CVE-2026-47774: Envoy vulnerable to HTTP/2 memory exhaustion via cookie header size bypass and HPACK amplification
A denial-of-service vulnerability was found in Envoy's HTTP/2 implementation. The attack chains two techniques: an HPACK compression bomb that exploits header compression to amplify a single byte on the wire into a full header allocation on the server (repeated thousands of times per request), and a Slowloris-style hold using a zero-byte HTTP/2 flow-control window that prevents the server from ever freeing the allocated memory.
A remote attacker can exploit this in Envoy's default HTTP/2 configuration to consume and hold large amounts of server memory (up to 32GB in approximately 20 seconds), rendering the server inaccessible.
This vulnerability exists in each server's default HTTP/2 configuration and requires no authentication to exploit.
External Reference: https://blog.calif.io/p/codex-discovered-a-hidden-http2-bomb
Other sources
Envoy is an open source edge and service proxy designed for cloud-native applications. Prior to versions 1.35.11, 1.36.7, 1.37.3, and 1.38.1, a vulnerability in Envoy's HTTP/2 downstream request processing allows an unauthenticated remote client to trigger excessive memory consumption, potentially resulting in OOM termination of the Envoy process and denial of service. The issue arises from the combination of two behaviors. First, cookie header bytes are not fully accounted for during request header size validation in Envoy. Second, HPACK header block limits in oghttp2/quiche are enforced on encoded bytes without a corresponding limit on total decoded header size. Together, these behaviors allow a malicious client to cause large decoded header allocations while bypassing the intended request header size protections. Versions 1.35.11, 1.36.7, 1.37.3, and 1.38.1 contain a fix. No complete workaround is known short of applying a fix. Possible temporary mitigations include disabling downstream HTTP/2 where operationally feasible; enforcing stricter request header and cookie limits before traffic reaches Envoy; and monitoring Envoy memory usage for abnormal growth under HTTP/2 traffic.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Envoy (HTTP/2 implementation)to a version that resolves this vulnerability.Fixed in 1.35.11 - Upgrade
Upgrade
Envoy (HTTP/2 implementation)to a version that resolves this vulnerability.Fixed in 1.36.7 - Upgrade
Upgrade
Envoy (HTTP/2 implementation)to a version that resolves this vulnerability.Fixed in 1.37.3 - Upgrade
Upgrade
Envoy (HTTP/2 implementation)to a version that resolves this vulnerability.Fixed in 1.38.1 - Configuration
Temporarily mitigate by disabling downstream HTTP/2 where operationally feasible so requests do not reach Envoy over HTTP/2 default configuration.
Envoy (HTTP/2 downstream configuration) Downstream HTTP/2 enablement = disable (where operationally feasible) - Configuration
Temporarily mitigate by enforcing stricter request header and cookie limits upstream (before traffic reaches Envoy), since no complete workaround is known short of applying a fix.
Envoy (request header/cookie limits enforcement before Envoy) Stricter request header and cookie limits = enforce stricter limits before traffic reaches Envoy - Compensating control
Monitor Envoy memory usage for abnormal growth under HTTP/2 traffic to detect exploitation attempts that consume/hold large amounts of server memory.
Event History
Frequently Asked Questions
What is the severity of CVE-2026-47774?
CVE-2026-47774 has a severity rating of high, with a score of 7.5.
How do I fix CVE-2026-47774?
To fix CVE-2026-47774, upgrade Envoy to versions 1.35.11, 1.36.7, 1.37.3, or 1.38.1 or later.
What type of attack does CVE-2026-47774 represent?
CVE-2026-47774 represents a vulnerability that allows unauthenticated remote clients to cause memory exhaustion via cookie header size bypass.
What is affected by CVE-2026-47774?
CVE-2026-47774 affects Envoy prior to versions 1.35.11, 1.36.7, 1.37.3, and 1.38.1.
What are the potential impacts of CVE-2026-47774?
The potential impacts of CVE-2026-47774 include excessive memory consumption, which can lead to service disruptions.