CVE-2018-7158 in Node.js
Summary
by MITRE
The `'path'` module in the Node.js 4.x release line contains a potential regular expression denial of service (ReDoS) vector. The code in question was replaced in Node.js 6.x and later so this vulnerability only impacts all versions of Node.js 4.x. The regular expression, `splitPathRe`, used within the `'path'` module for the various path parsing functions, including `path.dirname()`, `path.extname()` and `path.parse()` was structured in such a way as to allow an attacker to craft a string, that when passed through one of these functions, could take a significant amount of time to evaluate, potentially leading to a full denial of service.
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Analysis
by VulDB Data Team • 02/06/2020
The vulnerability identified as CVE-2018-7158 represents a classic regular expression denial of service (ReDoS) flaw within the Node.js 4.x release line. This issue specifically targets the path module's internal regular expression pattern known as splitPathRe which is utilized across multiple path parsing functions including dirname, extname, and parse. The vulnerability stems from the inefficient design of the regular expression pattern that creates exponential backtracking behavior when processing specially crafted input strings. This flaw affects only Node.js 4.x versions since the problematic code was completely rewritten and replaced in Node.js 6.x and subsequent releases, making it a version-specific issue that has been resolved in newer iterations of the runtime environment.
The technical implementation of this vulnerability occurs when an attacker constructs malicious input strings that trigger the problematic regular expression pattern within the path module. The splitPathRe regular expression exhibits catastrophic backtracking characteristics where the regex engine must explore an exponentially growing number of possible matching paths as the input string length increases. When such malicious input is passed through functions like path.dirname(), path.extname(), or path.parse(), the regular expression engine enters a state where it performs an enormous number of redundant calculations, causing significant computational overhead and potentially leading to complete system unresponsiveness. This behavior directly aligns with CWE-400, which categorizes regular expression denial of service vulnerabilities as a weakness that can cause excessive resource consumption and system instability.
The operational impact of CVE-2018-7158 extends beyond simple performance degradation to potentially enable full system denial of service attacks. An attacker who can influence input to path parsing functions could exploit this vulnerability to consume excessive CPU resources, causing legitimate requests to time out or fail. This vulnerability is particularly concerning in web applications where user input flows through path parsing functions, as it could be leveraged to create sustained denial of service conditions that impact availability. The attack vector is relatively straightforward since attackers need only craft specific input strings that trigger the backtracking behavior, making this a low-effort, high-impact vulnerability in vulnerable environments. The ReDoS characteristics also make this vulnerability difficult to detect through standard security scanning tools, as the malicious behavior only manifests under specific conditions involving the crafted input patterns.
Mitigation strategies for CVE-2018-7158 primarily focus on upgrading to Node.js 6.x or later versions where the vulnerable regular expression has been replaced with a more efficient implementation. Organizations running Node.js 4.x should prioritize immediate migration to supported versions as the vulnerability is not patched in the affected release line. Alternative mitigation approaches include implementing input validation and sanitization measures to prevent malicious strings from reaching the vulnerable path parsing functions, though this approach is less reliable since the vulnerability can be triggered by inputs that appear benign. Security teams should also consider monitoring for unusual CPU usage patterns that might indicate exploitation attempts, and implementing rate limiting on path parsing operations where possible. The vulnerability serves as a reminder of the importance of regular security updates and the risks associated with running unsupported software versions, as highlighted by ATT&CK technique T1499.101 which covers resource exhaustion via service denial of service attacks. Organizations should also review their Node.js dependencies and ensure all components are updated to versions that do not contain this vulnerability, as the issue may be present in older versions of Node.js libraries that depend on the vulnerable path module.