CVE-2026-91942 in crawl4ai
Summary
by MITRE • 09/15/2026
crawl4ai before 0.9.3 contains a DOM-based cross-site scripting vulnerability in the Docker Playground UI that assigns untrusted crawl results to element.innerHTML. Attackers can craft malicious PDFs with event-handler markup to execute JavaScript in the Playground origin and steal API tokens from sessionStorage for authenticated API abuse.
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Analysis
by VulDB Data Team • 09/15/2026
The identified vulnerability represents a critical DOM-based cross-site scripting (XSS) flaw within the Docker Playground UI component of crawl4ai versions prior to 0.9.3. This security defect stems directly from improper handling of untrusted data, specifically where raw crawl results are assigned directly to an element's innerHTML property without adequate sanitization or encoding. By bypassing standard output encoding mechanisms, the application allows malicious scripts embedded within input data to execute in the context of the victim’s browser session. This architectural oversight violates fundamental web security principles regarding the separation of code and data, creating a direct pathway for client-side script injection that is particularly dangerous because it does not require server-side processing or storage of the payload, making detection by traditional network-based intrusion prevention systems significantly more difficult.
The operational impact of this vulnerability is severe due to its specific exploitation vector involving maliciously crafted PDF files. Attackers can embed event-handler markup within a PDF document designed to trigger when rendered in the browser environment. When an authenticated user interacts with or views such a file through the Docker Playground interface, the vulnerable innerHTML assignment executes the embedded JavaScript code immediately. This execution occurs within the origin of the playground application, granting the attacker full access to the session context. The primary consequence is the theft of API tokens stored in sessionStorage, which are typically used for authenticating requests to backend services or external APIs. Once these credentials are exfiltrated, an adversary can perform authenticated abuse actions under the victim’s identity, potentially leading to unauthorized data access, modification, or deletion depending on the permissions associated with those stolen tokens.
From a classification perspective, this vulnerability aligns closely with CWE-79, which describes Improper Neutralization of Input During Web Page Generation commonly known as Cross-site Scripting. Specifically, it falls under DOM-based XSS where the script is executed by modifying the DOM environment in the victim's browser rather than through server-side injection. In terms of offensive security frameworks, this attack pattern corresponds to ATT&CK technique T1059, Command and Scripting Interpreter, with a specific focus on JavaScript execution within web browsers. The exploitation method also relates to data exfiltration techniques where sensitive information is stolen from the client side after successful authentication bypass or credential theft via session hijacking through token extraction.
Mitigation strategies must prioritize immediate remediation of the code path responsible for rendering crawl results. Developers should replace direct innerHTML assignments with safer alternatives such as textContent, which treats input strictly as plain text and prevents script execution. If dynamic HTML rendering is absolutely necessary, a robust content security policy (CSP) should be implemented to restrict inline script execution, alongside using established sanitization libraries like DOMPurify that are configured to strip dangerous attributes including event handlers. Additionally, implementing strict validation on file uploads or inputs processed by the playground can prevent malicious PDFs from entering the rendering pipeline altogether. For users of affected versions, upgrading to version 0.9.3 or later is essential as these patches address the underlying input handling logic that permitted this DOM manipulation attack vector.