What is CVE-2021-32778?

CVE-2021-32778 is a medium-severity vulnerability in Envoyproxy Envoy, classified under Excessive Iteration. CVSS score: 5.8/10. Published 2021-08-24.

How severe is CVE-2021-32778?

Medium severity. CVSS v3 base score is 5.8 out of 10.

Is CVE-2021-32778 known to be exploited?

1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Vulnerability in Envoyproxy Envoy

CVE-2021-32778

Envoy is an open source L7 proxy and communication bus designed for large modern service oriented architectures. In affected versions envoy’s procedure for resetting a HTTP/2 stream has O(N^2) complexity, leading to high CPU utilization when a large number of streams are reset. Deployments are susceptible to Denial of Service when Envoy is configured with high limit on H/2 concurrent streams. An attacker wishing to exploit this vulnerability would require a client opening and closing a large number of H/2 streams. Envoy versions 1.19.1, 1.18.4, 1.17.4, 1.16.5 contain fixes to reduce time complexity of resetting HTTP/2 streams. As a workaround users may limit the number of simultaneous HTTP/2 dreams for upstream and downstream peers to a low number, i.e. 100.

Published 2021-08-24 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 5.8 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:L.

EPSS exploit prediction

EPSS: 0.012 (67.7th percentile), scored 2026-09-30.

Low probability of exploitation in the next 30 days, but still tracked. 68th percentile — 67.7% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.012 · newest: 0.012 · change: 0.000

Affected products

Envoyproxy Envoy — versions >= 1.19.0, < 1.19.1, >= 1.16.0, < 1.16.5, >= 1.17.0, < 1.17.4

Weakness classification (CWE)

CWE-834 — Excessive Iteration

Public proof-of-concept exploits

n0-traces/cve_

References

www.envoyproxy.io/docs/envoy/v1.19.0/version_history/version_history (Release Notes, Vendor Advisory)
github.com/envoyproxy/envoy/security/advisories/GHSA-3xh3-33v5-chcc (Third Party Advisory)

Frequently asked questions

What is CVE-2021-32778?: CVE-2021-32778 is a medium-severity vulnerability in Envoyproxy Envoy, classified under Excessive Iteration. CVSS score: 5.8/10. Published 2021-08-24.
How severe is CVE-2021-32778?: Medium severity. CVSS v3 base score is 5.8 out of 10.
Is CVE-2021-32778 known to be exploited?: 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.