CVE-2021-20235: There's a flaw in the zeromq server in versions before 4.3.3 in src/decoder_allocators.hpp

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HIGH 8.1REMOTEUPGRADE

CVE-2021-20235 is a high-severity flaw (CVSS 8.1) affecting libzmq. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

There's a flaw in the zeromq server in versions before 4.3.3 in src/decoder_allocators.hpp. The decoder static allocator could have its sized changed, but the buffer would remain the same as it is a static buffer. A remote, unauthenticated attacker who sends a crafted request to the zeromq server could trigger a buffer overflow WRITE of arbitrary data if CURVE/ZAP authentication is not enabled. The greatest impact of this flaw is to application a

Why this priority

High severity
EPSS top 1%: very likely to be exploited
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	8.1 (High)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	43.9% (top 1%)
CISA exploited list	No
Weakness	CWE-120, CWE-787
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: >= 4.2.0 and < 4.3.3

Fixed in: 4.3.3

Safe to downgrade to: a release before 4.2.0

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.

3. If no update is available yet, DOWNGRADE to a release before 4.2.0 (Debian / Ubuntu)

apt-cache madison libzmq sudo apt install libzmq=<VERSION_BEFORE_4.2.0> sudo apt-mark hold libzmq

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list libzmq sudo dnf downgrade libzmq-<VERSION_BEFORE_4.2.0> sudo dnf versionlock add libzmq

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

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-2021-20235?

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

Is CVE-2021-20235 being exploited?

CVE-2021-20235 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-2021-20235 affect?

CVE-2021-20235 affects libzmq. Severity is high (CVSS 8.1). There's a flaw in the zeromq server in versions before 4.3.3 in src/decoder_allocators.hpp. The decoder static allocator could have its sized changed, but the buffer would remain the same as it is a static buffer. A remote, unauthenticated

How urgent is CVE-2021-20235?

Priority 58/100. High severity; EPSS top 1%: very likely to be exploited; Remotely exploitable, no login, no user action

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-2021-20234 MEDIUM 6.5 UPGRADE An uncontrolled resource consumption (memory leak) flaw was found in the ZeroMQ client in versions before 4.3.
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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