CVE-2026-73550: Envoy: HTTP/2 Discarded Host Header 200 GB Header-Copy OOM in Envoy

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-09-21 · updated 2026-10-05
Weakness: CWE-401

Description

Envoy is an open source edge and service proxy designed for cloud-native applications. Prior to 1.36.10, 1.37.6, 1.38.4, and 1.39.1, Envoy copies every decoded HTTP/2 Host header value before discarding it when :authority is already present. The discarded value bypasses saveHeader, so its bytes and count are not charged against request header limits. An unauthenticated client can use HPACK indexing to submit many references to a large Host value across a bounded number of streams, forcing extreme header-copy allocation and causing the proxy to be out-of-memory killed. The relevant scope boundary is that the demonstrated amplification uses HTTP/2 HPACK and the duplicate Host discard behavior. This issue is fixed in versions 1.36.10, 1.37.6, 1.38.4, and 1.39.1.

Affected products

envoyproxy envoy: before 1.36.10; from 1.37.0 before 1.37.6; from 1.38.0 before 1.38.4; from 1.39.0 before 1.39.1 — fixed in 1.36.10, 1.37.6, 1.38.4, 1.39.1

How to fix

Upgrade to a fixed version: envoyproxy envoy 1.36.10, 1.37.6, 1.38.4, 1.39.1. Check the vendor advisory for mitigations.

References

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