What is CVE-2021-26829?

CVE-2021-26829 is a medium-severity vulnerability in Linux Kernel, classified under Cross-site Scripting. CVSS score: 5.4/10. Published 2021-06-11.

How severe is CVE-2021-26829?

Medium severity. CVSS v3 base score is 5.4 out of 10.

Is CVE-2021-26829 known to be exploited?

Yes. CVE-2021-26829 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2025-11-28), indicating it is being actively exploited. 1 public proof-of-concept repositories are indexed.

XSS in Linux Kernel

CVE-2021-26829

OpenPLC ScadaBR through 0.9.1 on Linux and through 1.12.4 on Windows allows stored XSS via system_settings.shtm.

Vulnerability class: XSS (Cross-Site Scripting)

Published 2021-06-11 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 5.4 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.480 (98.8th percentile), scored 2026-10-18.

Moderate probability of exploitation in the next 30 days — watch for active exploitation reports. 99th percentile — 98.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.480 · newest: 0.480 · change: 0.000

Affected products

Weakness classification (CWE)

CWE-79 — Cross-site Scripting

CISA KEV (Known Exploited Vulnerabilities)

This CVE is on the CISA KEV catalog, added on 2025-11-28. CISA KEV inclusion means CISA has confirmed in-the-wild exploitation; US federal agencies are required to remediate within a published due date.

BOD 22-01 due date: 2025-12-19.

Required action: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

Public proof-of-concept exploits

ARPSyndicate/cve-scores

References

youtu.be/Xh6LPCiLMa8 (Exploit, Third Party Advisory)
forum.scadabr.com.br/t/report-falhas-de-seguranca-em-versoes-do-scadabr/3615/4 (Vendor Advisory)
134c704f-9b21-4f2e-91b3-4a467353bcc0 (US Government Resource)
134c704f-9b21-4f2e-91b3-4a467353bcc0 (Third Party Advisory)

Frequently asked questions

What is CVE-2021-26829?: CVE-2021-26829 is a medium-severity vulnerability in Linux Kernel, classified under Cross-site Scripting. CVSS score: 5.4/10. Published 2021-06-11.
How severe is CVE-2021-26829?: Medium severity. CVSS v3 base score is 5.4 out of 10.
Is CVE-2021-26829 known to be exploited?: Yes. CVE-2021-26829 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2025-11-28), indicating it is being actively exploited. 1 public proof-of-concept repositories are indexed.