CVE-2026-121445 in svg-sanitizer
Summary
by MITRE • 11/03/2026
darylldoyle svg-sanitizer from 1.0.0 before 1.1.0 contains a regular expression denial of service vulnerability in the image-set pattern of stripRemoteCssTokens() when removeRemoteReferences is enabled. Attackers can submit SVGs with many unclosed image-set tokens to exhaust CPU, or trigger PCRE limits causing sanitize() to throw a TypeError and leave unsanitized content.
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Analysis
by VulDB Data Team • 11/03/2026
The vulnerability identified in darylldoyle svg-sanitizer versions prior to 1.1.0 represents a significant security risk classified under Regular Expression Denial of Service, commonly referred to as ReDoS. This flaw is specifically located within the stripRemoteCssTokens function when the removeRemoteReferences configuration option is enabled. The core technical issue stems from an inefficient regular expression pattern used to match image-set tokens in SVG content. When processing input that contains a large number of unclosed or malformed image-set declarations, the regex engine exhibits catastrophic backtracking behavior. This occurs because the pattern lacks proper anchoring or possessive quantifiers, forcing the PCRE library to explore an exponential number of potential matches before determining failure. Consequently, this results in excessive CPU consumption and can lead to application unavailability due to resource exhaustion.
From a technical perspective, the vulnerability exploits the inherent complexity of backtracking regex engines when faced with ambiguous patterns. An attacker can craft malicious SVG files containing numerous unclosed image-set tokens designed specifically to trigger these worst-case scenarios for the regular expression engine. Upon processing such inputs, the sanitizer does not simply fail gracefully but instead enters an infinite loop or a prolonged computation state that monopolizes system resources. In some instances, this excessive computational load triggers internal limits within the PCRE library, causing the sanitize function to throw a TypeError. This exception handling failure is particularly dangerous as it may result in the sanitization process being aborted prematurely, leaving potentially malicious content unsanitized and passed through to downstream applications or users.
The operational impact of this vulnerability extends beyond simple service disruption. While the primary effect is a denial of service against the application utilizing the svg-sanitizer library, the secondary effect poses an integrity risk. If the TypeError causes the sanitization routine to exit without completing its validation checks, malicious scripts, external resource references, or other dangerous SVG elements may bypass security filters entirely. This undermines the fundamental purpose of using a sanitizer in web applications that process user-generated content. Attackers leveraging this flaw can effectively perform both availability attacks and potential injection attacks depending on how the application handles the resulting error state. The lack of robust input validation for regex complexity allows remote attackers to exploit these conditions with minimal effort, requiring only the submission of specially crafted SVG payloads via any endpoint that processes such files through the vulnerable library version.
Mitigation strategies must focus primarily on upgrading the darylldoyle svg-sanitizer package to version 1.1.0 or later, where this regular expression pattern has been optimized and fixed to prevent catastrophic backtracking. For organizations unable to immediately upgrade, implementing input size limits for SVG files can help mitigate the risk by preventing the submission of excessively large payloads that trigger the vulnerability. Additionally, deploying a Web Application Firewall with rules capable of detecting regex-based denial-of-service patterns may provide an additional layer of defense. It is also advisable to review application error handling mechanisms to ensure that exceptions thrown during sanitization do not result in partial processing or bypassing of security controls. Monitoring system metrics for unusual spikes in CPU usage associated with SVG parsing operations can aid in early detection and response to potential exploitation attempts. This vulnerability aligns with CWE-400, which covers uncontrolled resource consumption, and maps to the ATT&CK technique T1496, representing Resource Hijacking through computational exhaustion.