CVE-2026-117539 in svg-sanitizer
Summary
by MITRE • 10/27/2026
darylldoyle svg-sanitizer before 1.0.0 contains CSS injection and remote reference bypass vulnerabilities that allow attackers to embed external resources by uploading crafted SVG files. Attackers can use unsanitized style content, unquoted url() values, or image href attributes to exfiltrate page data, track users, or disclose visitor IP addresses.
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Analysis
by VulDB Data Team • 10/27/2026
The vulnerability in darylldoyle svg-sanitizer prior to version 1.0.0 represents a critical failure in input validation and sanitization logic for Scalable Vector Graphics files. This library is commonly used by web applications to process user-uploaded SVG images, yet the flawed implementation fails to adequately strip or neutralize malicious CSS properties and HTML attributes embedded within the vector markup. The core technical flaw lies in the parser's inability to correctly identify and sanitize unquoted url() values and specific image href attributes when they appear in certain contexts. Because the sanitizer does not strictly enforce a whitelist of safe SVG elements and attributes, it allows attackers to inject arbitrary Cascading Style Sheets (CSS) into the rendered output. This lack of rigorous filtering creates an environment where external resources can be referenced without proper validation, effectively bypassing security controls designed to prevent cross-site scripting and data exfiltration through image uploads.
From a technical perspective, the exploitation vector relies on the browser's native rendering engine interpreting the malformed SVG content as executable code rather than static imagery. By embedding unquoted url() directives within CSS style blocks or using specific href attributes in image tags, an attacker can force the victim's browser to make HTTP requests to domains controlled by the adversary. These requests often include sensitive data such as cookies, session tokens, and referrer headers containing page URLs. Furthermore, because these external resources are loaded from third-party servers, they enable sophisticated tracking mechanisms that can fingerprint users based on their IP addresses, user agent strings, and other network-level identifiers. This capability transforms a simple image upload feature into a potent tool for surveillance and data theft, as the victim's browser acts as an unwitting proxy for the attacker's infrastructure.
The operational impact of this vulnerability is severe, particularly in applications that allow users to generate or customize content using SVG files, such as avatar uploads, profile picture submissions, or document generation tools. Attackers can leverage these flaws to conduct cross-site tracking campaigns, stealing user identity information and session credentials without requiring any direct interaction beyond the initial upload. The ability to exfiltrate page data means that sensitive context surrounding the victim's activity at the time of exploitation is compromised. This not only violates user privacy but also undermines the integrity of the application by allowing malicious actors to harvest authentication tokens or other confidential state information stored in cookies or local storage, potentially leading to account takeover scenarios if combined with other attack vectors like cross-site scripting.
To mitigate this risk, organizations must immediately upgrade darylldoyle svg-sanitizer to version 1.0.0 or later, where these specific parsing flaws have been addressed through stricter validation rules and improved handling of unquoted values. For applications unable to update promptly, implementing a secondary layer of defense is essential. This includes configuring Content Security Policy headers that restrict the sources from which images and styles can be loaded, thereby neutralizing the impact of any injected external references even if they bypass the sanitizer. Additionally, deploying server-side validation that inspects uploaded SVG files for suspicious patterns such as unquoted url() or specific CSS injection attempts before processing them further adds a robust defense-in-depth measure. Regular security audits focusing on XML and SVG parsing libraries are recommended to ensure that future updates do not introduce similar weaknesses in input handling logic.
This vulnerability aligns with Common Weakness Enumeration CWE-79, which covers Improper Neutralization of Script-Related HTML Tags in Web Pages, as well as CWE-20 regarding improper input validation. In the context of the MITRE ATT&CK framework, this behavior maps to T1568.002, Dynamic Resolution, where attackers use domain generation algorithms or fast-flux techniques via external resource loading to establish command and control channels or exfiltrate data. The exploitation technique also reflects aspects of T1071 Application Layer Protocol abuse, specifically leveraging HTTP requests initiated by the victim's browser for malicious purposes. Understanding these classifications helps security teams prioritize remediation efforts based on established industry standards and threat intelligence models.