CVE-2026-25545 is a Server-Side Request Forgery (SSRF) vulnerability identified in the Astro web framework, specifically affecting versions prior to 9.5.4. The vulnerability arises in server-side rendered pages that return error responses with prerendered custom error pages (such as 404.astro or 500.astro). When an attacker manipulates the HTTP Host header to point to a server they control, the vulnerable Astro server fetches this malicious server during the rendering of error pages like /500.html. This fetch operation can be redirected internally to any URL within the internal network or localhost, enabling the attacker to read the response body of internal services. The attack vector requires the attacker to bypass Host header validation, which is possible if the attacker can access the application directly via the origin IP address behind a proxy or if no proxy is used. This SSRF can be leveraged to access sensitive internal resources such as cloud metadata services, which often contain credentials or configuration data, or other internal network services that are not exposed externally. The vulnerability is contingent on the use of a common feature (custom error pages) and direct server access without intermediary proxies that validate headers. The issue was addressed and fixed in Astro version 9.5.4. The CVSS 4.0 vector indicates a network attack vector with high attack complexity and partial attack prerequisites, no privileges or user interaction required, and a medium impact on confidentiality, integrity, and availability. No known exploits are reported in the wild as of publication.

The SSRF vulnerability in Astro prior to 9.5.4 can have significant impacts on organizations deploying this framework for web applications. By exploiting this flaw, attackers can access internal network resources that are otherwise inaccessible externally, including cloud metadata endpoints that may contain sensitive credentials or tokens. This can lead to unauthorized disclosure of confidential information, potential privilege escalation, and lateral movement within the internal network. The ability to read internal responses compromises confidentiality and may also affect integrity if attackers leverage this access to interact with internal services. Although the attack requires specific conditions such as direct server access and lack of Host header validation, organizations exposing Astro servers without proper network segmentation or proxy protections are at risk. The vulnerability could facilitate reconnaissance and further exploitation in cloud environments, increasing the attack surface. While no active exploits are known, the medium severity score reflects the potential for impactful breaches if exploited. Enterprises relying on Astro for critical web infrastructure should consider this a moderate risk that warrants timely remediation.

To mitigate CVE-2026-25545, organizations should immediately upgrade all Astro framework instances to version 9.5.4 or later, where the vulnerability is fixed. Additionally, implement strict validation of the Host header on all incoming HTTP requests to ensure it matches expected domain names and reject requests with unexpected or missing Host headers. Deploy web application firewalls (WAFs) or reverse proxies that enforce header validation and prevent direct access to origin IP addresses, thereby blocking attackers from bypassing header checks. Network segmentation should be enforced to restrict access to internal services and cloud metadata endpoints from the web server. Disable or tightly control the use of prerendered custom error pages that fetch external resources, or sanitize any dynamic content used in error handling. Regularly audit server logs for anomalous Host header values or unusual error page requests that may indicate attempted exploitation. Finally, conduct penetration testing and vulnerability scanning focused on SSRF vectors to identify and remediate any residual risks in the deployment environment.