CVE-2026-48044: Envoy Zstd Decompressor Memory Exhaustion DoS Vulnerability

Technical summary

CVE-2026-48044 affects Envoy's ZstdDecompressorImpl component across versions 1.23.0 through 1.38.0 (exclusive of patched releases). The vulnerability stems from inadequate bounds checking or resource limits during zstd decompression. An adversary crafting a highly compressed zstd payload can trigger unbounded memory allocation, violating the memory exhaustion safeguards expected in a proxy critical path. The flaw is classified as CWE-409 (Improper Restriction of Rendered UI Layers or Frames), though the underlying issue is fundamentally about uncontrolled resource consumption during decompression. The vulnerability carries a CVSS 3.1 score of 7.5 (HIGH), reflecting the ease of network exploitation and guaranteed availability impact.

Business impact

Any organization running Envoy as an ingress controller, sidecar proxy, or edge proxy in production faces availability risk. A single malicious or misconfigured upstream source can render the proxy unresponsive, potentially cascading to application outages if Envoy is single-threaded or resource-constrained. In Kubernetes environments, this may trigger pod eviction and cluster instability. Affected platforms include service meshes (Istio, Linkerd variants using Envoy), API gateways, and CDN edge nodes. Incident response teams will face service degradation alerts and difficulty correlating root cause without flow inspection.

Affected systems

Envoy versions 1.23.0 through 1.35.10, 1.36.0 through 1.36.6, 1.37.0 through 1.37.2, and 1.38.0 are vulnerable. The flaw only manifests when zstd decompression is explicitly enabled in Envoy's filter configuration. Organizations should inventory their Envoy deployments and check active decompression settings in their proxy configurations. Kubernetes distributions and service mesh operators bundling Envoy should verify their vendored versions against the patched releases.

Exploitability

The vulnerability is highly exploitable. No authentication or special privileges are required; any network-adjacent attacker with the ability to send HTTP requests (or other protocols Envoy proxies) can trigger the condition. The attack vector is straightforward: craft a zstd-compressed payload with specific compression ratios or patterns that amplify memory use. Tools for zstd compression are readily available. The only prerequisite is that zstd decompression must be enabled in Envoy's configuration, which is not the default but is common in environments handling compressed traffic from untrusted or third-party sources.

Remediation

Upgrade to Envoy 1.35.11, 1.36.7, 1.37.3, or 1.38.1 or later, depending on your deployed version line. If immediate upgrade is not possible, disable zstd decompression in your Envoy filter configuration if your workloads can tolerate uncompressed traffic, or implement strict compression payload size limits at the network ingress layer (e.g., HTTP Content-Length headers, WAF rules, or load balancer settings). Review your Envoy configuration to identify all active decompression filters and their scope.

Patch guidance

Patches are available in Envoy releases 1.35.11, 1.36.7, 1.37.3, and 1.38.1. Determine your current Envoy version (envoy --version) and identify which release line you are on. Most users should upgrade within the same minor version line to minimize compatibility risk. If you are running Envoy through a managed service (Kubernetes ingress controller, service mesh distribution, or cloud provider), check with your vendor for their patched image or release. Test patches in a staging environment before production rollout, particularly if you rely on compression for performance.

Detection guidance

Monitor for sudden spikes in memory usage correlated with HTTP requests or traffic patterns. Enable Envoy stats and watch for decompression-related metrics if available (consult your Envoy build documentation). At the system level, watch for OOM killer logs (dmesg, systemd journal) targeting Envoy processes. Network-based detection is challenging without deep packet inspection of zstd streams; instead, focus on behavioral signals: abnormal request sizes in logs, unusual compression ratios in flow telemetry, or requests from external/untrusted sources with Content-Encoding: zstd headers. Consider logging and alerting on decompression filter errors or timeouts.

Why prioritize this

This is a HIGH-severity denial-of-service vulnerability affecting core proxy infrastructure with trivial exploitation. Envoy is foundational in cloud-native stacks; its compromise directly impacts application availability. The lack of authentication, low complexity, and widespread use of zstd compression in modern HTTP scenarios make this a critical patch target. Organizations should prioritize remediation for internet-facing or untrusted-traffic-facing Envoy instances within 2–4 weeks.

Risk score, explained

The CVSS 3.1 score of 7.5 (HIGH) reflects: (1) Network-accessible attack vector with no authentication or interaction; (2) Low attack complexity—standard zstd compression tools suffice; (3) High availability impact—OOM crashes the proxy; (4) No confidentiality or integrity impact. The score does not account for the organizational or operational criticality of Envoy in your environment, which may warrant internal risk elevation.

Frequently asked questions