# CVE-2026-53666: React Router: Arbitrary Constructor Injection via deserializeErrors() in React Router SSR Hydration

> Attackers can trigger arbitrary constructor execution and outbound requests in React Router SSR apps if specific application code allows error manipulation.

- **Severity:** Medium 6.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 27 Jul 2026
- **Updated:** 28 Jul 2026
- **Also known as:** CLSA-2026-1789159895, CLSA-2026-1792253408, CLSA-2026-1792269434, GHSA-337j-9hxr-rhxg, MINI-v493-wr5j-jmqp, ROOT-APP-NPM-CVE-2026-53666

## What is CVE-2026-53666?

CVE-2026-53666 is a medium-severity code injection vulnerability in remix-run react-router, published on 27 Jul 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

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.

Weakness: CWE-470 (Use of Externally-Controlled Input to Select Classes or Code)

## Which versions are affected by CVE-2026-53666?

2 open source packages ship an affected version, including react-router. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `react-router` | npm | From 6.4.0 | 7.18.0 |
| `react-router` | npm | From 6.4.0 | 7.18.0 |

Products: Shopify React-Router.

## How do I fix CVE-2026-53666?

Upgrade react-router to 7.18.0 or later.

## Is CVE-2026-53666 being exploited?

There is no known exploitation of CVE-2026-53666 in the wild and no public exploit code.

## Other vulnerabilities in remix-run react-router

- [CVE-2026-55685](https://insights.sbomify.com/CVE-2026-55685/): React Router: Unauthenticated Denial of Service via Inefficient Route Matching (high 8.7)
- [CVE-2026-21884](https://insights.sbomify.com/CVE-2026-21884/): React Router SSR XSS in ScrollRestoration (high 8.2)
- [CVE-2026-42211](https://insights.sbomify.com/CVE-2026-42211/): React Router's vendored turbo-stream v2 allows arbitrary constructor invocation via TYPE_ERROR deserialization leading to Unauth RCE (high 8.1)
- [CVE-2026-33245](https://insights.sbomify.com/CVE-2026-33245/): React Router vulnerable to XSS in unstable RSC redirect handling via javascript: redirect targets (high 8.0)
- [CVE-2026-22029](https://insights.sbomify.com/CVE-2026-22029/): React Router vulnerable to XSS via Open Redirects (high 8.0)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-53666 and fix it if it is affected.

What it is: CVE-2026-53666 is a medium-severity code injection vulnerability in remix-run react-router, published on 27 Jul 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-53666 in the wild and no public exploit code.

Affected packages and fixed versions:
- react-router (npm): affected from 6.4.0; fixed in 7.18.0
- react-router (npm): affected from 6.4.0; fixed in 7.18.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-53666/ (Markdown: https://insights.sbomify.com/CVE-2026-53666/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-53666
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-53666
- OSV: https://osv.dev/vulnerability/CVE-2026-53666

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-53666/
Summaries are generated and may contain mistakes; the official description above is the record.