CVE-2024-53262

Executive Summary

CVE-2024-53262 is a Cross-Site Scripting (XSS) vulnerability in SvelteKit, a popular framework for building web applications with Svelte. The flaw resides in the static error.html template, which is rendered when the application encounters an unhandled error. This template uses placeholders (%sveltekit.status% and %sveltekit.error.message%) that are replaced with dynamic content without proper HTML escaping. An attacker who can control the error message (e.g., by providing malicious input that triggers an error) can inject arbitrary JavaScript into the error page, leading to XSS attacks. The vulnerability is rated with a CVSS score of 0.0 in the NVD data, but based on the description, it is a medium-severity XSS issue. The patch is available in version 2.8.3.

Supply Chain & Package Security

This vulnerability affects the SvelteKit npm package (@sveltejs/kit). Applications that use user-controlled input in error messages are at risk. The supply chain risk is moderate, as the vulnerability is in the framework itself, not a dependency. Developers should update to the patched version immediately.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The attack can be launched remotely.
• Attack Complexity (AC): Low (L) – No special conditions are required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): Required (R) – The victim must visit the crafted error page.
• Scope (S): Changed (C) – The injected script can affect the user’s session in the context of the application.
• Confidentiality Impact (C): Low (L) – Potential data leakage via script.
• Integrity Impact (I): Low (L) – Possible modification of content.
• Availability Impact (A): None (N) – No direct DoS.

CVSS Score: 6.1 (Medium) – Based on the vector above.

Threat Level: Medium. Exploitation requires user interaction and a specific application behavior (user input in error messages). However, if the application meets this condition, the impact can be significant.

Deep Dive & Mechanics

The vulnerability is in the error.html template, which is used as a fallback error page. The template contains two placeholders:

• %sveltekit.status% – The HTTP status code (e.g., 404, 500).
• %sveltekit.error.message% – The error message string.

In vulnerable versions, these placeholders are replaced with the actual values without any HTML escaping. If an attacker can cause an error with a message containing malicious HTML/JavaScript, that code will be rendered in the victim’s browser.

Example Attack Scenario:

1. The application uses user input to construct an error message, e.g., throw new Error(userInput).
2. The attacker provides input like <script>alert('XSS')</script>.
3. When the error occurs, the error.html template renders the message without escaping, executing the script.

The vulnerability is limited to applications that explicitly use user-controlled content in error messages. Most applications do not do this, but it is a common pattern in debugging or custom error handling.

Vulnerable vs. Patched Code

Vulnerable Code (SvelteKit < 2.8.3): The error.html template likely contains something like:

<!DOCTYPE html>
<html>
<head><title>Error</title></head>
<body>
  <h1>%sveltekit.status%</h1>
  <p>%sveltekit.error.message%</p>
</body>
</html>

The placeholders are replaced directly without escaping.

Patched Code (SvelteKit >= 2.8.3): The patched version escapes the placeholders before insertion. For example:

<!DOCTYPE html>
<html>
<head><title>Error</title></head>
<body>
  <h1>{{status}}</h1>
  <p>{{error.message}}</p>
</body>
</html>

Where the framework now HTML-encodes the values (e.g., < becomes <).

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• A SvelteKit application (version < 2.8.3) that uses user input in error messages.
• Example: throw new Error(req.query.msg).

Exploit Steps:

1. Identify an endpoint that triggers an error with user-controlled input.
2. Craft a payload: <img src=x onerror=alert(1)> or <script>alert(document.cookie)</script>.
3. Send the request: GET /?msg=<script>alert('XSS')</script>.
4. The error page will execute the script.

Lab Setup:

• Use a vulnerable SvelteKit app (e.g., from a CTF or intentionally vulnerable project).
• Run locally with npm run dev.
• Use a browser or curl to test.

Blue Team (Defense & Detection)

Detection:

• Monitor for unusual error messages containing HTML tags or JavaScript.
• Use WAF rules to block common XSS patterns in error responses.
• Implement Content Security Policy (CSP) headers to mitigate XSS impact.

Mitigation:

• Upgrade to SvelteKit 2.8.3 or later.
• Avoid using user input in error messages.
• If unavoidable, sanitize input before passing to Error constructor.

Indicators of Compromise (IOCs)

• Network: HTTP responses with status codes 4xx/5xx containing unescaped HTML/JavaScript in the body.
• Logs: Error logs showing user-controlled strings with HTML tags.
• File: Modified error.html template (if custom) without escaping.

Detection Engineering

YARA Rule

rule CVE_2024_53262_SvelteKit_XSS {
    meta:
        description = "Detects unescaped error messages in SvelteKit error.html"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "CVE-2024-53262"
    strings:
        $unescaped = /%sveltekit\.(status|error\.message)%/
    condition:
        $unescaped
}

Sigma Rule

title: SvelteKit Error Page XSS Detection
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects unescaped error messages in SvelteKit responses
author: Valters IT
logsource:
    category: webserver
    product: sveltekit
detection:
    selection:
        response.body|contains: '%sveltekit.'
    condition: selection
falsepositives:
    - Legitimate error messages with placeholders (if custom)
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-53262 SvelteKit XSS Attempt"; flow:to_client,established; content:"%sveltekit."; http_server_body; classtype:web-application-attack; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate:

• Upgrade SvelteKit to version 2.8.3 or later.
• If upgrade is not possible, manually escape error messages in custom error handling.

Long-term:

• Implement Content Security Policy (CSP) headers.
• Use input validation and output encoding for all user-controlled data.
• Regularly audit dependencies for vulnerabilities.

Workarounds:

• None officially provided. The only fix is to upgrade.

Valters IT
DevSecOps & Cyber Threat Intelligence Expert