CVE-2026-53666

Medium · 6.1

CVSS

6.1

EPSS

0.4%

Published

Jul 27, 2026

CISA due date

—

Cloudanix analysis

Monitor only SSVC: Track

In certain web applications that handle server errors in a specific way, an attacker could make users' browsers contact outside servers, but only with unusual code that most apps do not have and the issue is now fixed by an update.

Exploitation: Requires User Interaction
Attacker needs: User Interaction
Complexity: High
Impact: Other
Component: Web Framework
Class: Improper Input Validation
Remediation: Upgrade
Effort: Medium
Patch risk: Low

What it actually does

Improper input validation in SSR error handling allows attacker-supplied input to overwrite error object properties, triggering unexpected constructor execution during client-side hydration that initiates outbound network requests. Limited to Framework Mode and Data Mode applications performing manual SSR/hydration; Declarative Mode is unaffected.

Why it matters to the business

Low-probability risk of unauthorized client-initiated network callbacks in specific SSR setups, potentially enabling limited data leakage or external callbacks. Affects applications using shopify react-router 6.4.0-7.17.0 with custom error handling during hydration; low EPSS and no known exploitation reduce immediate operational urgency despite 91 days since publication.

How an attack unfolds

An attacker identifies an application using manual SSR/hydration in Framework or Data Mode, supplies crafted input that overwrites aspects of SSR-caught errors, and triggers error paths that propagate to client hydration where constructor execution causes an outbound network request.

How to fix it

Upgrade shopify react-router to version 7.18.0 or later. Review and avoid manual SSR error handling patterns that permit untrusted input to influence error objects.

How to confirm the fix worked

Inspect package.json and installed node_modules for react-router versions prior to 7.18.0; test SSR error scenarios with controlled inputs to verify constructor execution is no longer triggered on hydration.

Typical attack path

Identify target application using React Router Framework/Data Mode with manual SSR/hydration
Supply attacker-controlled input that reaches SSR error handling paths
Overwrite specific error object properties during SSR
Trigger client hydration that executes attacker-influenced constructor
Initiate outbound network request from the client

How to detect it

Examine server-side rendering logs for anomalous error inputs and client-side console or network logs for unexpected constructor invocations or hydration-triggered requests; correlate with application error handling paths.

What your security team should do

Upgrade to react-router 7.18.0 or newer in all affected applications
Audit custom SSR and error-handling code for untrusted input paths
Monitor client-side network activity for unexpected requests originating from hydration logic

The record

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	Introduced	Fixed