CVE-2018-3741 in rails-html-sanitizer Gem
Summary
by MITRE
There is a possible XSS vulnerability in all rails-html-sanitizer gem versions below 1.0.4 for Ruby. The gem allows non-whitelisted attributes to be present in sanitized output when input with specially-crafted HTML fragments, and these attributes can lead to an XSS attack on target applications. This issue is similar to CVE-2018-8048 in Loofah. All users running an affected release should either upgrade or use one of the workarounds immediately.
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Analysis
by VulDB Data Team • 02/24/2023
The CVE-2018-3741 vulnerability represents a critical cross-site scripting weakness in the rails-html-sanitizer gem, a widely-used Ruby library for sanitizing HTML content in web applications. This vulnerability specifically affects versions prior to 1.0.4 and stems from improper handling of HTML attributes during the sanitization process. The flaw allows malicious actors to inject harmful attributes that bypass the intended security controls, potentially leading to arbitrary code execution in users' browsers. The vulnerability is particularly concerning because it undermines the fundamental security assumptions of HTML sanitization, which is a critical defense mechanism against client-side attacks in web applications.
The technical root cause of this vulnerability lies in the gem's attribute filtering logic where non-whitelisted attributes are not being properly stripped from HTML fragments during sanitization. When developers use the rails-html-sanitizer gem to process user input, the library should remove potentially dangerous attributes such as onclick, onerror, or javascript: URLs. However, due to implementation flaws in versions below 1.0.4, certain attributes can slip through the sanitization process and remain in the output. This behavior creates a vector for attackers to inject malicious JavaScript code through carefully crafted HTML fragments that exploit the gap in the sanitization logic. The vulnerability aligns with CWE-79, which specifically addresses cross-site scripting flaws in input handling and output encoding.
The operational impact of CVE-2018-3741 extends beyond simple data theft or defacement, as it enables full exploitation of client-side attack vectors that can compromise user sessions, steal sensitive information, or redirect users to malicious sites. Applications using affected versions of the gem are at risk of having user-generated content processed through vulnerable sanitization routines, particularly in content management systems, comment sections, or any feature allowing HTML input. The vulnerability is especially dangerous in environments where users can submit HTML content that gets rendered on web pages without additional security layers. Attackers can craft malicious HTML fragments containing attributes that trigger XSS payloads when rendered in browsers, making this a significant threat to web application security.
Organizations should immediately upgrade to rails-html-sanitizer version 1.0.4 or later to remediate this vulnerability, as the patch addresses the core attribute filtering logic that was allowing dangerous attributes to persist. Security teams should conduct comprehensive audits of their applications to identify all instances where the affected gem is used, particularly in areas handling user-generated content or HTML input processing. Additional mitigations include implementing proper content security policies, using additional input validation layers, and considering alternative HTML sanitization libraries if immediate upgrades are not feasible. The vulnerability demonstrates the critical importance of maintaining up-to-date security dependencies and following the principle of least privilege in input handling, as outlined in the ATT&CK framework's defense against client-side attacks through proper input sanitization techniques.