CVE-2021-20236

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.9% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-05-28 · updated 2026-06-17
Weakness: CWE-120

Description

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.

Affected products

Fedoraproject Fedora: 33
Red Hat Ceph Storage: 2.0
Red Hat Enterprise Linux: 7.0
Zeromq: before 4.3.3 — fixed in 4.3.3

How to fix

Upgrade to a fixed version: Zeromq 4.3.3. Check the vendor advisory for mitigations.

References

https://bugzilla.redhat.com/show_bug.cgi?id=1921976
https://github.com/zeromq/libzmq/security/advisories/GHSA-qq65-x72m-9wr8
https://bugzilla.redhat.com/show_bug.cgi?id=1921976
https://github.com/zeromq/libzmq/security/advisories/GHSA-qq65-x72m-9wr8

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