CVE-2021-20236

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.

Published: 2021-05-28 Last update: 2024-11-21 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-20236 is rated High Risk (65.9/100): CVSS Critical severity, with medium exploitation likelihood (EPSS 1.60%). EPSS rose +1.29% over the last day, indicating growing attacker interest. High exploitation likelihood—assess exposure and prioritize remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2021-20236

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-15 0.32% 1.60% +1.29%
2 2026-02-11 0.41% 0.32% -0.09%
3 2026-01-28 — 0.41% —

Full EPSS history (14 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-20236

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
9.8 3.1 CRITICAL
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 5.9 [email protected]
7.5 2.0 HIGH
AV:N/AC:L/Au:N/C:P/I:P/A:P Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:L)
Exploitation conditions are straightforward and predictable.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:P)
Partial confidentiality impact.
Integrity impact (I:P)
Partial integrity impact.
Availability impact (A:P)
Partial availability impact.
 10.0 6.4 [email protected]

Weakness enumeration for CVE-2021-20236

OS Trackers for CVE-2021-20236

vendor priority summary link
alpine critical CVE-2021-20236: 1 source package rows (zeromq); 10 state rows across 5 repos (3.19-main, 3.20-main, 3.21-main, 3.22-main, edge-main); fixed 0, open 10. https://security.alpinelinux.org/vuln/CVE-2021-20236
debian not yet assigned CVE-2021-20236 not yet assigned priority: Debian including 1 source packages (zeromq3), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5. https://security-tracker.debian.org/tracker/CVE-2021-20236
redhat medium — https://access.redhat.com/security/cve/CVE-2021-20236
suse high CVE-2021-20236 severity important: SUSE including 1 source package names (zeromq), 2 product×package rows across 2 product lines (SUSE Manager Client Tools Beta for RHEL, Liberty and Clones 8, SUSE Manager Client Tools for RHEL, Liberty and Clones 8): Known Not Affected 2. https://www.suse.com/security/cve/CVE-2021-20236/
ubuntu medium CVE-2021-20236 medium priority: Ubuntu including 1 source packages (zeromq3), 16 status rows across 16 suites (bionic, focal, groovy, hirsute, impish, jammy, kinetic, lunar, mantic, noble, oracular, plucky, questing, trusty, upstream, xenial): not-affected 10, needed 4, ignored 1, released 1. https://ubuntu.com/security/CVE-2021-20236

Affected software / configurations for CVE-2021-20236

Vendor Product Version Raw CPE
zeromq zeromq < 4.3.3 cpe:2.3:a:zeromq:zeromq:*:*:*:*:*:*:*:*
redhat ceph_storage 2.0 cpe:2.3:a:redhat:ceph_storage:2.0:*:*:*:*:*:*:*
redhat enterprise_linux 7.0 cpe:2.3:o:redhat:enterprise_linux:7.0:*:*:*:*:*:*:*
fedoraproject fedora 33 cpe:2.3:o:fedoraproject:fedora:33:*:*:*:*:*:*:*

References for CVE-2021-20236

cvelogic Threat Intelligence