CVE-2025-59837: Astro Web Framework SSRF Vulnerability

CVE-2025-59837 Overview

CVE-2025-59837 is a server-side request forgery (SSRF) vulnerability in the Astro web framework's image proxy component. The image proxy enforces domain validation on the href parameter to restrict outbound requests to allowed hosts. Attackers can bypass this validation by injecting backslash characters into the href parameter, causing the proxy to issue server-side requests to arbitrary URLs. The flaw also creates conditions for cross-site scripting (XSS) when attacker-controlled content is returned through the proxy. The issue is an incomplete fix for CVE-2025-58179 and affects Astro versions 5.13.4 through 5.13.9. Astro 5.13.10 contains the corrected validation logic.

Critical Impact

Unauthenticated network attackers can coerce Astro servers into issuing arbitrary outbound HTTP requests, exposing internal services and enabling reflected XSS through the image proxy response.

Affected Products

• Astro framework versions 5.13.4 through 5.13.9 (Node.js)
• Applications using Astro's built-in image proxy with domain allowlists
• Server-rendered Astro deployments exposing the /_image endpoint

Discovery Timeline

• 2025-10-28 - CVE-2025-59837 published to NVD
• 2025-11-25 - Last updated in NVD database

Technical Details for CVE-2025-59837

Vulnerability Analysis

The Astro image proxy accepts an href parameter that specifies the source URL for image transformation. To prevent abuse, the proxy validates that the host in href matches a configured allowlist. The validation logic parses the URL and compares the extracted host against permitted domains. The patched code for CVE-2025-58179 addressed several bypass primitives but did not account for backslash characters during host extraction. Attackers supply a crafted href containing backslashes that the validation routine interprets as path separators while the underlying HTTP client treats them as part of the authority component. This parser differential routes the resulting request to an attacker-chosen host while passing the allowlist check. The classification falls under [CWE-79] in NVD, but the primary exploitation primitive is SSRF with secondary XSS depending on response handling.

Root Cause

The root cause is inconsistent URL parsing between the validation layer and the request-issuing layer. Backslashes in the href value are normalized differently by each component, allowing the validated host to differ from the requested host. The original patch for CVE-2025-58179 did not canonicalize backslashes before host comparison.

Attack Vector

An unauthenticated attacker sends a crafted GET request to the Astro image endpoint with a malicious href parameter. The crafted URL uses backslash characters to confuse the host parser. The Astro server then issues a request to an arbitrary internal or external host. Responses can be relayed back, enabling internal service enumeration, cloud metadata exfiltration, or reflected XSS when the proxied content includes script payloads.

No verified public proof-of-concept is published. See the GitHub Security Advisory GHSA-qcpr-679q-rhm2 and the corrective commits 1e2499e and 9ecf359 for the patch diff.

Detection Methods for CVE-2025-59837

Indicators of Compromise

• Requests to the Astro /_image endpoint containing backslash characters (\) or URL-encoded backslashes (%5C) in the href query parameter
• Outbound connections from Astro Node.js processes to internal RFC1918 addresses, link-local 169.254.169.254 metadata endpoints, or unexpected external hosts
• Image proxy responses with non-image Content-Type values such as text/html or application/json

Detection Strategies

• Inspect web server and reverse proxy logs for /_image?href= requests containing \, %5C, or mixed-slash hostnames
• Correlate Astro process network telemetry against the configured image domain allowlist and alert on deviations
• Apply web application firewall rules that decode and normalize the href parameter before allowlist enforcement

Monitoring Recommendations

• Monitor egress traffic from web tiers running Astro for connections to cloud instance metadata services
• Track the installed Astro version across build pipelines and runtime hosts to identify instances between 5.13.4 and 5.13.9
• Alert on image proxy response sizes or content types that deviate from baseline image traffic patterns

How to Mitigate CVE-2025-59837

Immediate Actions Required

• Upgrade Astro to version 5.13.10 or later in all build and deployment pipelines
• Audit the image service domain allowlist and remove any unnecessary entries that broaden the SSRF surface
• Review recent access logs for exploitation attempts targeting /_image with backslash payloads

Patch Information

The fix is delivered in Astro 5.13.10. The corrective changes are contained in commits 1e2499e8ea83ebfa233a18a7499e1ccf169e56f4 and 9ecf3598e2b29dd74614328fde3047ea90e67252, which normalize backslashes before host validation. Refer to the GitHub Security Advisory GHSA-qcpr-679q-rhm2 for full details.

Workarounds

• Block requests containing backslash characters or %5C in the href query parameter at a WAF or reverse proxy
• Disable the Astro image service or restrict it to local images only until the upgrade is deployed
• Enforce egress filtering on the Astro host to deny outbound connections to internal networks and metadata endpoints