CVE-2026-27579 – CORS Misconfiguration Leading to Authenticated Data Exposure

📌 Advisory Reference

• CVE ID: CVE-2026-27579
• GHSA: GHSA-qh5m-p8jh-hx88
• Vendor: karnop
• Product: realtime-collaboration-platform
• Severity: HIGH (CVSS 7.4)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N

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🧠 Vulnerability Overview

The realtime-collaboration-platform project contained a CORS misconfiguration in its Appwrite backend configuration.

The server:

• Allowed arbitrary origins
• Enabled Access-Control-Allow-Credentials: true

This combination allowed an attacker-controlled domain to:

• Send authenticated cross-origin requests
• Automatically include victim session cookies
• Read sensitive user account data from responses

This resulted in exposure of:

• Email address
• Account identifiers
• MFA status

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🔎 Root Cause

The backend failed to properly validate and restrict the Origin header.

Improper configuration:

Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true

Or dynamic reflection of arbitrary origins.

This violates CORS security principles because:

Browsers allow credentialed cross-origin requests only when the origin is explicitly trusted.

Instead, the application trusted any origin, creating a cross-origin data exfiltration vector.

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⚠️ Impact Analysis

Impact Area	Effect
Confidentiality	HIGH
Integrity	None
Availability	None
Scope	Changed

An attacker could:

1. Host a malicious webpage.
2. Lure a logged-in victim.
3. Issue authenticated XHR/fetch requests.
4. Read JSON responses containing sensitive account data.

No direct privilege required.

This makes it a powerful client-side attack with real-world exploitation potential.

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🧪 Proof-of-Concept Conceptual Flow

fetch("https://target-appwrite-endpoint/v1/account", {
    credentials: "include"
})
.then(res => res.json())
.then(data => {
    console.log("Exfiltrated:", data);
});

If the victim is logged in, their session cookie is included automatically.

Because of permissive CORS, the attacker can read the response.

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🛡️ Remediation

Proper mitigation requires:

• Restricting allowed origins explicitly
• Removing wildcard origin usage
• Disabling credentials for untrusted origins
• Enforcing strict origin validation

Correct configuration example:

Access-Control-Allow-Origin: https://trusted-domain.com
Access-Control-Allow-Credentials: true

Or:

Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: false

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📚 CWEs

• CWE-346: Origin Validation Error
• CWE-942: Permissive Cross-domain Policy with Untrusted Domains

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🔥 Key Security Lesson

CORS misconfiguration + credentials = Data breach.

Never allow:

* + credentials=true

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