 17, 2026Updated Sep 24, 2026Source checked Nov 1, 2026First seen by HOL Sep 17, 2026Material review Sep 24, 2026

Upstream Advisory

Key facts

Risk: High · CVSS 8.7
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Available

Why this deserves its current priority

CVSS is 8.7. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps ESP32Async/ESPAsyncWebServer (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
ESP32Async/ESPAsyncWebServergeneric	<3.11.1	3.11.1

Recommended response

Check inventory. Check lockfiles and deployed manifests for ESP32Async/ESPAsyncWebServer.
Review the reported fix. Update ESP32Async/ESPAsyncWebServer to 3.11.1 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
Sep 17, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Sep 24, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Sep 17, 2026

Sources and claim methodology

Upstream source 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.

Quoted source text, attributed separately from HOL analysis.