CVE-2021-20235: There's a flaw in the zeromq server in versions before 4.3.3 in src/decoder_allocators.hpp.
Noroxi assessment
Priority: Plan (action score 45/100)
- The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
- Probability of exploitation in the next 30 days: 43.8% (FIRST EPSS); higher than 98.7% of all records.
- Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. Special conditions such as timing or configuration are required.
- Fixed version: zeromq3 (Debian:12) 4.3.3-1; zeromq3 (Debian:9) 4.2.1-4+deb9u4.
Add it to your regular patch cycle. Upgrade to zeromq3 (Debian:12) 4.3.3-1; zeromq3 (Debian:9) 4.2.1-4+deb9u4 or later.
Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it
Primary sources: NVD · CVE.org · FIRST EPSS
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
- Apr 1, 2021
- Updated
- Jun 16, 2026
- EPSS
- 43.8% · 98th percentile
- CWE
- CWE-120
Report tools
Action score
45
Add it to your regular patch cycle.
- CVSS
- 32 / 40 · 8.1 / 10
- CISA KEV
- 0 / 30 · Not listed
- EPSS
- 13 / 30 · 43.8%
Get told when this record changes
- Not in KEV: no record of exploitation in the wild
- No public exploit code on record
- A fix has been released
If any of these changes, we'll tell you in the daily email digest.
Is your version affected? Check itNoroxi analysis
What does it mean in your environment?
The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.
We use this product, ask for helpAffected systems
Is your version affected?
Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.
| Vendor | Product | CPE |
|---|---|---|
| zeromq | libzmq | cpe:2.3:a:zeromq:libzmq |
Versions reported by the vendor
Affected version ranges reported by the assigning authority (redhat). Independent of NVD's CPE analysis and usually ahead of it.
n/a zeromq
- zeromq 4.3.3affected
Affected versions
NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.
- zeromq libzmq4.2.0 and later · before 4.3.3
Package-level exposure
OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.
| Ecosystem | Package | Affected range | Fix |
|---|---|---|---|
| Debian:12 | zeromq3 | before 4.3.3-1 | 4.3.3-1 |
| Debian:9 | zeromq3 | before 4.2.1-4+deb9u4 | 4.2.1-4+deb9u4 |
Same product
zeromq: all recordsOther highest-scoring records for the same primary product.
- CVE-2019-13132In 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…52Plan
- CVE-2020-36400ZeroMQ libzmq 4.3.3 has a heap-based buffer overflow in zmq::tcp_read, a different vulnerability than CVE-2021-20235.40Plan
- CVE-2019-6250A pointer overflow, with code execution, was discovered in ZeroMQ libzmq (aka 0MQ) 4.2.x and 4.3.x before 4.3.1.38Monitor
Published before and after this one in the same product
- ← EarlierCVE-2020-15166Sep 11, 2020 · Denial of Service in ZeroMQ31Monitor
- ← EarlierCVE-2021-20234Apr 1, 2021 · An uncontrolled resource consumption (memory leak) flaw was found in the ZeroMQ client in versions before 4.3.3 in src/pipe.cpp.26Monitor
- Later →CVE-2021-20237May 28, 2021 · An uncontrolled resource consumption (memory leak) flaw was found in ZeroMQ's src/xpub.cpp in versions before 4.3.3.31Monitor
Remediation
Which version to upgrade to
Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.
| Product / package | Fixed version | Source |
|---|---|---|
| debian:zeromq3 | 4.3.3-1 · Debian:12 | Package registry (OSV) |
Exploit status
No known public exploit
No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.
Research context
For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.
Timeline
From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.
No dated events beyond publication.
Not on record for this CVE: EPSS history · patch commit or PR link · researcher credit · variant or chain candidate · public bounty program.
Technical details
Attack conditions
- Anyone who can reach it over the internet can trigger it.
- No account or password is required.
- No user action is required.
- Special conditions such as timing or configuration are required.
If successful
- Confidentiality
- high · data can be read
- Integrity
- high · data or configuration can be modified
- Availability
- high · the service can be disrupted
- Attack vector
- Network
- Attack complexity
- High
- Privileges required
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality impact
- High
- Integrity impact
- High
- Availability impact
- High
Weakness class (CWE)
CWE-120 · Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')CVSS vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
nvd-primary
Root cause
When data is copied, the source length is not compared with the size of the destination buffer. Excess data overwrites adjacent memory.
The bug in code
The whole CWE-120 classA representative example of this vulnerability class. Not the vendor's source code; it shows the faulty pattern and its fix.
Vulnerable
char buf[256];
memcpy(buf, pkt->data, pkt->len);Fixed
char buf[256];
if (pkt->len > sizeof(buf)) {
return -EINVAL;
}
memcpy(buf, pkt->data, pkt->len);Attack context
MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.
Attack patterns (CAPEC)
- Buffer Overflow via Environment VariablesCAPEC-10likelihood: High · High
- Overflow BuffersCAPEC-100likelihood: High · Very High
- Client-side Injection-induced Buffer OverflowCAPEC-14likelihood: Medium · High
- Filter Failure through Buffer OverflowCAPEC-24likelihood: High · High
- MIME ConversionCAPEC-42likelihood: High · High
- Overflow Binary Resource FileCAPEC-44likelihood: High · Very High
- Buffer Overflow via Symbolic LinksCAPEC-45likelihood: High · High
- Overflow Variables and TagsCAPEC-46likelihood: High · High
ATT&CK techniques
—
Change log (1)
- Fix✗ → ✓
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References
Show all references (1 more)Show less
Vendor advisories and official records. Exploit/PoC links are deliberately left out.