CVE-2021-43824 is a medium-severity vulnerability affecting the Envoy proxy, an open-source edge and service proxy widely used in cloud-native applications for handling network traffic and service mesh architectures. The vulnerability is a NULL pointer dereference (CWE-476) that occurs when Envoy processes a specially crafted CONNECT request targeting the JWT (JSON Web Token) filter configured with regex matching. Specifically, when a CONNECT request is sent to Envoy with the JWT filter enabled and using regex match conditions, the code path fails to properly handle certain inputs, leading to a NULL pointer dereference. This results in a crash of the Envoy process, effectively causing a denial of service (DoS) condition. The affected versions include multiple releases spanning from versions prior to 1.18.6 up to versions before 1.21.1, with specific vulnerable ranges being >=1.19.0 and <1.19.3, >=1.20.0 and <1.20.2, and >=1.21.0 and <1.21.1. The vulnerability does not require authentication or user interaction beyond sending a crafted CONNECT request, which can be performed remotely. There are no known exploits in the wild at the time of reporting, but the impact is a straightforward DoS attack vector. The only workaround is to disable regex matching in the JWT filter configuration, which may reduce functionality or flexibility. Users are advised to upgrade to patched versions once available. No official patches were linked in the provided data, but upgrading to versions beyond the affected ranges is recommended. This vulnerability impacts the availability of services relying on Envoy proxies, potentially disrupting critical cloud-native infrastructure and service mesh deployments.

For European organizations, the impact of CVE-2021-43824 can be significant, especially for those relying heavily on Envoy proxies within their cloud-native environments, microservices architectures, or service mesh implementations. A successful exploitation leads to a denial of service, causing service interruptions that can affect business continuity, customer-facing applications, and internal services. This can degrade user experience, cause financial losses, and damage reputation. Critical sectors such as finance, telecommunications, healthcare, and government services that utilize Envoy for secure and reliable traffic routing may face operational disruptions. Additionally, the DoS could be leveraged as part of a larger attack chain to distract or delay incident response. Given that the vulnerability can be triggered remotely without authentication, attackers can exploit exposed Envoy endpoints on the internet or within internal networks if not properly segmented. The requirement to disable regex matching as a workaround may reduce the effectiveness of JWT-based security policies, potentially weakening access controls temporarily. Therefore, the vulnerability poses both direct availability risks and indirect security policy impacts.

1. Upgrade Envoy to a non-vulnerable version beyond 1.21.1 or the latest stable release that includes the fix for CVE-2021-43824. 2. If immediate upgrade is not feasible, disable regex matching in the JWT filter configuration to prevent the NULL pointer dereference, understanding this may reduce JWT filter flexibility. 3. Restrict access to Envoy management and proxy endpoints by implementing strict network segmentation and firewall rules to limit exposure to untrusted networks. 4. Monitor Envoy logs and metrics for unusual crashes or restarts that could indicate exploitation attempts. 5. Employ rate limiting and anomaly detection on CONNECT requests to reduce the risk of automated or repeated DoS attempts. 6. Integrate Envoy instances into centralized security monitoring and incident response workflows to enable rapid detection and mitigation of service disruptions. 7. Review and test JWT filter configurations to ensure they do not rely on complex regex patterns that could trigger the vulnerability. 8. Maintain an up-to-date inventory of Envoy deployments and versions to prioritize patching efforts effectively.