No exploitation reportedFix available

CVE-2021-3520

Q: Is CVE-2021-3520 actively exploited?

Active exploitation of CVE-2021-3520 has not been confirmed. Its EPSS score was 3.4% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

Q: What is the CVSS score for CVE-2021-3520?

CVE-2021-3520 has a CVSS v3 base score of 9.8 (CRITICAL severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

No exploitation reported by the sources we read.

Published2021-06-02
Fixed in9.0.6 · 8.2.12
WeaknessCWE-190 · CWE-787

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

There's a flaw in lz4. An attacker who submits a crafted file to an application linked with lz4 may be able to trigger an integer overflow, leading to calling of memmove() on a negative size argument, causing an out-of-bounds write and/or a crash. The greatest impact of this flaw is to availability, with some potential impact to confidentiality and integrity as well.

Timeline

dates in UTC

2021-06-0213:15CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 18:48 UTC

Indicators tagged with this CVE, per day0 indicators over 7 days

Block these indicators

Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2021-3520?format=txt
CSV export: https://api.ismalicious.com/edl/cve/CVE-2021-3520?format=csv
TAXII collection of exploited CVEs
The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD

Splunk universal forwarder

Branch	Affected	Fixed in	Source
9.0	≥ 9.0.0, < 9.0.6	9.0.6	NVD
8.2	≥ 8.2.0, < 8.2.12	8.2.12	NVD
9.1	= 9.1.0	no fix published	NVD

Lz4 project lz4

Branch	Affected	Fixed in	Source
1.x	≥ 1.8.3, < 1.9.4	1.9.4	NVD

Netapp active iq unified manager

Branch	Affected	Fixed in	Source
—	all versions	no fix published	NVD

4 more products in the sources.

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Fix

Update Splunk universal forwarder to 9.0.6 · 8.2.12

Advisories and patches

Detection

Nuclei template: none so far
Exploit-DB: no entry
Proof-of-concept repositories: none listed

Sources

CISA-ADPSSVC decisionnone2026-10-08
OpenCVESSVC decisionnone—
NVDCVSS 3.19.8read 2026-10-23
NVDCVSS 2.07.5read 2026-10-23
CNAweakness (CWE)CWE-190read 2026-10-23
NVDweakness (CWE)CWE-190, CWE-787read 2026-10-23
FIRST EPSSEPSS score3.4 %2026-10-22

Technical details

CVSS v3 Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Published: 2021-06-02
Last Modified: 2026-10-14
NVD status: Modified

Frequently asked questions

What is CVE-2021-3520?

Is CVE-2021-3520 actively exploited?

Active exploitation of CVE-2021-3520 has not been confirmed. Its EPSS score was 3.4% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-3520?

CVE-2021-3520 has a CVSS v3 base score of 9.8 (CRITICAL severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

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