CVE-2021-36755 in Web Monitor
Summary
by MITRE • 07/16/2021
Nightscout Web Monitor (aka cgm-remote-monitor) 14.2.2 allows XSS via a crafted X-Forwarded-For header.
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Analysis
by VulDB Data Team • 07/19/2021
The vulnerability identified as CVE-2021-36755 affects the Nightscout Web Monitor application, also known as cgm-remote-monitor version 14.2.2, which is a widely used open-source platform for monitoring continuous glucose monitor data. This application serves healthcare professionals and patients who rely on real-time glucose tracking capabilities, making its security critical for maintaining patient data integrity and privacy. The vulnerability manifests through improper input validation within the application's handling of HTTP headers, specifically the X-Forwarded-For header that is commonly used in web applications to identify the original IP address of a client connecting through a proxy or load balancer.
The technical flaw resides in the application's insufficient sanitization of the X-Forwarded-For header value before rendering it within the web interface. When an attacker crafts a malicious X-Forwarded-For header containing script tags or other malicious content, the application fails to properly escape or validate this input, allowing the malicious payload to be executed within the context of a victim's browser session. This represents a classic cross-site scripting vulnerability that falls under CWE-79, which specifically addresses cross-site scripting flaws in web applications. The vulnerability is particularly concerning because it can be exploited through a simple HTTP header manipulation without requiring authentication or direct interaction with the application's core functionality.
The operational impact of this vulnerability extends beyond simple data theft or manipulation, as it enables attackers to potentially execute arbitrary JavaScript code within the browser of authenticated users. An attacker could leverage this vulnerability to steal session cookies, redirect users to malicious sites, modify application content, or even perform actions on behalf of the user if the application has sufficient privileges. The attack vector is particularly dangerous because it can be delivered through standard HTTP headers that are automatically included by proxies, load balancers, or other network infrastructure components, making it difficult to detect and prevent through traditional network monitoring approaches. This vulnerability specifically aligns with ATT&CK technique T1566.001, which covers the use of malicious headers in web application attacks.
Mitigation strategies for this vulnerability should include immediate implementation of input validation and output encoding for all HTTP headers, particularly the X-Forwarded-For header, before any rendering occurs. Organizations should ensure that all user-supplied data, including HTTP headers, is properly sanitized and escaped before being displayed in web interfaces. The recommended approach involves implementing a robust header validation mechanism that strips or encodes potentially dangerous characters, along with regular security audits of the application's input handling processes. Additionally, maintaining up-to-date versions of the Nightscout application and implementing proper web application firewall rules that can detect and block malicious header content will significantly reduce the risk of exploitation. The vulnerability also underscores the importance of following secure coding practices as outlined in OWASP Top Ten and the CWE guidelines for preventing cross-site scripting attacks through proper input validation and output encoding mechanisms.