CVE-2025-65019 is a Cross-Site Scripting (XSS) vulnerability identified in the Astro web framework, specifically affecting versions prior to 5.15.9 when deployed with the @astrojs/cloudflare adapter configured for server output mode. The vulnerability resides in the image optimization endpoint (_/image), where the isRemoteAllowed() function fails to properly validate URLs, unconditionally allowing data: protocol URLs. This improper input neutralization (CWE-79) enables attackers to craft malicious SVG payloads that can be injected and executed in the context of the vulnerable web application. Because the vulnerability bypasses domain restrictions and Content Security Policy (CSP) protections, traditional defenses against XSS are ineffective. The attack vector requires no authentication but does require user interaction, such as clicking a malicious link or visiting a crafted page. The CVSS v3.1 score is 5.4 (medium severity), reflecting the limited impact on confidentiality and integrity but no impact on availability. Although no known exploits are currently in the wild, the vulnerability poses a significant risk to applications using the affected Astro versions with Cloudflare adapter. The issue was addressed and patched in Astro version 5.15.9 by tightening URL validation in the isRemoteAllowed() function to block unsafe data: protocol URLs.

For European organizations, this vulnerability could lead to the execution of malicious scripts in users' browsers, potentially resulting in theft of sensitive information such as authentication tokens, session cookies, or personal data. This can facilitate account takeover, phishing, or further exploitation within the affected web applications. Since Astro is a modern web framework gaining adoption for building performant websites, organizations using it with the Cloudflare adapter in server output mode are at risk. The bypass of CSP protections increases the attack surface, making traditional mitigation less effective. Although the vulnerability does not affect availability, the compromise of confidentiality and integrity can damage organizational reputation, lead to regulatory non-compliance (e.g., GDPR), and cause financial losses. The medium CVSS score indicates moderate risk, but the ease of exploitation without authentication and the potential for user-targeted attacks warrant prompt remediation.

European organizations should immediately upgrade all Astro framework instances to version 5.15.9 or later to apply the official patch that corrects the URL validation logic. Review and tighten Content Security Policy configurations to restrict allowable sources for images and scripts, explicitly disallowing data: protocol URLs where possible. Implement input validation and sanitization on any user-supplied content that may be processed by the image optimization endpoint. Conduct thorough security testing, including automated scanning and manual penetration testing, focusing on SVG and image handling functionalities. Educate developers about the risks of improper input neutralization and the importance of adhering to secure coding practices. Monitor web application logs for suspicious requests targeting the _/image endpoint and consider deploying Web Application Firewalls (WAF) with custom rules to detect and block malicious payloads. Finally, maintain an incident response plan to quickly address any exploitation attempts.