CVE-2017-16119 in Fresh
Summary
by MITRE
Fresh is a module used by the Express.js framework for HTTP response freshness testing. It is vulnerable to a regular expression denial of service when it is passed specially crafted input to parse. This causes the event loop to be blocked causing a denial of service condition.
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Analysis
by VulDB Data Team • 02/16/2020
The vulnerability identified as CVE-2017-16119 affects the Fresh module within the Express.js framework, which is designed to handle HTTP response freshness testing. This module plays a critical role in determining whether a cached response can be considered fresh enough to serve to a client without revalidation. The flaw manifests when the module processes specially crafted input through regular expression patterns that are susceptible to catastrophic backtracking. This vulnerability falls under the category of Regular Expression Denial of Service as classified by CWE-400, where malicious input causes the regular expression engine to consume excessive computational resources. The Fresh module's implementation uses regular expressions to parse cache-control headers and other HTTP freshness-related parameters, making it an attractive target for attackers seeking to disrupt service availability.
The technical exploitation of this vulnerability occurs when an attacker crafts malicious input that triggers catastrophic backtracking in the regular expression engine. When the Fresh module processes such input, the regular expression matching algorithm enters an exponential time complexity state where it repeatedly backtracks through the same input positions, causing the event loop to become blocked indefinitely. This behavior is characteristic of ReDoS attacks as defined in the ATT&CK framework under T1499.3, specifically targeting application availability through resource exhaustion. The vulnerability is particularly dangerous in Node.js environments because the event loop is single-threaded, meaning that blocking operations directly impact the entire application's responsiveness. The module's reliance on regular expressions for parsing HTTP headers creates a direct pathway for attackers to exploit this weakness without requiring elevated privileges or complex attack vectors.
The operational impact of CVE-2017-16119 extends beyond simple service disruption to potentially affecting the entire application ecosystem. When exploited, the vulnerability can cause the affected Express.js application to become unresponsive, leading to denial of service for legitimate users while the event loop remains occupied with processing the malicious regular expression input. This vulnerability is especially concerning in high-traffic web applications where the Fresh module is frequently invoked during request processing cycles. The attack surface includes any application using Express.js with the Fresh module, particularly those that accept user-provided input in HTTP headers or response parameters. Organizations running vulnerable versions of Express.js and related modules face significant risk of service degradation or complete application unavailability, making this vulnerability a critical priority for remediation.
Mitigation strategies for CVE-2017-16119 focus on both immediate patching and defensive programming approaches. The primary recommendation involves updating to patched versions of the Fresh module and Express.js framework where the vulnerable regular expressions have been replaced with more efficient alternatives or properly bounded patterns. Organizations should also implement input validation and sanitization measures to filter out potentially malicious headers before they reach the Fresh module. Additional defensive measures include implementing timeouts for regular expression operations, rate limiting header parsing, and monitoring for unusual processing patterns that might indicate ReDoS attempts. The vulnerability highlights the importance of secure coding practices and regular security assessments, particularly for modules that process untrusted input through regular expression engines. Security teams should also consider implementing network-level protections such as web application firewalls that can detect and block suspicious header patterns before they reach vulnerable application components.