CVE-2026-53666 is a medium-severity (CVSS 6.1) CWE-470 vulnerability in react-router. A fix is available for react-router — see the affected versions and patch details below.
React Router: Arbitrary Constructor Injection via deserializeErrors() in React Router SSR Hydration
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-53666.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-53666 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 395,249 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
react-routernpmDescription
React Router is a router for React. In versions 6.4.0 through 7.17.0, if application code was written in a way that allows attacker-supplied input to overwrite certain aspects of errors caught by the SSR process, then it was possible for an attacker to trigger unexpected constructor execution on the client, which would in turn trigger an outbound network request. This is only possible with very specific (and unlikely) application-layer code. Note that this does not impact an application if it is using Declarative Mode. It only impacts Framework Mode and Data Mode applications that perform manual SSR/hydration. This issue has been fixed in version 7.18.0.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | react-router | ≥ 6.4.0&&< 7.18.0 | 7.18.0npm install react-router@7.18.0 |
Affected Products
react-routershopifyDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for react-router, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update react-router to 7.18.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-53666 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This Moderate impact flaw in React Router affects applications configured to allow attacker-controlled input to modify server-side rendering (SSR) errors. Exploitation requires specific application-layer code in Framework Mode or Data Mode with manual SSR/hydration, leading to unexpected client-side code execution and…
To mitigate this issue, ensure that custom application code does not permit attacker-controlled input to modify error objects during server-side rendering (SSR) when using React Router in Framework or Data Modes. Review and harden application-layer error handling to prevent such overwrites. Applications not requiring Framework or Data Mode SSR should consider utilizing Declarative Mode, which is unaffected.Source: Red Hat security advisory for CVE-2026-53666 (CC BY 4.0)
Frequently Asked Questions
Is CVE-2026-53666 in your dependencies?
Find it across npm, including transitive dependencies.