Input validation error in React Router - CVE-2026-53666
Published: July 22, 2026
Vulnerability identifier: #VU139180
CSH Severity: Medium
CVSS v4.0: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N/E:U/U:Green
CVE-ID: CVE-2026-53666
CWE-ID: CWE-20
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vendor: Remix
Affected software:
React Router
React Router
Detailed vulnerability description
The vulnerability allows a remote attacker to trigger unexpected constructor execution on the client.
The vulnerability exists due to improper input validation in the SSR hydration process when processing attacker-supplied input that overwrites certain aspects of errors caught by SSR. A remote attacker can supply crafted input to trigger unexpected constructor execution on the client.
This only affects Framework Mode and Data Mode applications performing manual SSR and hydration, and does not impact Declarative Mode.
How to mitigate CVE-2026-53666
Install security update from vendor's website.