Affected versions
Symfony versions >=6.4, <6.4.41, >=7.0, <7.4.13, >=8.0, <8.0.13 of the Symfony HTTP Client and Symfony HTTP Foundation components are affected by this security issue.
The issue has been fixed in Symfony 5.4.53, 6.4.41, 7.4.13, 8.0.13.
Description
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.
Resolution
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).
Credits
We would like to thank tonghuaroot (童话) for discovering the issue and Nicolas Grekas for providing the fix.