CVE-2017-16118 in forwarded
Summary
by MITRE
The forwarded module is used by the Express.js framework to handle the X-Forwarded-For header. It is vulnerable to a regular expression denial of service when it's 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-2017-16118 represents a critical security flaw within the Express.js framework's forwarded module, which processes the X-Forwarded-For HTTP header commonly used in web applications to identify the original IP address of a client connecting through proxies or load balancers. This module serves as a fundamental component in web application architectures where traffic routing and client identification are essential for proper functionality. The vulnerability manifests when the module processes specially crafted input that triggers a regular expression denial of service condition, fundamentally compromising the application's availability and operational integrity.
The technical implementation of this vulnerability stems from the module's use of regular expressions to parse the X-Forwarded-For header values, where the regular expression pattern is susceptible to catastrophic backtracking when confronted with maliciously constructed input. This particular class of vulnerability aligns with CWE-400, which categorizes regular expression denial of service as a weakness that can lead to resource exhaustion and system unresponsiveness. The flaw occurs because the regular expression engine enters into an exponential time complexity scenario when processing inputs designed to exploit the backtracking behavior inherent in poorly constructed regular expressions. This results in the event loop being monopolized for extended periods, effectively blocking all other operations and rendering the application unresponsive to legitimate requests.
The operational impact of CVE-2017-16118 extends beyond simple service disruption, as it can be exploited by attackers to perform denial of service attacks against web applications built on the Express.js framework. The vulnerability's exploitation requires minimal privileges and can be executed through simple HTTP requests containing malicious X-Forwarded-For headers, making it particularly dangerous in production environments where such headers are commonly processed. From an attacker's perspective, this vulnerability maps to ATT&CK technique T1499.004, which involves network denial of service attacks, and T1595.001, which encompasses network infiltration through common application vulnerabilities. The vulnerability affects applications where the forwarded module is utilized for header parsing, potentially impacting numerous web services and APIs that rely on proper client IP address identification for security policies, rate limiting, and access control mechanisms.
Mitigation strategies for this vulnerability involve immediate patching of the forwarded module to version 0.1.5 or later, which addresses the regular expression vulnerability through improved input validation and more efficient parsing algorithms. Organizations should implement comprehensive monitoring and logging of X-Forwarded-For header processing to detect potential exploitation attempts and establish rate limiting mechanisms to prevent abuse of the vulnerable functionality. Additionally, security teams should conduct thorough vulnerability assessments of their Express.js applications to identify all instances where the forwarded module is utilized and ensure that proper input sanitization measures are implemented at the application level. The vulnerability also underscores the importance of regular security audits and dependency updates as part of the software development lifecycle to prevent similar issues from emerging in other components of the web application stack.