CVE-2021-43523 is a critical vulnerability affecting versions of uClibc and uClibc-ng prior to 1.0.39. These are lightweight C standard libraries commonly used in embedded systems and resource-constrained environments. The vulnerability arises from improper handling of special characters in domain names returned by DNS servers when functions such as gethostbyname, getaddrinfo, gethostbyaddr, and getnameinfo are called. These functions are part of the stub resolver implementation responsible for translating domain names to IP addresses and vice versa. The core issue is the absence of a validation step that should sanitize or verify the domain name strings returned by DNS responses. As a result, maliciously crafted DNS responses containing special characters can cause the resolver to output incorrect hostnames, potentially leading to domain hijacking scenarios. Furthermore, these malformed domain names can be injected into applications that use these resolver functions, enabling remote code execution, cross-site scripting (XSS), application crashes, or other forms of exploitation. The vulnerability is classified under CWE-79 (Improper Neutralization of Input During Web Page Generation), highlighting the risk of injection attacks. The CVSS v3.1 base score is 9.6 (critical), reflecting the high impact on confidentiality, integrity, and availability, with no privileges required and low attack complexity, although user interaction is needed. No known exploits are publicly reported yet, but the potential for severe impact remains significant, especially in embedded devices and networked applications relying on these libraries for DNS resolution.

For European organizations, the impact of CVE-2021-43523 can be substantial, particularly for industries relying on embedded systems such as telecommunications, industrial control systems, IoT devices, and network infrastructure equipment. Compromised DNS resolution can lead to domain hijacking, redirecting traffic to malicious servers, which can facilitate data theft, espionage, or malware distribution. Injection vulnerabilities may allow attackers to execute arbitrary code remotely or cause application crashes, disrupting critical services. Given the widespread use of uClibc and uClibc-ng in embedded Linux environments, organizations with operational technology (OT) networks or IoT deployments are at heightened risk. The vulnerability can undermine trust in DNS responses, affecting secure communications and potentially enabling man-in-the-middle attacks. The critical severity and the potential for remote exploitation without privileges make this a pressing concern for European entities managing critical infrastructure, telecommunications, and connected devices.

To mitigate CVE-2021-43523, European organizations should: 1) Identify all systems and devices using uClibc or uClibc-ng libraries, especially embedded and IoT devices. 2) Upgrade affected systems to uClibc-ng version 1.0.39 or later, where the vulnerability is patched. 3) Where upgrades are not immediately feasible, implement network-level DNS filtering and validation to block or flag suspicious DNS responses containing special characters or malformed domain names. 4) Employ application-level input validation and sanitization for any data derived from DNS resolution functions to prevent injection attacks. 5) Monitor DNS traffic and application logs for anomalies indicative of exploitation attempts, such as unexpected domain names or application crashes. 6) Collaborate with vendors of embedded devices to ensure timely firmware updates incorporating the patched libraries. 7) Consider deploying DNS security extensions (DNSSEC) to enhance the authenticity and integrity of DNS responses, reducing the risk of malicious DNS data injection. These steps go beyond generic advice by focusing on embedded systems inventory, network DNS traffic controls, and vendor coordination.