CVE-2026-25545 is a Server-Side Request Forgery (SSRF) vulnerability identified in the Astro web framework before version 9.5.4. Astro uses server-side rendering for its web pages, including custom error pages such as 404.astro or 500.astro. The vulnerability arises when these error pages fetch resources based on the HTTP Host header without proper validation. An attacker who can manipulate the Host header to point to an attacker-controlled server can cause the Astro server to fetch that server's content during error page rendering. More critically, the attacker can redirect this fetch to internal network addresses, including cloud provider metadata IPs or localhost services, thereby reading sensitive internal responses. This SSRF exploit requires the attacker to have direct access to the application server without intermediary proxies that validate or restrict Host headers. The flaw leverages a common feature of Astro’s error page rendering, making it broadly exploitable in affected versions. The vulnerability was assigned CVE-2026-25545 and has a CVSS 4.0 score of 6.9, indicating medium severity. The attack vector is network-based but requires high attack complexity and no privileges or user interaction. The scope is changed as internal resources can be accessed, impacting confidentiality partially. The vulnerability was fixed in Astro version 9.5.4. No known active exploits have been reported, but the potential for sensitive data exposure and internal network reconnaissance is significant.

Organizations running vulnerable versions of the Astro framework (prior to 9.5.4) face risks of internal network exposure and data leakage. Attackers exploiting this SSRF can access internal services that are normally inaccessible externally, such as cloud metadata endpoints, which often contain sensitive credentials and configuration data. This can lead to further compromise of cloud infrastructure, lateral movement within internal networks, and exposure of confidential information. The vulnerability can also be used to interact with localhost services, potentially exploiting other local vulnerabilities or accessing sensitive APIs. Since the flaw requires direct server access without Host header validation, environments exposing origin IPs or lacking proper proxy configurations are at higher risk. The impact is particularly critical for cloud-hosted applications and enterprises relying on Astro for web services, as it can undermine the confidentiality and integrity of internal resources. Although no active exploits are known, the medium severity score and the nature of SSRF attacks suggest a significant threat if exploited. Organizations could face data breaches, service disruptions, and compliance violations if this vulnerability is exploited.

To mitigate CVE-2026-25545, organizations should immediately upgrade Astro to version 9.5.4 or later, where the vulnerability is patched. Additionally, implement strict Host header validation on all incoming requests to ensure only legitimate hostnames are accepted, preventing attackers from injecting malicious Host values. Deploy web application firewalls (WAFs) with rules to detect and block suspicious SSRF patterns, especially requests with unusual Host headers or error page triggers. Restrict direct access to application origin IPs by enforcing proxy usage and network segmentation, ensuring that internal services and metadata endpoints are not accessible from untrusted networks. Employ network-level controls such as firewall rules to block outbound requests from the application server to internal IP ranges unless explicitly required. Monitor logs for unusual error page requests or unexpected internal resource fetches. Finally, conduct regular security assessments and penetration testing focusing on SSRF and header injection vectors to proactively identify and remediate similar issues.