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CVE-2021-26829

From Exploited flaws, HARD’s guide to the flaws being used in attacks · Checked
This article is about a flaw in OpenPLC ScadaBR. For every flaw being used in attacks, see Exploited flaws.

CVE-2021-26829 is a security flaw in OpenPLC ScadaBR. Attackers can run code in the browser of someone using OpenPLC ScadaBR and act as them. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 28 Nov 2025.[1]

OpenPLC ScadaBR is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff.

Description

CISA calls it a cross-site scripting in OpenPLC ScadaBR. In its words: “OpenPLC ScadaBR contains a cross-site scripting vulnerability via system_settings.shtm.”[1]

The underlying weakness is cross-site scripting (CWE-79), in MITRE’s list of common software weaknesses.[2]

It scores 5.4 out of 10 on the Common Vulnerability Scoring System, which rates it medium, scored by NVD using CVSS 3.1.[2]

Affected versions

NVD’s record doesn’t list version ranges for it. CISA names the product as OpenPLC ScadaBR.

Most families won’t have OpenPLC ScadaBR; it’s for whoever looks after IT at work.

Exploitation

CISA added it to its catalog of known exploited vulnerabilities on 28 Nov 2025, 1,631 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 19 Dec 2025 to fix it.[1]

FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 48%. That’s in the top 2% of the flaws it scores.[3]

CISA assesses it as not automatable: each attack takes some work.

CISA doesn’t say who was attacked, or how.

Fix

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.”[1]

  1. Check whether you use it.OpenPLC ScadaBR is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
  2. Send this page to your IT person.Ask them to confirm it’s fixed, or not in use. CISA gave US government agencies until 19 Dec 2025 to fix it.
  3. If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.

You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.

Timeline

DateEvent
The flaw is made public in the National Vulnerability Database, with a severity score of 5.4.[2]
CISA adds it to its catalog of flaws known to be used in attacks.[1]
CISA’s deadline for US government agencies to fix it.[1]
FIRST puts the odds of it being used in attacks in the next 30 days at 48%.[3]

See also

Other OpenPLC ScadaBR flaws

  • CVE-2021-26828: unrestricted upload of file with dangerous type ·

Other cross-site scripting flaws

  • CVE-2026-42897, Microsoft: exchange server cross-site scripting ·
  • CVE-2025-48700, Synacor Zimbra Collaboration Suite (ZCS): cross-site scripting ·
  • CVE-2025-66376, Synacor Zimbra Collaboration Suite (ZCS): cross-site scripting ·
  • CVE-2025-68461, Roundcube Webmail: cross-site scripting ·
  • CVE-2025-27915, Synacor Zimbra Collaboration Suite (ZCS): cross-site scripting ·

References

  1. ^ Cybersecurity and Infrastructure Security Agency. “OpenPLC ScadaBR Cross-site Scripting Vulnerability”. Known Exploited Vulnerabilities Catalog, added 28 Nov 2025.
  2. ^ National Institute of Standards and Technology. “CVE-2021-26829 Detail”. National Vulnerability Database, published 11 Jun 2021.
  3. ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.480, percentile 98.9, 31 Oct 2026.
  4. CVE Program. “CVE-2021-26829”. The official CVE record.

This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.