Koel: Full-read SSRF via podcast enclosure URL: isPublicHost() filter_var guard does not reject NAT64 (64:ff9b::/96) or 6to4 (2002::/16) IPv6-transition wrappers of internal IPv4
Summary
Koel's outbound-URL guard App\Helpers\Network::isPublicHost() classifies an IP as "public" using PHP's filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE). That flag set does not recognise IPv6 transition-address forms that embed a private/loopback/link-local IPv4: NAT64 well-known prefix 64:ff9b::/96 (RFC 6052) and 6to4 2002::/16 (RFC 3056). An address such as 64:ff9b::7f00:1 (= 127.0.0.1), 64:ff9b::a9fe:a9fe (= 169.254.169.254, the cloud metadata endpoint), or 2002:a00:1:: (= 10.0.0.1) is reported as a public address, so the guard returns true and Koel proceeds to fetch the URL.
The guard is the only SSRF defense in front of App\Values\Podcast\EpisodePlayable::createForEpisode(), which downloads a podcast episode with Http::sink($file)->get($url) and streams the response body back to the requesting user. Because an attacker fully controls the <enclosure url> of any RSS feed they host (and any authenticated user can subscribe to a feed), they can publish an enclosure whose hostname has an AAAA record that is a NAT64/6to4 wrapper of an internal IP. On hosts with NAT64 or 6to4/dual-stack routing (the standard configuration on IPv6-only AWS/GCP subnets and 6to4-relayed networks), the kernel routes the wrapper to the embedded IPv4, and Koel performs a full-read SSRF against the internal endpoint — returning the response body to the attacker.
This is a server-side request forgery with full response disclosure (CWE-918) against internal services and cloud instance metadata.
Vulnerable code
app/Helpers/Network.php — isPublicHost() (the literal-IP branch and the per-resolved-record branch use the identical predicate):
```php
public function isPublicHost(string $host): bool
{
if (filter_var($host, FILTER_VALIDATE_IP)) {
return (
filter_var($host, FILTER_VALIDATE_IP, FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE) !== false
);
}
try {
$records = array_merge(dns_get_record($host, DNS_A) ?: [], dns_get_record($host, DNS_AAAA) ?: []);
} catch (Throwable) {
return false;
}
if ($records === []) {
return false;
}
foreach ($records as $record) {
$ip = $record['ip'] ?? $record['ipv6'] ?? null;
if (
!$ip
|| filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE) === false
) {
return false;
}
}
return true;
}
```
PHP's FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE rejects RFC 1918, loopback, link-local and IPv4-mapped IPv6 (::ffff:a.b.c.d), but treats NAT64 64:ff9b::/96 and 6to4 2002::/16 as ordinary global addresses — even though both forms deterministically embed an IPv4 the kernel will route to.
The sink, app/Values/Podcast/EpisodePlayable.php — createForEpisode():
```php
$network = app(Network::class);
$url = (string) $episode->path;
if (!$network->isSafeUrl($url)) { // isSafeUrl() -> isPublicHost(), the only guard
throw UnsafeUrlException::forUrl($url);
}
Http::sink($file)
->withOptions([
'allow_redirects' => [
'max' => 5,
'on_redirect' => static function (
RequestInterface $request,
ResponseInterface $response,
UriInterface $uri,
) use ($network): void {
if (!$network->isSafeUrl((string) $uri)) { // same guard on redirects -> same bypass
throw UnsafeUrlException::forUrl((string) $uri);
}
},
],
])
->get($url) // full-read SSRF: response streamed into $file
->throw();
```
$episode->path is the <enclosure url> from the subscribed podcast RSS feed. The redirect callback reuses the same isSafeUrl(), so a redirect to a NAT64/6to4 host is also accepted.
Attack scenario / How input reaches the sink
1. Attacker hosts a podcast RSS feed and serves an item whose enclosure is <enclosure url="http://int.attacker.example/secret" type="audio/mpeg"/>, where int.attacker.example publishes AAAA = 64:ff9b::a9fe:a9fe (NAT64 wrapper of 169.254.169.254) or 2002:a00:1:: (6to4 wrapper of 10.0.0.1). The attacker may also use a bare IPv6-literal enclosure host directly.
2. A Koel user subscribes to the feed (a standard, intended feature — the podcast subscription endpoint accepts an arbitrary feed URL) and plays / streams the episode.
3. EpisodePlayable::createForEpisode() calls isSafeUrl($url). The host resolves to the NAT64/6to4 address; isPublicHost() runs filter_var(NO_PRIV_RANGE | NO_RES_RANGE) over the embedded-IPv4 transition form and returns true.
4. Http::sink($file)->get($url) conn
