CVE-2021-32780: Incorrect handling of H/2 GOAWAY followed by SETTINGS frames

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-08-24 · updated 2026-06-17
Weakness: CWE-754

Description

Envoy is an open source L7 proxy and communication bus designed for large modern service oriented architectures. In affected versions Envoy transitions a H/2 connection to the CLOSED state when it receives a GOAWAY frame without any streams outstanding. The connection state is transitioned to DRAINING when it receives a SETTING frame with the SETTINGS_MAX_CONCURRENT_STREAMS parameter set to 0. Receiving these two frames in the same I/O event results in abnormal termination of the Envoy process due to invalid state transition from CLOSED to DRAINING. A sequence of H/2 frames delivered by an untrusted upstream server will result in Denial of Service in the presence of untrusted **upstream** servers. Envoy versions 1.19.1, 1.18.4 contain fixes to stop processing of pending H/2 frames after connection transition to the CLOSED state.

Affected products

envoyproxy envoy: from 1.18.0 before 1.18.4; 1.19.0 — fixed in 1.18.4

How to fix

Upgrade to a fixed version: envoyproxy envoy 1.18.4. Check the vendor advisory for mitigations.

References

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