What is CVE-2021-42756?

CVE-2021-42756 is a critical-severity vulnerability in Fortinet Fortiweb, classified under Stack-based Buffer Overflow. CVSS score: 9.8/10. Published 2023-02-16.

How severe is CVE-2021-42756?

Critical severity. CVSS v3 base score is 9.8 out of 10.

Is CVE-2021-42756 known to be exploited?

5 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Buffer overflow in Fortinet Fortiweb

CVE-2021-42756

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.

Vulnerability class: Buffer Overflow

Published 2023-02-16 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.350 (98.4th percentile), scored 2026-09-29.

Moderate probability of exploitation in the next 30 days — watch for active exploitation reports. 98th percentile — 98.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.350 · newest: 0.350 · change: 0.000

Affected products

Fortinet Fortiweb — versions 6.4.0, 6.3.0, 6.2.0

Weakness classification (CWE)

Public proof-of-concept exploits

References

fortiguard.com/psirt/FG-IR-21-186 (Vendor Advisory)

Frequently asked questions

What is CVE-2021-42756?: CVE-2021-42756 is a critical-severity vulnerability in Fortinet Fortiweb, classified under Stack-based Buffer Overflow. CVSS score: 9.8/10. Published 2023-02-16.
How severe is CVE-2021-42756?: Critical severity. CVSS v3 base score is 9.8 out of 10.
Is CVE-2021-42756 known to be exploited?: 5 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.