CVE-2026-30837: Elysia TypeScript Framework DoS Vulnerability

CVE-2026-30837 Overview

CVE-2026-30837 is a Regular Expression Denial of Service (ReDoS) vulnerability in Elysia, a TypeScript framework designed for request validation, type inference, OpenAPI documentation, and client-server communication. Prior to version 1.4.26, the t.String({ format: 'url' }) validation function is vulnerable to ReDoS attacks. By repeating a partial URL format (protocol and hostname) multiple times, attackers can cause the underlying regex to slow down significantly, leading to denial of service conditions.

Critical Impact

Attackers can exploit this ReDoS vulnerability to cause severe application slowdown or complete denial of service by sending specially crafted URL strings to validation endpoints, consuming excessive CPU resources and blocking legitimate requests.

Affected Products

• Elysia Framework versions prior to 1.4.26
• Applications using t.String({ format: 'url' }) validation
• TypeScript/JavaScript services built on vulnerable Elysia versions

Discovery Timeline

• 2026-03-10 - CVE CVE-2026-30837 published to NVD
• 2026-03-11 - Last updated in NVD database

Technical Details for CVE-2026-30837

Vulnerability Analysis

This vulnerability (CWE-1333: Inefficient Regular Expression Complexity) exists in Elysia's URL format validation mechanism. The regular expression used to validate URL strings contains patterns that exhibit catastrophic backtracking when processing maliciously crafted input. When an attacker submits a string that repeats partial URL components (such as the protocol scheme and hostname portions) multiple times, the regex engine enters an exponential time complexity state, consuming excessive CPU cycles.

The vulnerability is particularly dangerous in web application contexts where user-supplied input is validated against the URL format. Since the validation occurs synchronously on the main thread in Node.js/Bun environments, a single malicious request can effectively freeze the entire application, blocking all other requests from being processed.

Root Cause

The root cause is an inefficiently designed regular expression pattern used for URL format validation within Elysia's type system. The regex contains nested quantifiers or overlapping alternation patterns that create multiple paths for the regex engine to explore when processing partial matches. When input contains repeated URL-like fragments that nearly match but ultimately fail, the regex engine must exhaustively try all possible combinations before determining the string is invalid, resulting in exponential time complexity.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by sending HTTP requests containing specially crafted strings to any endpoint that uses t.String({ format: 'url' }) validation. The malicious payload consists of repeated partial URL patterns that trigger catastrophic backtracking in the validation regex.

The attack is particularly effective because:

1. It requires only a single malicious request to impact application availability
2. The attacker needs no special privileges or authentication
3. The payload can be embedded in any field validated with the URL format constraint

For detailed technical information and proof-of-concept examples, refer to the GitHub PoC Repository and the GitHub Security Advisory.

Detection Methods for CVE-2026-30837

Indicators of Compromise

• Unusual CPU spikes during request processing, particularly on validation endpoints
• HTTP requests with abnormally long URL-formatted string parameters containing repeated patterns
• Application response time degradation or timeouts correlating with specific request patterns
• Node.js/Bun event loop blocking events in application monitoring

Detection Strategies

• Implement request timeout monitoring to detect validation operations exceeding normal thresholds
• Monitor CPU utilization per request and flag requests causing disproportionate processing time
• Deploy web application firewall (WAF) rules to detect and block inputs with suspicious repetitive patterns
• Use application performance monitoring (APM) to track regex validation duration anomalies

Monitoring Recommendations

• Configure alerts for sustained high CPU usage in Elysia-based application processes
• Implement request logging with timing metrics to identify slow validation operations
• Monitor for increased request timeouts or error rates on endpoints using URL validation
• Set up anomaly detection for request body sizes and pattern characteristics

How to Mitigate CVE-2026-30837

Immediate Actions Required

• Upgrade Elysia to version 1.4.26 or later immediately to address this vulnerability
• Review application code to identify all usages of t.String({ format: 'url' }) validation
• Implement request timeouts and rate limiting as defense-in-depth measures
• Consider input length restrictions on URL-validated fields as a temporary mitigation

Patch Information

The vulnerability has been addressed in Elysia version 1.4.26. The fix involves replacing the vulnerable regular expression with a more efficient pattern that eliminates catastrophic backtracking scenarios. Users should update their package.json dependencies and run npm update elysia or the equivalent command for their package manager. For more details, see the GitHub Security Advisory.

Workarounds

• Implement input length validation before URL format validation to limit potential ReDoS impact
• Use alternative URL validation libraries with known-safe regex implementations as a temporary measure
• Deploy rate limiting and request timeouts at the infrastructure level to contain DoS impact
• Consider using URL parsing libraries (like the WHATWG URL API) instead of regex-based validation

# Configuration example
# Upgrade Elysia to the patched version
npm update elysia@1.4.26

# Or update package.json and reinstall
# "elysia": "^1.4.26"
npm install