CVE-2021-32778

What it is

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.

At a glance

Exploitability

Can be exploited over the network without logging in. Limited direct impact.

Exploit odds

1.2% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2021-08-24 21:15 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-06-17 03:53 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 1.2% in the next 30 days (EPSS).
• Severity: 5.8 MEDIUM (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2021-08-24 21:15 UTC

Last modified

2026-06-17 03:53 UTC

Exploit probability

1.2% · EPSS, next 30 days

Weakness

CWE-834

Record status

Modified

Affected

• Envoyproxy Envoy < 1.16.5
• Envoyproxy Envoy < 1.17.4
• Envoyproxy Envoy < 1.18.4
• Envoyproxy Envoy 1.19.0

Attack profile CVSS 3.1 · 5.8

Attack vector

Network

Complexity

Low

Privileges needed

None

User interaction

None

Scope

Changed

Confidentiality

None

Integrity

None

Availability

Low

Is it in your code?

Hand this to your coding agent. It searches your project for the affected products and checks versions. Nothing leaves your machine.

Read https://cve-list.com/agent/cve/CVE-2021-32778.md and follow it. Check whether this project is affected by CVE-2021-32778. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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