CVE-2021-32780Network attack vector

Proxy crash via invalid HTTP/2 state transition

Published Aug 24, 2021 · Updated Aug 3, 2024

Denial of service in Envoy 1.18.0 through 1.18.3 and 1.19.0 allows remote attackers controlling an upstream server to crash the proxy. Envoy closes an HTTP/2 connection after a GOAWAY frame with no outstanding streams, then wrongly processes a SETTINGS_MAX_CONCURRENT_STREAMS value of 0 from the same I/O event and attempts an invalid CLOSED-to-DRAINING transition. Exploitation requires control of an HTTP/2 upstream server; the resulting abnormal process termination interrupts proxied traffic.

CVSS severity8.6
High
EPSS probability1.23%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in Envoy 1.18.0 through 1.18.3 and 1.19.0 allows remote attackers controlling an upstream server to crash the proxy. Envoy closes an HTTP/2 connection after a GOAWAY frame with no outstanding streams, then wrongly processes a SETTINGS_MAX_CONCURRENT_STREAMS value of 0 from the same I/O event and attempts an invalid CLOSED-to-DRAINING transition. Exploitation requires control of an HTTP/2 upstream server; the resulting abnormal process termination interrupts proxied traffic.

The record

CVE
CVE-2021-32780
Published
Aug 24, 2021
Updated
Aug 3, 2024
Vendor
Envoy Proxy
Product
Envoy
Classifications
CWE-754, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 24, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=>= 1.18.0, < 1.18.4
  2. Affected versionversion=>= 1.19.0, < 1.19.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Envoy Proxy · Envoyversion=>= 1.18.0, < 1.18.4; version=>= 1.19.0, < 1.19.1

Published CVSS scores

5.0NVDAV:N/AC:L/Au:N/C:N/I:N/A:P
8.6GitHub CNACVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H
7.5NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-754Improper Check for Unusual or Exceptional Conditions
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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