CVE-2019-18857 in svg-sanitizer
Summary
by MITRE
darylldoyle svg-sanitizer before 0.12.0 mishandles script and data values in attributes, as demonstrated by unexpected whitespace such as in the javascript	:alert substring.
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Analysis
by VulDB Data Team • 02/11/2024
The vulnerability identified as CVE-2019-18857 affects the darylldoyle svg-sanitizer library version 0.12.0 and earlier, representing a critical security flaw in SVG sanitization mechanisms. This issue stems from inadequate handling of script and data values within SVG attributes, specifically failing to properly sanitize whitespace characters that can be used to obfuscate malicious code execution. The vulnerability manifests when the sanitizer encounters unexpected whitespace sequences such as the javascript	:alert substring, where the 	 represents a tab character that bypasses standard sanitization checks. This flaw falls under the category of improper input validation and sanitization, which is classified as CWE-20 by the CWE database, specifically addressing weaknesses in the input validation process. The sanitizer's failure to properly handle encoded whitespace characters creates a pathway for attackers to inject malicious JavaScript code that would otherwise be blocked by conventional security measures.
The technical exploitation of this vulnerability relies on the ability to manipulate whitespace characters within SVG attribute values to bypass sanitization filters. When the svg-sanitizer processes SVG content containing encoded whitespace such as tab characters, it fails to normalize these sequences properly, allowing malicious code to persist in the sanitized output. The specific case involves the javascript	:alert substring where the tab character encoded as 	 is not properly stripped or normalized during the sanitization process. This represents a classic example of encoding bypass techniques that are commonly employed in web application security attacks and aligns with tactics described in the MITRE ATT&CK framework under T1203 - Exploitation for Client Execution. The vulnerability demonstrates how seemingly benign whitespace characters can be weaponized to circumvent security controls designed to prevent cross-site scripting attacks.
The operational impact of this vulnerability extends beyond simple code injection, potentially allowing attackers to execute arbitrary JavaScript within contexts that trust the sanitized SVG content. When applications rely on the svg-sanitizer library for processing user-provided SVG data, malicious actors can craft SVG files containing tab-encoded JavaScript payloads that bypass security controls. This creates a significant risk for web applications that process SVG uploads, render user-generated content, or integrate SVG elements into dynamic web pages. The vulnerability is particularly dangerous because it can be exploited across multiple attack vectors including file upload restrictions, content management systems, and web applications that process SVG data from untrusted sources. Security professionals should note that this issue affects not just the immediate application but can potentially compromise entire user sessions and data integrity when exploited in conjunction with other vulnerabilities.
Mitigation strategies for CVE-2019-18857 require immediate implementation of library updates to version 0.12.0 or later, which contain proper handling of whitespace characters in attribute values. Organizations should also implement additional layers of security including comprehensive input validation, strict content type restrictions, and regular security scanning of SVG content. The fix addresses the core sanitization logic to properly normalize and remove encoded whitespace characters from SVG attributes before processing. Security teams should also consider implementing web application firewalls with specific rules targeting encoded whitespace patterns and conducting thorough code reviews for SVG processing functions. Additionally, organizations should establish monitoring for suspicious SVG content patterns and maintain up-to-date threat intelligence feeds that track similar encoding bypass techniques. The vulnerability highlights the importance of thorough testing for encoding edge cases in security libraries and the need for comprehensive sanitization that accounts for various character encoding scenarios beyond basic ASCII characters.