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Vulnerability record · CVE-2021-20235 · published 1 April 2021

CVE-2021-20235: Zeromq libzmq classic buffer overflow vulnerability

Zeromq · Libzmq

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 availability, data integrity, and confidentiality.

8.1 CVSS 3.1 High EPSS 44% · top 1.3% CWE-120 · Classic buffer overflowCWE-787 · Out-of-bounds write
8.1CVSS 3.1 base score, v2 6.8
44%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
4References
17 Jun 2026Last modified by NVD

Description

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 availability, data integrity, and confidentiality.

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://bugzilla.redhat.com/show_bug.cgi?id=1921983 Issue TrackingPatchThird Party Advisory
https://github.com/zeromq/libzmq/security/advisories/GHSA-fc3w-qxf5-7hp6 PatchThird Party Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=1921983 Issue TrackingPatchThird Party Advisory
https://github.com/zeromq/libzmq/security/advisories/GHSA-fc3w-qxf5-7hp6 PatchThird Party Advisory

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

9.8CVE-2020-36400Zeromq libzmq out-of-bounds write vulnerabilityZeroMQ libzmq 4.3.3 has a heap-based buffer overflow in zmq::tcp_read, a different vulnerability than CVE-2021-20235.EPSS 1.8%9.8CVE-2019-13132Zeromq libzmq out-of-bounds write vulnerabilityIn ZeroMQ libzmq before 4.0.9, 4.1.x before 4.1.7, and 4.2.x before 4.3.2, a remote, unauthenticated client connecting to a libzmq application, runni…EPSS 42%8.8CVE-2019-6250Zeromq libzmq integer overflow vulnerabilityA pointer overflow, with code execution, was discovered in ZeroMQ libzmq (aka 0MQ) 4.2.x and 4.3.x before 4.3.1. A v2_decoder.cpp zmq::v2_decoder_t::…EPSS 9.4%7.5CVE-2021-20237Zeromq libzmq uncontrolled resource consumption vulnerabilityAn uncontrolled resource consumption (memory leak) flaw was found in ZeroMQ's src/xpub.cpp in versions before 4.3.3. This flaw allows a remote unauth…EPSS 1.7%7.5CVE-2020-15166Zeromq libzmq uncontrolled resource consumption vulnerabilityIn ZeroMQ before version 4.3.3, there is a denial-of-service vulnerability. Users with TCP transport public endpoints, even with CURVE/ZAP enabled, a…EPSS 3.4%6.5CVE-2021-20234Zeromq libzmq uncontrolled resource consumption vulnerabilityAn 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 …EPSS 1.1%5.5CVE-2025-43520Apple OS kernel memory corruption via malicious appA memory corruption flaw (classic buffer overflow) in Apple's kernel was fixed across iOS, iPadOS, macOS, tvOS, visionOS and watchOS. A malicious app…KEVEPSS 0.43%analysed8.8CVE-2025-31277Apple WebKit memory corruption via malicious web contentApple WebKit fails to handle memory correctly when processing crafted web content, leading to memory corruption across Safari, iOS, iPadOS, macOS, tv…KEVEPSS 1.6%analysed

Source: NIST National Vulnerability Database (record CVE-2021-20235), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.