CVE-2025-59837: Astro Web Framework SSRF Vulnerability

CVE-2025-59837 Overview

CVE-2025-59837 is a Server-Side Request Forgery (SSRF) vulnerability in Astro, a popular web framework for building content-driven websites. The vulnerability exists in Astro's image proxy functionality, where domain validation can be bypassed using backslashes in the href parameter. This bypass allows attackers to make server-side requests to arbitrary URLs, potentially leading to SSRF attacks and Cross-Site Scripting (XSS) exploitation.

This vulnerability represents an incomplete fix for CVE-2025-58179, indicating that the original mitigation was insufficient in preventing all forms of URL manipulation attacks.

Critical Impact

Attackers can bypass image proxy domain validation to access internal resources, exfiltrate sensitive data, or chain with XSS for client-side attacks against users.

Affected Products

• Astro versions 5.13.4 through 5.13.9
• Astro Node.js integrations using the image proxy feature
• Applications utilizing Astro's remote image handling capabilities

Discovery Timeline

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

Technical Details for CVE-2025-59837

Vulnerability Analysis

The vulnerability resides in Astro's isRemotePath function within the internal helpers package. This function is responsible for determining whether a given path should be treated as a remote URL, which is critical for the image proxy's domain validation logic.

The original implementation only checked for standard URL protocols and protocol-relative URLs (starting with //). However, it failed to account for backslash variants that can be normalized to valid remote URLs by browsers and URL parsers. For example, a path like \\example.com or %5C%5Cexample.com (URL-encoded backslashes) would not be detected as a remote path, allowing it to bypass the domain allowlist validation.

This bypass is particularly dangerous because when these malformed URLs are processed by downstream components or rendered in browsers, the backslashes may be normalized to forward slashes, resulting in valid remote URL requests.

Root Cause

The root cause stems from insufficient input validation in the URL path detection logic. The original isRemotePath function used a simple regex check (URL_PROTOCOL_REGEX) and a data: prefix check, but did not handle:

1. Backslash-prefixed URLs (e.g., \\attacker.com)
2. URL-encoded backslash variants (e.g., %5C%5Cexample.com)
3. Protocol schemes with backslashes instead of forward slashes (e.g., http:\\attacker.com)

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can craft a malicious URL with backslash characters in the href parameter when requesting images through Astro's image proxy. The malformed URL bypasses domain validation, causing the server to make requests to attacker-controlled or internal resources.

  * - `ws://`
  * - `//` (protocol-relative URLs)
  * - `data:` (base64 images)
+ * - Backslash variants (e.g., `\\example.com`) that could normalize to remote URLs
+ * - URL-encoded backslash variants (e.g., `%5C%5Cexample.com`)
  * @param src
  */
 export function isRemotePath(src: string) {
-	return URL_PROTOCOL_REGEX.test(src) || src.startsWith('data:');
+	// First decode any URL-encoded backslashes
+	const decoded = src.replace(/%5C/gi, '\\');
+	
+	// Check for any backslash at the start (single or multiple)
+	// These can be normalized to protocol-relative URLs
+	if (decoded[0] === '\\') {
+		return true;
+	}
+	
+	// Check for protocols with backslashes (e.g., http:\\ or https:\\)
+	if (/^(?:http|https|ftp|ws):\\/.test(decoded)) {
+		return true;
+	}
+	
+	// Check standard URL patterns
+	return URL_PROTOCOL_REGEX.test(decoded) || decoded.startsWith('data:');
 }
 
 export function slash(path: string) {

Source: GitHub Commit Changes

Detection Methods for CVE-2025-59837

Indicators of Compromise

• Unusual image proxy requests containing backslash characters (\ or %5C) in URL parameters
• Server-side requests to unexpected internal IP ranges (e.g., 127.0.0.1, 10.x.x.x, 192.168.x.x, 169.254.169.254)
• Outbound connections from the Astro application server to external domains not in the configured allowlist
• Web application firewall (WAF) logs showing encoded backslash patterns in image-related endpoints

Detection Strategies

• Deploy WAF rules to detect and block requests containing %5C or %5c URL-encoded backslashes in image proxy parameters
• Implement network monitoring to alert on server-initiated requests to cloud metadata endpoints (e.g., 169.254.169.254)
• Review application logs for anomalous patterns in image proxy request paths, particularly those with non-standard URL formats
• Configure SentinelOne Singularity to monitor for SSRF-related network behaviors from web application processes

Monitoring Recommendations

• Enable verbose logging for Astro's image proxy component to capture all incoming request URLs
• Set up alerts for any outbound HTTP requests from the application server to internal network ranges
• Monitor for XSS payload patterns in server responses that may indicate successful exploitation chaining
• Implement request rate limiting on the image proxy endpoint to slow down potential automated exploitation attempts

How to Mitigate CVE-2025-59837

Immediate Actions Required

• Upgrade Astro to version 5.13.10 or later immediately, which contains the complete fix for this vulnerability
• If immediate upgrade is not possible, disable the image proxy feature temporarily until the patch can be applied
• Review server logs for any signs of exploitation attempts using backslash-based URL bypass patterns
• Audit any applications using Astro's image proxy to ensure they are not exposed to untrusted user input

Patch Information

The vulnerability has been fixed in Astro version 5.13.10. The fix enhances the isRemotePath function to properly detect and handle backslash variants, including URL-encoded backslashes. The patched version decodes potential URL-encoded characters before performing validation checks and explicitly checks for backslash-prefixed paths that could be normalized to remote URLs.

For detailed information about the security fix, see the GitHub Security Advisory. The fix was implemented across two commits: commit 1e2499e and commit 9ecf359.

Workarounds

• Disable remote image handling in Astro configuration by removing domains from the image.remotePatterns configuration
• Implement a reverse proxy or WAF rule to sanitize and reject requests containing backslash characters before they reach the Astro application
• Restrict network egress from the application server to only necessary external domains using firewall rules
• Use a content security policy (CSP) to limit the scope of potential XSS impact if the vulnerability is chained

# Example: Upgrade Astro to the patched version
npm update astro@5.13.10

# Or using yarn
yarn upgrade astro@5.13.10

# Verify the installed version
npx astro --version