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fix: check AAAA records in DNS rebinding protection (IPv6 bypass) #742

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fix: check AAAA records in DNS rebinding protection (IPv6 bypass) #742

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25 changes: 20 additions & 5 deletions browse/src/url-validation.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,8 @@
const BLOCKED_METADATA_HOSTS = new Set([
'169.254.169.254', // AWS/GCP/Azure instance metadata
'fd00::', // IPv6 unique local (metadata in some cloud setups)
'fe80::1', // IPv6 link-local (metadata in some setups)
'::ffff:169.254.169.254', // IPv4-mapped IPv6 form of link-local metadata
'metadata.google.internal', // GCP metadata
'metadata.azure.internal', // Azure IMDS
]);
Expand Down Expand Up @@ -45,15 +47,28 @@ function isMetadataIp(hostname: string): boolean {
}

/**
* Resolve a hostname to its IP addresses and check if any resolve to blocked metadata IPs.
* Mitigates DNS rebinding: even if the hostname looks safe, the resolved IP might not be.
* Resolve a hostname to its IP addresses (both A and AAAA records) and check if any
* resolve to blocked metadata IPs. Mitigates DNS rebinding: even if the hostname looks
* safe, the resolved IP might not be. Checks IPv6 AAAA records in addition to IPv4 A
* records to prevent bypass via IPv6-only DNS responses.
*/
async function resolvesToBlockedIp(hostname: string): Promise<boolean> {
try {
const dns = await import('node:dns');
const { resolve4 } = dns.promises;
const addresses = await resolve4(hostname);
return addresses.some(addr => BLOCKED_METADATA_HOSTS.has(addr));
const { resolve4, resolve6 } = dns.promises;

// Run both A and AAAA lookups concurrently; ignore individual failures
const [v4Addrs, v6Addrs] = await Promise.allSettled([
resolve4(hostname),
resolve6(hostname),
]);

const allAddresses: string[] = [
...(v4Addrs.status === 'fulfilled' ? v4Addrs.value : []),
...(v6Addrs.status === 'fulfilled' ? v6Addrs.value : []),
];

return allAddresses.some(addr => BLOCKED_METADATA_HOSTS.has(addr));
} catch {
// DNS resolution failed — not a rebinding risk
return false;
Expand Down