Sources: National Vulnerability Database, CVE.org, CISA KEV, FIRST EPSS
CVE-2026-123566 Overview
Sveltejs devalue before 5.9.3 contains a denial of service vulnerability that allows attackers to cause CPU amplification by supplying sparse arrays to uneval. Attackers who influence sparse values passed to uneval can force synchronous work proportional to the declared array length, blocking the event loop.
Critical Impact
Affected Products
- Sveltejs devalue: see the vendor advisory for the exact affected versions
Discovery Timeline
- 2026-11-08: CVE-2026-123566 published to the National Vulnerability Database by disclosure@vulncheck.com
- 2026-11-08: last updated in the NVD database
Technical Details for CVE-2026-123566
Vulnerability Analysis
CVE-2026-123566 is a Uncontrolled Resource Consumption weakness (CWE-400: Uncontrolled Resource Consumption) in Sveltejs devalue. Based on its CVSS vector, the flaw can be reached over the network, needs no login, and needs no user interaction. An attacker can exhaust CPU, memory or connections.
With a CVSS base score of 8.7, apply the vendor fix and confirm exposure on internet facing and business critical assets first.
Root Cause
The weakness is classified as CWE-400: Uncontrolled Resource Consumption. An attacker can exhaust CPU, memory or connections.
Attack Vector
Based on the CVSS vector, the flaw can be reached over the network, needs no login, and needs no user interaction. 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:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/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-2026-123566
Detection Strategies
- Inventory every instance of Sveltejs devalue 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 Sveltejs devalue 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-2026-123566
Immediate Actions Required
- Apply the fixed version of Sveltejs devalue 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 Sveltejs devalue.
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
- CWE-400: Uncontrolled Resource Consumption More CVEs with this weakness
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
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