CVE-2024-27919 affects the Envoy proxy, a widely used open-source edge and service proxy in cloud-native environments. Specifically, the vulnerability resides in the HTTP/2 codec implementation in versions 1.29.0 and 1.29.1. Envoy's HTTP/2 stack fails to properly reset requests when header map limits are exceeded. An attacker can exploit this by sending a sequence of CONTINUATION frames without the END_HEADERS bit set, which causes Envoy to continuously allocate memory without bounds. This results in uncontrolled memory consumption leading to denial of service through resource exhaustion. The issue is classified under CWE-390, indicating detection of an error condition without appropriate action. This vulnerability is a regression introduced in the specified versions, meaning it was not present in earlier releases. The recommended fix is to upgrade to Envoy 1.29.2 or later, where the issue has been addressed. Alternatively, users can downgrade to 1.28.1 or disable HTTP/2 for downstream connections as temporary workarounds. The vulnerability has a CVSS v3.1 score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H), indicating it can be exploited remotely without authentication or user interaction and primarily impacts availability. No public exploits have been reported so far, but the nature of the flaw makes it a significant risk for denial of service attacks in environments relying on vulnerable Envoy versions.

For European organizations, this vulnerability poses a significant risk to the availability of services that rely on Envoy proxy versions 1.29.0 and 1.29.1, especially in cloud-native and microservices architectures common in sectors like finance, telecommunications, and government. A successful exploitation could lead to service outages, impacting business continuity and potentially causing financial and reputational damage. Since Envoy is often deployed at the edge or as a service mesh component, the DoS could affect critical infrastructure and customer-facing applications. The lack of confidentiality or integrity impact limits data breach risks, but the availability disruption alone can have severe operational consequences. Organizations with high traffic volumes or exposed HTTP/2 endpoints are particularly vulnerable. The absence of known exploits in the wild provides a window for proactive mitigation, but the ease of exploitation (no authentication or user interaction required) means attackers could quickly weaponize this flaw if it becomes publicly exploited.

European organizations should prioritize upgrading Envoy proxies to version 1.29.2 or later to fully remediate the vulnerability. If immediate upgrading is not feasible, downgrading to version 1.28.1 or earlier is a viable temporary measure. Additionally, disabling HTTP/2 protocol support for downstream connections can mitigate the risk, though this may impact performance or functionality. Network-level protections such as rate limiting and anomaly detection on HTTP/2 traffic can help detect and block abnormal CONTINUATION frame floods. Monitoring memory usage and setting resource limits on Envoy instances can reduce the impact of potential attacks. Organizations should audit their environments to identify all instances of Envoy running vulnerable versions and ensure patch management processes are updated to prevent regression vulnerabilities. Finally, integrating this vulnerability into incident response plans and threat hunting activities will improve readiness against exploitation attempts.