Public proof of conceptFix available

CVE-2021-42756

Q: Is CVE-2021-42756 actively exploited?

Active exploitation of CVE-2021-42756 has not been confirmed. Its EPSS score was 35.0% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

Q: What is the CVSS score for CVE-2021-42756?

CVE-2021-42756 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.

A public proof of concept exists; no exploitation attested.

Published2023-02-16
Fixed in6.3.17 · 6.2.7 · 6.1.3 · 6.0.8
WeaknessCWE-121 · 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

Multiple stack-based buffer overflow vulnerabilities [CWE-121] in the proxy daemon of FortiWeb 5.x all versions, 6.0.7 and below, 6.1.2 and below, 6.2.6 and below, 6.3.16 and below, 6.4 all versions may allow an unauthenticated remote attacker to achieve arbitrary code execution via specifically crafted HTTP requests.

Timeline

dates in UTC

2023-02-1619:15CVE publishedNVD
2026-10-2319:23First public proof of conceptPoC-in-GitHub
2026-10-2319:23EPSS crossed 10 %FIRST EPSS

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-2021-42756?format=txt
CSV export: https://api.ismalicious.com/edl/cve/CVE-2021-42756?format=csv
TAXII collection of exploited CVEs
The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD

Fortinet fortiweb

Branch	Affected	Fixed in	Source
6.3	≥ 6.3.0, < 6.3.17	6.3.17	NVD
6.2	≥ 6.2.0, < 6.2.7	6.2.7	NVD
6.1	≥ 6.1.0, < 6.1.3	6.1.3	NVD
6.0	≥ 5.6.0, < 6.0.8	6.0.8	NVD
6.4	≥ 6.4.0, ≤ 6.4.2	no fix published	NVD

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

Fix

Update Fortinet fortiweb to 6.3.17 · 6.2.7 · 6.1.3 · 6.0.8

Advisories and patches

fortiguard.comVendor Advisory

Detection

read 2026-10-23 19:23 UTC

Nuclei template: none so far
Exploit-DB: no entry
Proof-of-concept repositories: 1 repository, unverified3ndorph1n/CVE-2021-42756

Sources

CISA-ADPSSVC decisionnone2024-10-23
OpenCVESSVC decisionnone—
CNACVSS 3.19.8read 2026-10-23
NVDCVSS 3.19.8read 2026-10-23
CNAweakness (CWE)CWE-121read 2026-10-23
NVDweakness (CWE)CWE-787read 2026-10-23
FIRST EPSSEPSS score35.0 %read 2026-10-23
PoC-in-GitHubproof of concept1read 2026-10-23

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: 2023-02-16
Last Modified: 2026-06-17
NVD status: Modified

Frequently asked questions

What is CVE-2021-42756?

Is CVE-2021-42756 actively exploited?

Active exploitation of CVE-2021-42756 has not been confirmed. Its EPSS score was 35.0% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-42756?

CVE-2021-42756 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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