CVE-2021-26829

OpenPLC ScadaBR cross-site scripting, confirmed exploited in the wild.

OpenPLC ScadaBR contains a cross-site scripting vulnerability via system_settings.shtm.

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48%chance of exploitation in the next 30 days (EPSS), 98th percentile

28 Novadded to CISA's Known Exploited Vulnerabilities list, 2025

19 Decfederal remediation deadline, already passed

Passedpast the deadline: patch now

CISA's name for it	OpenPLC ScadaBR Cross-site Scripting Vulnerability
Vendor and product	OpenPLC · ScadaBR
Type	cross-site scripting · CWE-79
Exploited	Confirmed by CISA listed 28 November 2025
Likelihood	one of the most likely to be exploited of every vulnerability tracked
Deadline	19 December 2025 · already passed
Used in ransomware	Not reported
Advisories	github.com ↗ · nvd.nist.gov ↗

What to do

Three steps, in this order

Step 1

Apply the vendor's fix or mitigation; CISA lists only vulnerabilities seen exploited.

Step 2

If you cannot patch now, restrict network access to the affected product.

Step 3

Check for signs of compromise since the date it was added to the list.

CISA's 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.

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Questions

CVE-2021-26829 in short

Is CVE-2021-26829 being exploited?

Yes. CISA added CVE-2021-26829 to its Known Exploited Vulnerabilities catalogue on 28 November 2025, which it does only on evidence of exploitation in the wild.

What is the CISA deadline for CVE-2021-26829?

US federal agencies must remediate it by 19 December 2025. Everyone else should treat that date as the latest safe point to patch an internet-facing system.

How likely is CVE-2021-26829 to be exploited in the next 30 days?

FIRST's Exploit Prediction Scoring System gives it 48%, the 98th percentile of every CVE it scores.

How do I fix CVE-2021-26829?

Apply the vendor's fix or mitigation; CISA lists only vulnerabilities seen exploited. If you cannot patch now, restrict network access to the affected product. Check for signs of compromise since the date it was added to the list.

Same vendor

Other OpenPLC vulnerabilities being exploited

CVE	Product	Type	Listed
CVE-2021-26828	ScadaBR	exploited vulnerability	3 December 2025

Sources: CISA Known Exploited Vulnerabilities catalogue (released 10 November 2026) and FIRST EPSS. Page compiled automatically by SAK T.L.M, the threat-intelligence tool of SAK T.E.C, on 13 November 2026.