CVE-2021-43826Network attack vector

Proxy crash via post-disconnect upstream retry

Published Feb 22, 2022 · Updated Apr 23, 2025

Use-after-free in Envoy allows remote attackers to crash the proxy by aborting a tunneled TCP connection during upstream setup. The TCP proxy records no downstream-closed state, so an upstream event can call initializeUpstreamConnection after the downstream object is gone. Reachability requires upstream tunneling, more than one connection attempt, and a downstream disconnect before the upstream HTTP/2 stream finishes establishing.

CVSS severity6.1
Medium
EPSS probability1.06%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Use-after-free in Envoy allows remote attackers to crash the proxy by aborting a tunneled TCP connection during upstream setup. The TCP proxy records no downstream-closed state, so an upstream event can call initializeUpstreamConnection after the downstream object is gone. Reachability requires upstream tunneling, more than one connection attempt, and a downstream disconnect before the upstream HTTP/2 stream finishes establishing.

The record

CVE
CVE-2021-43826
Published
Feb 22, 2022
Updated
Apr 23, 2025
Vendor
Envoy Proxy
Product
Envoy
Classifications
CWE-416, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 22, 2022CVE publishedPublication date reported by the CVE source.
  2. Apr 23, 2025Record 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=< 1.18.6
  2. Affected versionversion=>= 1.19.0, < 1.19.3
  3. Affected versionversion=>= 1.20.0, < 1.20.2
  4. Affected versionversion=>= 1.21.0, < 1.21.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Envoy Proxy · Envoyversion=< 1.18.6; version=>= 1.19.0, < 1.19.3; version=>= 1.20.0, < 1.20.2; version=>= 1.21.0, < 1.21.1

Published CVSS scores

7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:N/A:P
6.1GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:N/I:L/A:L

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-416Use After Free
T1499Endpoint Denial of Service

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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