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Technical Analysis: How CVE-2026-42211 Works
This vulnerability arises from a deserialization flaw in React Router’s vendored copy of turbo-stream v2, an internal serialization library used exclusively in Framework Mode. The core issue is that the library’s custom serialization logic for `Error` objects is overly permissive, allowing an attacker to control which constructor is invoked during the deserialization process. Specifically, the vulnerability is triggered when an object of type `TYPE_ERROR` is deserialized. Normally, this would simply reconstruct a `TypeError` object. However, the `turbo-stream` serializer does not validate the constructor being called. If an attacker can first inject a property into the `Error.prototype` chain (via a separate prototype pollution vulnerability), they can replace the intended constructor with any other constructor present in the JavaScript environment, such as the `Function` constructor. A classic prototype pollution gadget can be used to inject a payload into Error.prototype.constructor. For example:
// Step 1: Attacker pollutes the prototype of a built-in object.
Object.prototype.polluted = "Value";
// More specifically, they target Error.constructor:
Error.prototype.constructor = function() { / malicious code / };
With this pollution in place, the second step—deserializing a specially crafted `TYPE_ERROR` object—will invoke the malicious constructor instead of the legitimate one. Because the `Function` constructor can execute arbitrary JavaScript code when passed a string argument, this leads to Remote Code Execution (RCE) on the server. The attack chain is therefore:
1. Prototype Pollution – The attacker must first find a way to pollute the prototype of a built-in object (like Error) in the server-side JavaScript environment. This could be through a separate vulnerability in the application or a dependency.
2. Deserialization Gadget – Once the prototype is polluted, the attacker sends a crafted `turbo-stream` payload that deserializes a TYPE_ERROR. The polluted constructor is invoked, executing the attacker’s code.
The vulnerability exists in React Router versions 7.0.0 through 7.14.1 when running in Framework Mode. It does not affect applications using Declarative Mode (<BrowserRouter>) or Data Mode (createBrowserRouter). The fix, implemented in version 7.14.2, removes the undocumented custom error serialization logic, preventing this gadget chain from being exploitable.
DailyCVE Form:
Platform: npm react-router
Version: 7.0.0-7.14.1
Vulnerability: Deserialization RCE
Severity: 8.1 High
date: 2026-06-02
Prediction: 2026-06-03
What Undercode Say:
Check your current React Router version npm list react-router Update to the patched version npm install [email protected] Verify the update npm list react-router
// Simulated vulnerability check (do not use in production)
const isVulnerable = (version) => {
const [major, minor, patch] = version.split('.').map(Number);
return major === 7 && (minor < 14 || (minor === 14 && patch < 2));
};
console.log(isVulnerable('7.14.1') ? 'Vulnerable' : 'Safe');
Exploit:
The exploit chain requires two conditions:
- A separate prototype pollution vulnerability in the target application.
- The application must be using React Router v7 in Framework Mode.
An attacker would first pollute the prototype (e.g., via a JSON merge operation) and then send a crafted request containing a serialized `TYPE_ERROR` to the server, triggering RCE.
Protection:
- Upgrade to React Router v7.14.2 or later.
- If upgrading is not immediately possible, avoid using Framework Mode and switch to Declarative or Data Mode.
- Sanitize all user input to prevent prototype pollution vulnerabilities.
- Implement strict input validation for any data that is serialized.
Impact:
- Full Remote Code Execution (RCE) on the server, allowing an attacker to:
- Run arbitrary system commands.
- Read, modify, or delete sensitive data.
- Pivot to internal networks.
- Install backdoors for persistent access.
- Unauthenticated Attack: The RCE can be triggered without prior authentication.
- High Confidentiality, Integrity, and Availability Impact (CVSS 8.1).
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Sources:
Reported By: github.com
Extra Source Hub:
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