CVE-2025-55166 is a medium-severity vulnerability affecting versions of the darylldoyle svg-sanitizer PHP library prior to 0.22.0. Svg-sanitizer is designed to sanitize SVG and XML content to prevent malicious code injection, particularly in web applications that accept user-supplied SVG files. The vulnerability arises from improper neutralization of input during web page generation, specifically a Cross-site Scripting (XSS) flaw categorized under CWE-79. The root cause is that the sanitization logic in the cleanXlinkHrefs method only inspects attribute names in lowercase, allowing attackers to bypass the isHrefSafeValue check by using uppercase or mixed-case attribute names. This bypass enables injection of malicious scripts or links to external domains within sanitized SVG content. Exploiting this vulnerability requires no privileges and no authentication but does require user interaction, such as viewing or uploading malicious SVG content. The CVSS 4.0 base score is 5.1 (medium), reflecting network attack vector, low complexity, no privileges required, user interaction needed, and limited impact on confidentiality and integrity. The issue has been addressed in version 0.22.0 by correcting the attribute name case handling to ensure all variants are properly sanitized. No known exploits are currently reported in the wild. This vulnerability is significant for web applications that rely on svg-sanitizer to safely process SVG files, as successful exploitation can lead to XSS attacks, enabling attackers to execute arbitrary scripts in the context of the victim's browser, potentially leading to session hijacking, data theft, or redirection to malicious sites.

For European organizations, especially those operating web platforms that allow user-generated SVG content or integrate svg-sanitizer in their PHP-based web stacks, this vulnerability poses a tangible risk. Successful exploitation can compromise user confidentiality and integrity by executing malicious scripts, potentially leading to credential theft, unauthorized actions, or distribution of malware. The impact on availability is limited but could be leveraged in phishing or social engineering campaigns targeting European users. Given the widespread use of PHP and SVG in web applications across Europe, organizations in sectors such as e-commerce, government portals, media, and online collaboration tools are at risk. The vulnerability's exploitation does not require authentication, increasing exposure. However, the need for user interaction (e.g., viewing malicious SVG content) somewhat limits automated mass exploitation. Nonetheless, attackers could craft targeted attacks against European users or employees, especially in countries with high internet penetration and digital service usage. The vulnerability also raises compliance concerns under GDPR if personal data is compromised through XSS attacks. Therefore, European organizations must prioritize patching and monitoring for suspicious SVG content uploads or rendering.

1. Immediate upgrade of svg-sanitizer to version 0.22.0 or later to ensure the sanitization logic correctly handles attribute name casing and prevents bypasses. 2. Implement additional server-side validation and sanitization of SVG files beyond relying solely on svg-sanitizer, such as using multiple sanitization layers or alternative libraries with robust security track records. 3. Employ Content Security Policy (CSP) headers to restrict script execution and mitigate the impact of potential XSS attacks originating from SVG content. 4. Monitor and log SVG file uploads and usage patterns to detect anomalous or suspicious activity indicative of exploitation attempts. 5. Educate developers and security teams about the risks of improper SVG sanitization and encourage secure coding practices when handling user-generated content. 6. For web applications, consider disabling or restricting SVG uploads if not strictly necessary, or convert SVGs to safer formats after sanitization. 7. Conduct regular security assessments and penetration testing focusing on file upload and rendering functionalities to identify similar weaknesses. 8. Ensure incident response plans include procedures for handling XSS incidents and potential data breaches stemming from such vulnerabilities.