CVE-2026-27829 Overview
A Server-Side Request Forgery (SSRF) vulnerability exists in the Astro web framework versions 9.0.0 through 9.5.3. The vulnerability resides in Astro's image pipeline where the inferSize option bypasses image.domains and image.remotePatterns restrictions, allowing the server to fetch content from unauthorized remote hosts without proper domain validation.
Critical Impact
Attackers who can influence image URLs (e.g., via CMS content or user-supplied data) can cause the server to make arbitrary requests to internal network services and cloud metadata endpoints, potentially leading to sensitive information disclosure and further compromise of backend infrastructure.
Affected Products
- Astro web framework versions 9.0.0 through 9.5.3
Discovery Timeline
- 2026-02-26 - CVE CVE-2026-27829 published to NVD
- 2026-02-26 - Last updated in NVD database
Technical Details for CVE-2026-27829
Vulnerability Analysis
The vulnerability is classified as CWE-918 (Server-Side Request Forgery). Astro provides an inferSize option that fetches remote images at render time to determine their dimensions. Under normal operation, remote image fetches are restricted to domains that site developers have manually authorized through the image.domains or image.remotePatterns configuration options. However, when inferSize is used, no domain validation is performed — the image is fetched from any host regardless of the configured restrictions.
This oversight allows an attacker who can control or influence image URLs to cause the Astro server to make HTTP requests to arbitrary destinations, effectively turning the web server into an open proxy for outbound requests.
Root Cause
The root cause is missing domain validation in the remote image loading functionality when the inferSize option is enabled. The image pipeline fails to check incoming URLs against the configured image.domains or image.remotePatterns allowlists before making fetch requests. Additionally, the original implementation followed HTTP redirects automatically, which could be leveraged to bypass restrictions even if initial URL validation was performed.
Attack Vector
The attack is network-based and requires no authentication. An attacker needs the ability to influence image URL values processed by Astro's image pipeline — this can be achieved through CMS content injection, user-controlled input fields, or other data sources that feed into image components using inferSize. Once the attacker controls the URL, they can:
- Target internal network services that are not accessible from the internet
- Access cloud metadata endpoints (e.g., http://169.254.169.254/) to retrieve instance credentials
- Scan internal network ports and services
- Exfiltrate data through DNS or HTTP channels
The following code shows the security patch applied in version 9.5.4:
export async function loadRemoteImage(src: string) {
const req = new Request(src);
- const res = await fetch(req);
+ const res = await fetch(req, { redirect: 'manual' });
+
+ if (res.status >= 300 && res.status < 400) {
+ throw new Error(`Failed to load remote image ${src}. The request was redirected.`);
+ }
if (!res.ok) {
throw new Error(
Source: GitHub Commit
Additional patch in the generic endpoint handler:
const res = await fetch(src, {
// Forward all headers from the original request
headers,
+ redirect: 'manual',
});
+ if (res.status >= 300 && res.status < 400) {
+ return undefined;
+ }
+
if (!res.ok) {
return undefined;
}
Source: GitHub Commit
Detection Methods for CVE-2026-27829
Indicators of Compromise
- Outbound HTTP requests from the Astro server to internal IP ranges (e.g., 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16)
- Requests to cloud metadata endpoints such as 169.254.169.254
- Unexpected outbound connections to external hosts not in the configured image.domains allowlist
- Server logs showing image fetch failures with unusual or internal URLs
Detection Strategies
- Monitor server-side HTTP request logs for connections to internal network addresses or cloud metadata services
- Implement network egress filtering and alerting for unauthorized outbound connections from web servers
- Review Astro application logs for image loading errors that reference unexpected domains
- Deploy Web Application Firewall (WAF) rules to detect SSRF patterns in user-controlled input
Monitoring Recommendations
- Enable verbose logging for Astro's image pipeline to capture all remote fetch attempts
- Set up alerts for outbound requests to RFC 1918 private address ranges from production web servers
- Monitor for unusual patterns in image URL parameters, particularly those containing internal hostnames or IP addresses
- Implement network segmentation to limit the blast radius of potential SSRF exploitation
How to Mitigate CVE-2026-27829
Immediate Actions Required
- Upgrade Astro to version 9.5.4 or later immediately
- Review application code for usage of inferSize option with user-controllable image URLs
- Implement network-level egress filtering to restrict outbound connections from web servers
- Audit CMS content and user-submitted data for potentially malicious image URLs
Patch Information
Astro version 9.5.4 addresses this vulnerability by adding redirect handling with { redirect: 'manual' } to prevent redirect-based bypasses and properly rejecting requests that return redirect status codes (300-399). The fix ensures that the image.domains and image.remotePatterns restrictions are properly enforced.
For complete details, see the GitHub Security Advisory and the patch commit.
Workarounds
- Avoid using the inferSize option with untrusted or user-controllable image URLs until patched
- Implement strict input validation and URL allowlisting at the application layer before passing URLs to Astro's image pipeline
- Deploy network-level controls to block outbound requests to internal networks and cloud metadata endpoints
- Consider using a proxy or image CDN service to fetch and process remote images, isolating the web server from direct outbound requests
# Network egress filtering example (iptables)
# Block outbound connections to internal networks from web server
iptables -A OUTPUT -d 10.0.0.0/8 -j DROP
iptables -A OUTPUT -d 172.16.0.0/12 -j DROP
iptables -A OUTPUT -d 192.168.0.0/16 -j DROP
iptables -A OUTPUT -d 169.254.169.254 -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
