CVE-2026-121446 in svg-sanitizer
Summary
by MITRE • 11/03/2026
svg-sanitizer from 0.5.0 before 1.1.0 contains an incomplete filtering vulnerability that allows attackers to bypass remote URL removal by using CSS escape sequences inside url() in SVG presentation attributes. Attackers can supply SVG files with attributes like fill containing escaped slashes that browsers resolve to remote URLs, triggering external requests and enabling data exfiltration.
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Analysis
by VulDB Data Team • 11/03/2026
The svg-sanitizer library, specifically versions prior to 1.1.0, suffers from a critical input validation flaw classified as an incomplete filtering vulnerability. This security defect arises because the sanitization logic fails to adequately process CSS escape sequences embedded within url() functions in SVG presentation attributes. While the primary objective of the sanitizer is to strip remote URLs to prevent external network requests and potential data leakage, it incorrectly assumes that standard string matching or basic parsing is sufficient for all valid representations of a URL. By leveraging CSS escaping mechanisms, such as using backslashes followed by hexadecimal digits to represent special characters like slashes, attackers can construct payloads where the raw text appears safe but resolves to a malicious remote address when interpreted by the rendering engine.
The technical mechanism behind this bypass relies on how web browsers parse SVG attributes containing url() values. When an attacker supplies an SVG file with presentation attributes such as fill or stroke set to a value like url(\002f\002e\002e), the sanitizer may not recognize these escaped sequences as representing forward slashes and relative path components. Consequently, the library permits the attribute to pass through its filtering rules intact. However, when this SVG is rendered in a modern web browser or an SVG-capable viewer, the CSS parser interprets the escape sequences according to standard specifications, resolving them into actual URL paths. This discrepancy between the sanitizer's static analysis and the runtime behavior of the client-side rendering engine creates a significant security gap that allows malicious content to bypass intended restrictions.
The operational impact of this vulnerability is severe, primarily facilitating unauthorized data exfiltration and potentially enabling remote code execution depending on the context in which the SVG is processed. Because the browser successfully resolves the escaped sequences into valid remote URLs, it initiates HTTP requests to attacker-controlled servers. These requests can carry sensitive information from the victim's environment as part of the URL query parameters or headers, allowing attackers to steal cookies, session tokens, or other confidential data stored in the client context. Furthermore, if the SVG is used in a server-side rendering pipeline that does not perform its own independent validation, this vulnerability could lead to Server-Side Request Forgery (SSRF) scenarios where internal network resources are accessed from an external-facing application layer.
This flaw aligns with Common Weakness Enumeration CWE-20, which describes improper input validation, and specifically relates to the failure to handle encoded or escaped inputs correctly during sanitization processes. In terms of offensive security frameworks, this vulnerability supports techniques associated with ATT&CK T1567, known as Exfiltration Over Web Service, where data is stolen by sending it over HTTP/HTTPS protocols disguised as legitimate traffic. The use of CSS escapes to obfuscate the malicious intent also touches upon aspects of defense evasion, making detection by simple signature-based systems more difficult since the payload does not contain obvious indicators of compromise in its raw form.
To mitigate this risk, organizations relying on svg-sanitizer must immediately upgrade to version 1.1.0 or later, where the filtering logic has been hardened to correctly decode and evaluate CSS escape sequences before determining whether a URL is safe. For environments that cannot update promptly, it is advisable to implement additional server-side validation layers that normalize all input strings by decoding any Unicode or CSS escapes prior to processing them through sanitization routines. Developers should also consider adopting defense-in-depth strategies by validating SVG content against strict allow-lists of permitted attributes and values rather than relying solely on blacklist-based filtering mechanisms which are prone to bypasses via encoding tricks. Regular security audits focusing on input handling in XML and SVG parsers will further reduce the likelihood of similar vulnerabilities being introduced or exploited in related components.