CVE-2026-40074: SvelteKit DoS Vulnerability

CVE-2026-40074 Overview

CVE-2026-40074 is a Denial of Service (DoS) vulnerability affecting SvelteKit, a popular framework for rapidly developing robust, performant web applications using Svelte. The vulnerability exists in versions prior to 2.57.1 and occurs when the redirect function is called from inside the handle server hook with a location parameter containing characters that are invalid in an HTTP header. This results in an unhandled TypeError that can crash the application.

Critical Impact

Applications using SvelteKit versions prior to 2.57.1 that pass unsanitized user input to the redirect function within the handle server hook are vulnerable to denial of service attacks. Attackers can craft malicious requests containing invalid HTTP header characters to crash the server.

Affected Products

• SvelteKit versions prior to 2.57.1
• SvelteKit applications running on Node.js
• Applications using the handle server hook with redirect functionality

Discovery Timeline

• April 10, 2026 - CVE-2026-40074 published to NVD
• April 15, 2026 - Last updated in NVD database

Technical Details for CVE-2026-40074

Vulnerability Analysis

This vulnerability falls under CWE-755 (Improper Handling of Exceptional Conditions). The core issue lies in the redirect function's failure to properly validate and sanitize the location parameter before using it in HTTP header construction. When the location contains characters that are invalid in HTTP headers, the framework throws an unhandled TypeError instead of gracefully handling the error.

The vulnerability is particularly dangerous when applications pass unsanitized user input to the redirect function. An attacker can exploit this by providing specially crafted input containing invalid HTTP header characters, causing the server to crash and resulting in a denial of service condition.

Root Cause

The root cause of this vulnerability is improper exception handling in the SvelteKit framework's redirect mechanism. When the redirect function is invoked within the handle server hook, it attempts to set HTTP headers with the provided location value. If this value contains characters that are not permitted in HTTP headers (such as certain control characters or non-ASCII characters), the underlying HTTP library throws a TypeError. SvelteKit versions prior to 2.57.1 do not catch this exception, allowing it to propagate up and crash the application.

Attack Vector

The attack vector is network-based, requiring no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Identifying an endpoint or application flow where user-controlled input is passed to the redirect function within the handle server hook
2. Crafting a malicious request containing invalid HTTP header characters (e.g., newline characters, null bytes, or other control characters)
3. Sending the crafted request to the vulnerable SvelteKit application
4. The unhandled TypeError causes the Node.js process to crash, resulting in service disruption

The vulnerability is exploited through carefully crafted input containing characters that violate HTTP header specifications (RFC 7230). When such characters reach the redirect location, the HTTP library's header validation fails, throwing an uncaught exception.

Detection Methods for CVE-2026-40074

Indicators of Compromise

• Unexpected application crashes or restarts with TypeError exceptions in logs
• Error messages referencing invalid characters in HTTP headers within the handle server hook
• Unusual patterns of requests containing non-standard characters in URL parameters or form fields
• Increased crash frequency correlated with specific request patterns

Detection Strategies

• Monitor application logs for unhandled TypeError exceptions related to HTTP header operations
• Implement request logging to identify patterns of malicious input containing control characters
• Use application performance monitoring (APM) tools to detect sudden process terminations
• Review SvelteKit application code for instances where user input flows to redirect functions

Monitoring Recommendations

• Configure alerts for abnormal application restart rates or crash loops
• Enable detailed error logging for the handle server hook to capture exception details
• Implement request validation logging to flag requests with suspicious character patterns
• Set up health checks that can detect and alert on service unavailability

How to Mitigate CVE-2026-40074

Immediate Actions Required

• Upgrade SvelteKit to version 2.57.1 or later immediately
• Audit application code to identify any use of the redirect function with user-controlled input
• Implement input sanitization for any data passed to redirect functions before patching
• Review and validate all redirect locations in the handle server hook

Patch Information

The vulnerability has been fixed in SvelteKit version 2.57.1. The fix is available through the GitHub commit 10d7b44. Users should upgrade by updating their package.json dependencies and running the appropriate package manager update command. For detailed information, see the GitHub Security Advisory GHSA-3f6h-2hrp-w5wx and the official release notes.

Workarounds

• Sanitize all user input before passing it to the redirect function to remove invalid HTTP header characters
• Implement a wrapper function around redirect that validates and encodes the location parameter
• Use try-catch blocks in the handle hook to gracefully handle potential errors from redirect operations
• Implement URL encoding on redirect locations to escape potentially problematic characters

# Configuration example
# Upgrade SvelteKit to the patched version
npm update @sveltejs/kit@2.57.1

# Or explicitly install the fixed version
npm install @sveltejs/kit@2.57.1

# Verify the installed version
npm list @sveltejs/kit