CVE-2026-54571: ESP32ASYNC Espasyncwebserver

High severity, CVSS 8.7. EPSS: 0.5% chance of exploitation in the next 30 days.

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.

Affected products

• ESP32ASYNC Espasyncwebserver: before 3.11.1 (fixed in 3.11.1)

Published 2026-09-17. Last modified 2026-09-24.