CVE-2025-67647

Executive Summary

CVE-2025-67647 is a Server-Side Request Forgery (SSRF) and Denial of Service (DoS) vulnerability affecting SvelteKit, a popular framework for building web applications with Svelte. The vulnerability arises from insufficient validation of the Host header when handling prerendered routes. An attacker can craft a malicious Host header to cause the server to make requests to internal or external resources (SSRF) or to trigger a crash (DoS). The issue is present in versions 2.19.0 through 2.49.4, with a more severe DoS variant affecting versions 2.44.0 through 2.49.4 when at least one prerendered route exists. The vulnerability is fixed in version 2.49.5.

Supply Chain & Package Security

This vulnerability affects the SvelteKit npm package (@sveltejs/kit). Organizations using SvelteKit should immediately audit their package.json and lock files to ensure they are not using a vulnerable version. The supply chain risk is moderate, as the vulnerability is in the framework itself, not a dependency. However, any application using a vulnerable version is at risk. The official patch is available via npm registry. Developers should update to version 2.49.5 or later and verify the integrity of the package using checksums provided by the SvelteKit maintainers.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (NVD) – However, based on analysis, a more accurate score is 7.5 (High).
• Attack Vector (AV): Network (N)
• Attack Complexity (AC): Low (L)
• Privileges Required (PR): None (N)
• User Interaction (UI): None (N)
• Scope (S): Unchanged (U)
• Confidentiality (C): None (N) – SSRF can lead to information disclosure, but primary impact is availability.
• Integrity (I): None (N)
• Availability (A): High (H) – DoS is the primary impact.
• Threat Level: High. The vulnerability is easily exploitable remotely without authentication, and can cause significant service disruption.

Deep Dive & Mechanics

The vulnerability lies in how SvelteKit handles prerendered routes. When a route is prerendered (export const prerender = true), the server generates static HTML at build time. However, during runtime, the server may still process requests for these routes, particularly when handling redirects or rewriting URLs. The issue is that the server uses the Host header from the incoming request to construct URLs for internal redirects or to fetch prerendered content. An attacker can manipulate the Host header to point to an arbitrary host, causing the server to:

1. SSRF: Make requests to internal services (e.g., http://localhost:8080/admin) or external malicious servers.
2. DoS: Cause the server to enter an infinite loop or crash by providing a malformed or self-referencing Host header.

The vulnerability is particularly dangerous in environments without a reverse proxy that validates the Host header, or when the ORIGIN environment variable is not set (for adapter-node). The fix in version 2.49.5 introduces proper validation of the Host header against a whitelist of allowed origins.

Vulnerable vs. Patched Code

Vulnerable Code (SvelteKit < 2.49.5):

// In the server-side request handler, the Host header is used directly
// to construct redirect URLs or fetch prerendered content.
const host = request.headers.get('host');
const url = new URL(request.url);
url.host = host; // No validation
// ... use url for internal operations

Patched Code (SvelteKit >= 2.49.5):

// The Host header is validated against a whitelist of allowed origins.
const allowedOrigins = ['example.com', 'localhost:3000']; // Configurable
const host = request.headers.get('host');
if (!allowedOrigins.includes(host)) {
    return new Response('Invalid Host header', { status: 400 });
}
const url = new URL(request.url);
url.host = host;
// ... use url for internal operations

Exploitation & Lab Setup

Red Team

Prerequisites:

• A SvelteKit application with at least one prerendered route (e.g., src/routes/+page.svelte with export const prerender = true).
• The application is running on a vulnerable version (2.19.0 to 2.49.4).
• No reverse proxy or ORIGIN environment variable configured.

Exploitation Steps:

1. Identify a vulnerable endpoint: Find a prerendered route, e.g., https://target.com/about.
2. Craft a malicious request: Use curl or a similar tool to send a request with a manipulated Host header.

   curl -H "Host: 127.0.0.1:8080" https://target.com/about
   

3. Observe SSRF: The server may attempt to fetch content from http://127.0.0.1:8080/about, potentially exposing internal services.
4. Trigger DoS: Use a Host header that causes an infinite loop or crash.

   curl -H "Host: target.com" https://target.com/about
   

   If the server uses the Host header to construct a redirect back to itself, it may enter an infinite loop.

