CVE-2020-36400: ZeroMQ libzmq 4.3.3 has a heap-based buffer overflow in zmq::tcp_read, a different vulnerability than CVE-2021

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CRITICAL 9.8REMOTEDOWNGRADE

CVE-2020-36400 is a critical-severity flaw (CVSS 9.8) affecting libzmq. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

ZeroMQ libzmq 4.3.3 has a heap-based buffer overflow in zmq::tcp_read, a different vulnerability than CVE-2021-20235.

Why this priority

Critical severity
Remotely exploitable, no login, no user action

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.8% (top 21%)
CISA exploited list	No
Weakness	CWE-787
Published	2021-07-01
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: zeromq

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

Am I affected?

Timeline

2021-07-01 03:15 UTCPublished CVE record published
2026-06-17 03:15 UTCUpdated Record last modified

zeromq libzmq DOWNGRADE

Affected: 4.3.3

Fixed in: no fixed release known

Safe to downgrade to: a release before 4.3.3

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 4.3.3 (Debian / Ubuntu)

apt-cache madison libzmq sudo apt install libzmq=<VERSION_BEFORE_4.3.3> sudo apt-mark hold libzmq

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list libzmq sudo dnf downgrade libzmq-<VERSION_BEFORE_4.3.3> 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-2020-36400?

libzmq: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2020-36400 being exploited?

CVE-2020-36400 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-2020-36400 affect?

CVE-2020-36400 affects libzmq. Severity is critical (CVSS 9.8). ZeroMQ libzmq 4.3.3 has a heap-based buffer overflow in zmq::tcp_read, a different vulnerability than CVE-2021-20235.

How urgent is CVE-2020-36400?

Priority 59/100. Critical severity; 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-2021-20235 HIGH 8.1 UPGRADE There's a flaw in the zeromq server in versions before 4.3.3 in src/decoder_allocators.hpp
CVE-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-2020-15166 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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