A flaw was found in the ZeroMQ server in versions before 4.3.3.

published 2021-05-28 5 y ago modified 2026-07-18 3 mo ago also EUVD-2021-7675 , JLSEC-2026-516

cvss: 9.8; version 3.1, NVD
cisa kev: Not listed; not in CISA's catalog
epss: 1.9%; EPSS 1.9% chance of exploitation in 30 days, percentile 79%
public exploits: 0; none known

A flaw was found in the ZeroMQ server in versions before 4.3.3. This flaw allows a malicious client to cause a stack buffer overflow on the server by sending crafted topic subscription requests and then unsubscribing. The highest threat from this vulnerability is to confidentiality, integrity, as well as system availability.

Patch / fix

fixed in 2 distros 1 links 1

julia ZeroMQ_jll 4.3.4+0
zeromq zeromq 4.3.3

zeromq3 Debian 14, Debian 13, Debian 12 4.3.3-1

debian advisories DEBIAN-CVE-2021-20236

github.com/zeromq/libzmq/security/advisories/GHSA-qq65-x72m-9wr8 advisory

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-20236 updated 2026-06-26 cvss 7.5

zeromq: Stack overflow on server running PUB/XPUB socket

Red Hat Ceph Storage 2 zeromq3
affected: no fix planned “Out of support scope”

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

1.87% chance of exploitation in the next 30 days, higher than 79% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
NVD	3.1	9.8	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
NVD	2.0	7.5	AV:N/AC:L/Au:N/C:P/I:P/A:P
OSV	3.1	9.8	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Affected packages

ecosystem	package	affected	fixed in
julia	ZeroMQ_jll	<4.3.4+0	4.3.4+0

Affected products

n/a zeromq

Classification

Weakness

CWE-120 Buffer Copy without Checking Size of Input
CWE-787 Out-of-bounds Write

References

github.com /zeromq/libzmq/security/advisories/GHSA-qq65-x72m-9wr8
patch, third-party-advisory, web
bugzilla.redhat.com /show_bug.cgi?id=1921976
issue-tracking, third-party-advisory, web

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary