The webhook URL guard's _ip_is_private() only checks a hardcoded
_PRIVATE_NETWORKS list, which misses several addresses that route
internally. validate_webhook_url() therefore ALLOWED:
- http://[::]/ (IPv6 unspecified, reaches localhost)
- http://[::ffff:127.0.0.1]/ (IPv4-mapped IPv6 loopback = 127.0.0.1)
- http://[::ffff:169.254.169.254]/ (IPv4-mapped cloud metadata endpoint)
The last one is the dangerous case: a webhook pointed at the mapped
169.254.169.254 can pull cloud instance credentials (SSRF -> credential
theft).
Harden _ip_is_private(): first unwrap IPv4-mapped IPv6 to its embedded IPv4
(addr.ipv4_mapped), then reject via the stdlib address properties
(is_private, is_loopback, is_link_local, is_reserved, is_multicast,
is_unspecified) in addition to the existing network list. Public addresses
still pass.
tests/test_webhook_ssrf_resilience.py asserts validate_webhook_url raises
for the three IPv6 bypasses plus 127.0.0.1 and 0.0.0.0, and still accepts a
public IP literal. The IPv6 cases fail before this change.
