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CVE-2026-126800: Hydro @hydrooj/ui-default Cross Site Scripting Vulnerability

CVE-2026-126800 explained: critical Cross Site Scripting in Hydro @hydrooj/ui-default. CVSS 9.3. Affected versions, detection, fixes and CTEM priority.

Last updated: Nov 11, 2026

CVSS
9.3 CRITICAL
EPSS
n/a
CISA KEV
Not listed
CWE
CWE-79

Sources: National Vulnerability Database, CVE.org, CISA KEV, FIRST EPSS

CVE-2026-126800 Overview

Hydro @hydrooj/ui-default before 4.59.1 contains a stored cross-site scripting vulnerability that allows registered users to inject script by placing double quotes in language- class tokens within markdown-it-xss.ts. Attackers can store a payload in their profile bio that executes when administrators or other users view it, stealing the non-httpOnly sid session cookie for session hijacking.

Critical Impact

Remote, authenticated attackers who persuade a user to act can exploit this Cross Site Scripting in Hydro @hydrooj/ui-default. The CVSS base score is 9.3 (Critical).

Affected Products

  • Hydro @hydrooj/ui-default: see the vendor advisory for the exact affected versions

Discovery Timeline

  • 2026-11-11: CVE-2026-126800 published to the National Vulnerability Database by disclosure@vulncheck.com
  • 2026-11-11: last updated in the NVD database

Technical Details for CVE-2026-126800

Vulnerability Analysis

CVE-2026-126800 is a Cross Site Scripting weakness (CWE-79: Cross Site Scripting) in Hydro @hydrooj/ui-default. Based on its CVSS vector, the flaw can be reached over the network, needs some level of access, and relies on a user taking an action. Attacker supplied script runs in another user's browser inside the trusted site.

With a CVSS base score of 9.3, apply the vendor fix and confirm exposure on internet facing and business critical assets first.

Root Cause

The weakness is classified as CWE-79: Cross Site Scripting. Attacker supplied script runs in another user's browser inside the trusted site.

Attack Vector

Based on the CVSS vector, the flaw can be reached over the network, needs some level of access, and relies on a user taking an action. Attack complexity is low complexity.

No exploitation in the wild is confirmed by CISA at the time of the last update. Check the references below for any vendor or researcher proof of concept.

CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:H/VI:H/VA:N/SC:H/SI:H/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Detection Methods for CVE-2026-126800

Detection Strategies

  • Inventory every instance of Hydro @hydrooj/ui-default and compare the running version with the affected range above.
  • Check external attack surface scans for internet facing instances of the component.
  • Run an authenticated vulnerability scan once your scanner ships a check for this CVE.

Monitoring Recommendations

  • Forward Hydro @hydrooj/ui-default application, access and audit logs to your SIEM for retrospective hunting.
  • Alert on new processes, configuration changes or outbound connections from the affected service.
  • Track the vendor advisory and the CISA KEV catalog for changes in exploitation status.

How to Mitigate CVE-2026-126800

Immediate Actions Required

  • Apply the fixed version of Hydro @hydrooj/ui-default named in the vendor advisory.
  • Prioritize internet facing and business critical instances first.
  • Rescan after the change to confirm the exposure is closed.

Patch Information

Use the fixed version listed in the vendor advisory for Hydro @hydrooj/ui-default.

Workarounds

  • If patching must wait, restrict network access to the affected service and place it behind authentication or a web application firewall rule.

CWE References

Technical References

This entry is built directly from NVD, CISA KEV and EPSS data. Verify fixed versions in the vendor advisory before you deploy.

Frequently asked questions

CVE-2026-126800 is a critical severity Cross Site Scripting in Hydro @hydrooj/ui-default with a CVSS score of 9.3. Apply the vendor fix and confirm exposure on internet facing and business critical assets first.

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