| Package | Affected versions | Patched versions |
|---|---|---|
| symfony/http-client | ≥ 5.4.0, < 5.4.53 | 5.4.53 |
| symfony/http-foundation | ≥ 6.4.0, < 6.4.41≥ 7.0.0, < 7.4.13≥ 8.0.0, < 8.0.13 | 6.4.417.4.138.0.13 |
| symfony/symfony | ≥ 5.4.0, < 5.4.53≥ 6.4.0, < 6.4.41≥ 7.0.0, < 7.4.13≥ 8.0.0, < 8.0.13 | 5.4.536.4.417.4.138.0.13 |
| symfony | ≥ 5.4.0, < 5.4.43≥ 6.4.0, < 6.4.41≥ 7.0.0, < 7.4.13≥ 8.0.0, < 8.0.13 | 5.4.436.4.417.4.138.0.13 |
Symfony\Component\HttpClient\NoPrivateNetworkHttpClient is documented as a decorator that blocks requests to private networks by default. The list of blocked subnets (Symfony\Component\HttpFoundation\IpUtils::PRIVATE_SUBNETS on 6.4+, a private constant in NoPrivateNetworkHttpClient on 5.4) enumerates RFC1918, loopback, link-local and IPv4-mapped IPv6 (::ffff:0:0/96) prefixes, but omits the remaining IPv6 transition forms that can embed a private IPv4 destination: 6to4 (2002::/16, RFC 3056), Teredo (2001::/32, RFC 4380), NAT64 (64:ff9b::/96, RFC 6052 and 64:ff9b:1::/48, RFC 8215) and IPv4-compatible IPv6 (::/96, RFC 4291 §2.5.5.1).
IpUtils::checkIp6() is a pure bitwise CIDR comparison against the constants list and never extracts the embedded IPv4, so an attacker who can supply a URL writes the loopback / RFC1918 IPv4 target as e.g. http://[2002:7f00:1::]/ (6to4 → 127.0.0.1), http://[64:ff9b::7f00:1]/ (NAT64 → 127.0.0.1), http://[::7f00:1]/ (IPv4-compatible → 127.0.0.1) or http://[2001::1]/ (Teredo). IpUtils::isPrivateIp() returns false and NoPrivateNetworkHttpClient dispatches the request.
Real-world reachability of the embedded IPv4 depends on the deploy's IPv6 routing (6to4 tunnel interface, upstream NAT64 gateway, kernel handling of IPv4-compatible addresses), but the security boundary the decorator promises — the dispatch decision — is crossed regardless of whether the packet ultimately lands on the embedded IPv4.
The private-subnet list now includes ::/96, 2002::/16, 2001::/32, 64:ff9b::/96 and 64:ff9b:1::/48. Blanket blocking of these prefixes matches the policy applied by Chromium and Mozilla's Private Network Access; server-side HTTPS APIs are not legitimately published on these prefixes.
The patches for this issue are available here for branch 5.4 and here for branch 6.4 (and forward-ported to 7.4, 8.0 and 8.1).
Symfony would like to thank tonghuaroot for reporting the issue and Nicolas Grekas for providing the fix.
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