CVE-2021-20234: An uncontrolled resource consumption (memory leak) flaw was found in the ZeroMQ client in versions before 4.3.

Q: How do I fix CVE-2021-20234?

libzmq: upgrade to 4.3.3. Exact commands for each OS are on this page.

Q: Is CVE-2021-20234 being exploited?

CVE-2021-20234 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-20234 affect?

CVE-2021-20234 affects libzmq. Severity is medium (CVSS 6.5). An uncontrolled resource consumption (memory leak) flaw was found in the ZeroMQ client in versions before 4.3.3 in src/pipe.cpp. This issue causes a client that connects to multiple malicious or compromised servers to crash. The highest thr

Q: How urgent is CVE-2021-20234?

Priority 44/100. Public exploit reference

This month

MEDIUM 6.5UPGRADE

CVE-2021-20234 is a medium-severity flaw (CVSS 6.5) affecting libzmq. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

An uncontrolled resource consumption (memory leak) flaw was found in the ZeroMQ client in versions before 4.3.3 in src/pipe.cpp. This issue causes a client that connects to multiple malicious or compromised servers to crash. The highest threat from this vulnerability is to system availability.

Why this priority

Public exploit reference

A fixed release or distro patch exists.

Risk facts

CVSS score	6.5 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	1.1% (top 35%)
CISA exploited list	No
Weakness	CWE-400, CWE-401
Published	2021-04-01
Last modified	2026-06-17

Publisher (CNA): Red Hat, Inc. · Vendor: zeromq

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-04-01 14:15 UTCPublished CVE record published
2026-06-17 03:33 UTCUpdated Record last modified

zeromq libzmq UPGRADE

Affected: < 4.3.3

Fixed in: 4.3.3

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy libzmq || rpm -q libzmq || libzmq --version

2. UPGRADE to 4.3.3 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade libzmq

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade libzmq

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-20234?

libzmq: upgrade to 4.3.3. Exact commands for each OS are on this page.

Is CVE-2021-20234 being exploited?

CVE-2021-20234 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-20234 affect?

CVE-2021-20234 affects libzmq. Severity is medium (CVSS 6.5). An uncontrolled resource consumption (memory leak) flaw was found in the ZeroMQ client in versions before 4.3.3 in src/pipe.cpp. This issue causes a client that connects to multiple malicious or compromised servers to crash. The highest thr

How urgent is CVE-2021-20234?

Priority 44/100. Public exploit reference

Related CVEs

CVE-2021-20237 HIGH 7.5 UPGRADE An uncontrolled resource consumption (memory leak) flaw was found in ZeroMQ's src/xpub.cpp in versions before
CVE-2021-20235 HIGH 8.1 UPGRADE There's a flaw in the zeromq server in versions before 4.3.3 in src/decoder_allocators.hpp
CVE-2020-36400 CRITICAL 9.8 DOWNGRADE ZeroMQ libzmq 4.3.3 has a heap-based buffer overflow in zmq::tcp_read, a different vulnerability than CVE-2021
CVE-2020-15166 HIGH 7.5 UPGRADE MITIGATE In ZeroMQ before version 4.3.3, there is a denial-of-service vulnerability
CVE-2019-6250 HIGH 8.8 UPGRADE MITIGATE A pointer overflow, with code execution, was discovered in ZeroMQ libzmq (aka 0MQ) 4.2.x and 4.3.x before 4.3.

References

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