CVE-2026-27612: Repostat React Component XSS Vulnerability

CVE-2026-27612 Overview

CVE-2026-27612 is a Reflected Cross-Site Scripting (XSS) vulnerability affecting the Repostat React component, a library used to fetch and display GitHub repository information. The vulnerability exists in the RepoCard component, which uses React's dangerouslySetInnerHTML to render the repository name (repo prop) during the loading state without any sanitization. If a developer using this package passes unvalidated user input directly into the repo prop (for example, reading it from a URL query parameter), an attacker can execute arbitrary JavaScript in the context of the user's browser.

Critical Impact

Attackers can execute arbitrary JavaScript in users' browsers, potentially leading to session hijacking, credential theft, and unauthorized actions on behalf of authenticated users.

Affected Products

• Repostat versions prior to 1.0.1

Discovery Timeline

• 2026-02-25 - CVE CVE-2026-27612 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-27612

Vulnerability Analysis

This vulnerability is a classic example of improper use of React's dangerouslySetInnerHTML API. When the RepoCard component is in a loading state, it renders a message containing the repository name passed via the repo prop. The problematic code directly interpolates user-supplied input into an HTML string and renders it using dangerouslySetInnerHTML, bypassing React's built-in XSS protections.

The impact of this vulnerability depends on how developers integrate the Repostat component into their applications. If the repo prop value is sourced from user-controllable input (such as URL query parameters, form fields, or other untrusted sources), an attacker can craft a malicious payload that executes arbitrary JavaScript when the component renders.

Root Cause

The root cause is the use of dangerouslySetInnerHTML to render user-controllable input without proper sanitization. React's dangerouslySetInnerHTML exists specifically for cases where developers need to render raw HTML, but it explicitly bypasses React's automatic escaping of HTML entities. When the repo prop containing the repository name is interpolated directly into an HTML template string, any HTML or JavaScript code in that prop value gets executed as-is.

Attack Vector

The attack vector requires user interaction, as the victim must visit a page containing the vulnerable component with a malicious repo prop value. A typical attack scenario involves:

1. Attacker identifies an application using Repostat that sources the repo prop from URL parameters
2. Attacker crafts a malicious URL containing JavaScript payload in the repository parameter
3. Victim clicks the malicious link or is redirected to the URL
4. The RepoCard component renders during loading state, executing the attacker's payload
5. Malicious JavaScript runs in the victim's browser context with full access to the page's cookies, session data, and DOM

The following patch shows how the vulnerability was remediated in version 1.0.1:

     backgroundColor: '#fff'
   };
 
-  if (loading) {
-    return (
-      <div style={cardStyle} dangerouslySetInnerHTML={{ 
-        __html: `Loading data for: <strong>${repo}</strong>...` 
-      }} />;
-    );
-  }
+if (loading) {
+  return (
+    <div style={cardStyle}>
+      Loading data for: <strong>{repo}</strong>...
+    </div>
+  );
+}
 
   if (error) {
     return <div style={{ ...cardStyle, color: 'red' }}>Error: {error}</div>;

Source: GitHub Commit Update

Detection Methods for CVE-2026-27612

Indicators of Compromise

• Presence of suspicious script tags or event handlers in URL query parameters (e.g., ?repo=<script>alert(1)</script>)
• Unusual JavaScript execution errors in browser console logs related to injected content
• Reports of unexpected browser behavior or pop-ups when users access pages with the RepoCard component

Detection Strategies

• Review application code to identify if the Repostat RepoCard component receives input from user-controllable sources such as URL parameters, cookies, or form inputs
• Implement Content Security Policy (CSP) headers to detect and block inline script execution attempts
• Use static code analysis tools to identify uses of dangerouslySetInnerHTML with unsanitized input

Monitoring Recommendations

• Enable web application firewall (WAF) rules to detect XSS payloads in URL parameters and request bodies
• Monitor application logs for requests containing common XSS attack patterns such as <script>, javascript:, or event handlers like onerror
• Implement client-side error monitoring to detect unexpected JavaScript execution errors that may indicate XSS exploitation attempts

How to Mitigate CVE-2026-27612

Immediate Actions Required

• Upgrade Repostat to version 1.0.1 or later immediately
• Audit application code to ensure the repo prop is not being populated with unvalidated user input
• Implement input validation and sanitization for any user-controlled values passed to third-party components

Patch Information

The vulnerability has been addressed in Repostat version 1.0.1. The fix removes the use of dangerouslySetInnerHTML and instead uses standard React JSX data binding ({repo}), which automatically escapes HTML entities. For more information, refer to the GitHub Security Advisory GHSA-fm8c-6m29-rp6j.

Workarounds

• If upgrading is not immediately possible, implement server-side validation to sanitize the repository name before passing it to the RepoCard component
• Wrap user input in a sanitization function that removes or escapes HTML special characters before passing to the repo prop
• Implement a strict Content Security Policy (CSP) that disallows inline scripts to reduce the impact of XSS vulnerabilities

# Example: Update Repostat to patched version
npm update repostat@1.0.1
# Or using yarn
yarn upgrade repostat@1.0.1