CVE-2025-53094 is a high-severity vulnerability affecting the ESPAsyncWebServer library, a widely used asynchronous HTTP and WebSocket server implementation for embedded devices such as ESP32, ESP8266, RP2040, and RP2350 microcontrollers. The vulnerability arises from improper neutralization of CRLF (Carriage Return Line Feed) sequences in HTTP header construction and output within the AsyncWebHeader.cpp component. Specifically, versions up to and including 3.7.8 do not sanitize user-supplied input that is incorporated into HTTP header names or values, allowing attackers to inject CR (\r) or LF (\n) characters. This CRLF injection can lead to arbitrary HTTP header or response manipulation, enabling a variety of attacks such as HTTP response splitting, cache poisoning, cross-site scripting (XSS), and session fixation. The vulnerability requires no authentication or user interaction and can be exploited remotely over the network. The CVSS 4.0 base score is 8.7, reflecting its high impact on confidentiality and integrity, with network attack vector, low attack complexity, and no privileges or user interaction required. Although no known exploits are currently reported in the wild, the vulnerability's presence in popular IoT device libraries makes it a significant risk. A fix has been proposed (pull request 211) and is expected in version 3.7.9 of the library, which sanitizes CRLF characters in headers to prevent injection.

For European organizations deploying IoT devices or embedded systems using ESP32, ESP8266, RP2040, or RP2350 microcontrollers with the vulnerable ESPAsyncWebServer library, this vulnerability poses a substantial risk. Exploitation could allow attackers to manipulate HTTP responses, potentially leading to unauthorized access, data leakage, or injection of malicious content into web interfaces. This is particularly critical for industrial control systems, smart building infrastructure, healthcare devices, and consumer IoT products prevalent in Europe. Compromise of such devices can disrupt operations, violate data protection regulations (e.g., GDPR), and damage organizational reputation. Since these devices often operate unattended and are connected to critical networks, the vulnerability could be leveraged as a foothold for lateral movement or to launch further attacks within enterprise or critical infrastructure environments.

European organizations should immediately identify all devices and systems running ESPAsyncWebServer versions 3.7.8 or earlier. They should prioritize upgrading to version 3.7.9 or later once available, which includes the patch neutralizing CRLF injection. Until patches are applied, network-level mitigations such as strict input validation on HTTP headers, deployment of Web Application Firewalls (WAFs) with custom rules to detect CRLF injection patterns, and segmentation of IoT devices from critical networks can reduce exposure. Device manufacturers and integrators should audit their firmware and software stacks for use of this library and coordinate timely updates. Additionally, monitoring HTTP traffic for anomalous header patterns and implementing intrusion detection systems tuned for CRLF injection attempts can help detect exploitation attempts. Finally, organizations should enforce secure development lifecycle practices to prevent similar injection vulnerabilities in future embedded web servers.