Flowise: SSRF Protection Bypass (TOCTOU & Default Insecure)

Summary

The core security wrappers (secureAxiosRequest and secureFetch) intended to prevent Server-Side Request Forgery (SSRF) contain multiple logic flaws. These flaws allow attackers to bypass the allow/deny lists via DNS Rebinding (Time-of-Check Time-of-Use) or by exploiting the default configuration which fails to enforce any deny list.

Details

The flaws exist in packages/components/src/httpSecurity.ts.

Default Insecure: If process.env.HTTP_DENY_LIST is undefined, checkDenyList returns immediately, allowing all requests (including localhost).

DNS Rebinding (TOCTOU): The function performs a DNS lookup (dns.lookup) to validate the IP, and then the HTTP client performs a new lookup to connect. An attacker can serve a valid IP first, then switch to an internal IP (e.g., 127.0.0.1) for the second lookup.

PoC

Ensure HTTP_DENY_LIST is unset (default behavior).

Use any node utilizing secureFetch to access http://127.0.0.1.

Result: Request succeeds.

Scenario 2: DNS Rebinding

Attacker controls domain attacker.com and a custom DNS server.

Configure DNS to return 1.1.1.1 (Safe IP) with TTL=0 for the first query.

Configure DNS to return 127.0.0.1 (Blocked IP) for subsequent queries.

Flowise validates attacker.com -> 1.1.1.1 (Allowed).

Flowise fetches attacker.com -> 127.0.0.1 (Bypass).

Run the following for manual verification

// PoC for httpSecurity.ts Bypasses
import * as dns from 'dns/promises';

// Mocking the checkDenyList logic from Flowise
async function checkDenyList(url: string) {
    const deniedIPs = ['127.0.0.1', '0.0.0.0']; // Simplified deny list logic

    if (!process.env.HTTP_DENY_LIST) {
        console.log(\"⚠️  HTTP_DENY_LIST not set. Returning allowed.\");
        return; // Vulnerability 1: Default Insecure
    }

    const { hostname } = new URL(url);
    const { address } = await dns.lookup(hostname);

    if (deniedIPs.includes(address)) {
        throw new Error(`IP ${address} is denied`);
    }
    console.log(`✅ IP ${address} allowed check.`);
}

async function runPoC() {
    console.log(\"--- Test 1: Default Configuration (Unset HTTP_DENY_LIST) ---\");
    // Ensure env var is unset
    delete process.env.HTTP_DENY_LIST;
    try {
        await checkDenyList('http://127.0.0.1');
        console.log(\"[PASS] Default config allowed localhost access.\");
    } catch (e) {
        console.log(\"[FAIL] Blocked:\", e.message);
    }

    console.log(\"\
--- Test 2: 'private' Keyword Bypass (Logic Flaw) ---\");
    process.env.HTTP_DENY_LIST = 'private'; // User expects this to block localhost
    try {
        await checkDenyList('http://127.0.0.1');
        // In real Flowise code, 'private' is not expanded to IPs, so it only blocks the string \"private\"
        console.log(\"[PASS] 'private' keyword failed to block localhost (Mock simulation).\");
    } catch (e) {
        console.log(\"[FAIL] Blocked:\", e.message);
    }
}

runPoC();

Impact

Confidentiality: High (Access to internal services if protection is bypassed).

Integrity: Low/Medium (If internal services allow state changes via GET).

Availability: Low.