How severe is CVE-2021-20235?

CVE-2021-20235 has a CVSS 3.x base score of 8.1, rated high severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2021-20235 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 44% (99th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-20235?

CVE-2021-20235 affects Zeromq Libzmq. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-20235?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

When was CVE-2021-20235 published?

CVE-2021-20235 was published on 2021-04-01 and last updated on 2026-06-17.

CVE-2021-20235

CVE-2021-20235 is a high-severity vulnerability in Zeromq Libzmq with a CVSS 3.x base score of 8.1. Its EPSS exploit-prediction score of 44% places it in the 99th percentile, indicating an elevated likelihood of exploitation. The underlying weakness is classified as CWE-787.

Key facts

Severity: High (CVSS 3.x base score 8.1)
CVSS v2: 6.8
EPSS exploit prediction: 44% (99th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-787
Affected product: Zeromq Libzmq
Published: 2021-04-01
Last modified: 2026-06-17

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.

Frequently asked questions

What is 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.
How severe is CVE-2021-20235?: CVE-2021-20235 has a CVSS 3.x base score of 8.1, rated high severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2021-20235 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 44% (99th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-20235?: CVE-2021-20235 affects Zeromq Libzmq. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-20235?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2021-20235 published?: CVE-2021-20235 was published on 2021-04-01 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:zeromq:libzmq:*:*:*:*:*:*:*:*

More vulnerabilities in Zeromq Libzmq

CVE-2020-36400 — Critical (CVSS 9.8): ZeroMQ libzmq 4.3.3 has a heap-based buffer overflow in zmq::tcp_read, a different vulnerability than CVE-2021-20235.
CVE-2019-13132 — Critical (CVSS 9.8): In 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…
CVE-2019-6250 — High (CVSS 8.8): A pointer overflow, with code execution, was discovered in ZeroMQ libzmq (aka 0MQ) 4.2.x and 4.3.x before 4.3.1. A…
CVE-2021-20237 — High (CVSS 7.5): An uncontrolled resource consumption (memory leak) flaw was found in ZeroMQ's src/xpub.cpp in versions before 4.3.3.…
CVE-2020-15166 — High (CVSS 7.5): In ZeroMQ before version 4.3.3, there is a denial-of-service vulnerability. Users with TCP transport public endpoints,…
CVE-2021-20234 — Medium (CVSS 6.5): An uncontrolled resource consumption (memory leak) flaw was found in the ZeroMQ client in versions before 4.3.3 in…

All CVEs affecting Zeromq Libzmq →

Other CWE-787 (Out-of-bounds Write) vulnerabilities

CVE-2026-42369 — Critical (CVSS 10.0): GV-VMS V20 is a Video Monitoring Software used to gather the feeds of many surveillance cameras and manage other…
CVE-2026-4746 — Critical (CVSS 10.0): Out-of-bounds Write vulnerability in timeplus-io proton (base/poco/Foundation/src‎ modules). This vulnerability is…
CVE-2025-43300 — Critical (CVSS 10.0): An out-of-bounds write issue was addressed with improved bounds checking. This issue is fixed in iOS 15.8.5 and iPadOS…
CVE-2025-24201 — Critical (CVSS 10.0): An out-of-bounds write issue was addressed with improved checks to prevent unauthorized actions. This issue is fixed in…
CVE-2024-42479 — Critical (CVSS 10.0): llama.cpp provides LLM inference in C/C++. The unsafe `data` pointer member in the `rpc_tensor` structure can cause…
CVE-2024-39791 — Critical (CVSS 10.0): Stack-based buffer overflow vulnerabilities affecting Vonets industrial wifi bridge relays and wifi bridge…

Browse all CWE-787 (Out-of-bounds Write) vulnerabilities →