Note: No public exploit code is available at this time.

Blue Team

Detection:

• Monitor for unusual Host headers in incoming requests, especially those pointing to internal IP addresses (e.g., 127.0.0.1, 10.0.0.1, 192.168.x.x).
• Look for repeated requests to the same prerendered route with varying Host headers.
• Use the detection rules provided in the Detection Engineering section.

Mitigation:

• Immediately upgrade to SvelteKit 2.49.5 or later.
• If upgrade is not possible, configure a reverse proxy (e.g., Nginx, Apache) to validate the Host header.
• Set the ORIGIN environment variable for adapter-node deployments.
• Implement network segmentation to limit the impact of SSRF.

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP requests with Host headers containing internal IP addresses (e.g., 127.0.0.1, 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16).
  • HTTP requests with Host headers pointing to external malicious domains.
  • Unusual 5xx error responses from the server, indicating a crash.
• File IOCs:
  • Log entries showing requests to prerendered routes with unexpected Host headers.
  • Core dump files (if the server crashes) in the application directory.
• Behavioral IOCs:
  • Server making outbound connections to internal services that are not normally accessed.
  • Server becoming unresponsive or returning errors for prerendered routes.

Detection Engineering

YARA Rule

rule CVE_2025_67647_SvelteKit_SSRF_DoS {
    meta:
        description = "Detects SvelteKit SSRF/DoS exploitation attempts via Host header manipulation"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-67647"
        severity = "high"
    strings:
        $host_header = /Host:\s*(127\.0\.0\.1|10\.\d{1,3}\.\d{1,3}\.\d{1,3}|172\.(1[6-9]|2\d|3[01])\.\d{1,3}\.\d{1,3}|192\.168\.\d{1,3}\.\d{1,3})/ nocase
        $prerender_route = /\/[a-zA-Z0-9_-]+\/?(?=\s|$)/ // Adjust to match prerendered routes
    condition:
        $host_header and #prerender_route > 0
}

Sigma Rule

title: SvelteKit SSRF/DoS via Host Header Manipulation
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects attempts to exploit CVE-2025-67647 by manipulating the Host header in requests to prerendered SvelteKit routes.
author: Valters IT
date: 2025/04/10
references:
    - https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2025-67647
logsource:
    category: webserver
    product: apache
    service: access_combined
detection:
    selection:
        cs-host|contains:
            - '127.0.0.1'
            - '10.'
            - '172.16.'
            - '172.17.'
            - '172.18.'
            - '172.19.'
            - '172.20.'
            - '172.21.'
            - '172.22.'
            - '172.23.'
            - '172.24.'
            - '172.25.'
            - '172.26.'
            - '172.27.'
            - '172.28.'
            - '172.29.'
            - '172.30.'
            - '172.31.'
            - '192.168.'
        cs-uri-stem|contains: '/prerendered-route' # Adjust to actual prerendered routes
    condition: selection
falsepositives:
    - Legitimate internal network requests (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-67647 SvelteKit SSRF/DoS Attempt"; flow:to_server; content:"Host|3a|"; nocase; pcre:"/Host:\s*(127\.0\.0\.1|10\.\d{1,3}\.\d{1,3}\.\d{1,3}|172\.(1[6-9]|2\d|3[01])\.\d{1,3}\.\d{1,3}|192\.168\.\d{1,3}\.\d{1,3})/Hi"; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Action: Upgrade SvelteKit to version 2.49.5 or later.

   npm install @sveltejs/kit@latest
   

2. Workaround (if upgrade is not possible):
   • Configure a reverse proxy (e.g., Nginx) to validate the Host header.

     server {
         listen 80;
         server_name example.com;
         if ($host != $server_name) {
             return 400;
         }
         # ... rest of configuration
     }
     

   • Set the ORIGIN environment variable for adapter-node deployments.

     export ORIGIN=https://example.com
     

3. Long-term:
   • Implement network segmentation to limit the impact of SSRF.
   • Regularly update dependencies and monitor for security advisories.
   • Use a Web Application Firewall (WAF) to filter malicious Host headers.

Valters IT
Securing the digital frontier, one CVE at a time.