CVE-2021-20235

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.

Published: 2021-04-01 Last update: 2024-11-21 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-20235 is rated Moderate Risk (61.2/100): CVSS High severity, with high exploitation likelihood (EPSS 6.58%, 91th percentile). EPSS ranks this CVE among the most likely to be exploited in the near term. 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-2021-20235

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 2026-04-25 9.70% 6.58% -3.12%
2 2026-04-09 9.49% 9.70% +0.21%
3 2025-11-23 — 9.49% —

Full EPSS history (23 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-20235

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
8.1 3.1 HIGH
CVSS:3.1/AV:N/AC:H/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:H)
Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work.
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.
 2.2 5.9 [email protected]
6.8 2.0 MEDIUM
AV:N/AC:M/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:M)
Exploitation needs some favorable conditions, but not exceptional ones.
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.
 8.6 6.4 [email protected]

Weakness enumeration for CVE-2021-20235

OS Trackers for CVE-2021-20235

vendor priority summary link
debian not yet assigned CVE-2021-20235 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-2021-20235
redhat medium — https://access.redhat.com/security/cve/CVE-2021-20235
suse high CVE-2021-20235 severity important: SUSE including 5 source package names (libzmq3, libzmq5, libzmq5-32bit, zeromq, zeromq-devel), 51 product×package rows across 22 product lines (SUSE CaaS Platform 4.0, SUSE Enterprise Storage 6, … (22 product lines)): Known Not Affected 51. https://www.suse.com/security/cve/CVE-2021-20235/
ubuntu medium CVE-2021-20235 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 12, released 3, ignored 1. https://ubuntu.com/security/CVE-2021-20235

Affected software / configurations for CVE-2021-20235

Vendor Product Version Raw CPE
zeromq libzmq >= 4.2.0, < 4.3.3 cpe:2.3:a:zeromq:libzmq:*:*:*:*:*:*:*:*

References for CVE-2021-20235

cvelogic Threat Intelligence