The R09 scholarly hardening routes every hop through check_outbound_url with
block_private=True. On a DNS64/NAT64 network, an IPv4-only host can resolve
through the RFC 6052 Well-Known Prefix. For example, export.arxiv.org resolved
to 64:ff9b::924b:5b2a in the reproduced environment.
CPython classifies that outer IPv6 prefix as reserved, so the URL guard rejected
the request before examining the effective IPv4 destination.
Decode addresses in exactly 64:ff9b::/96 to their embedded IPv4 destination and
evaluate that destination under the strict outbound policy.
The translated target is always checked with private-address blocking enabled.
This prevents the NAT64 prefix from becoming a path to loopback, private,
shared/CGNAT, link-local, multicast, unspecified, or other non-global IPv4
space.
Network-specific translation prefixes are not decoded. In particular,
64:ff9b:1::/48 remains subject to the existing IPv6 policy.
Coverage includes:
- public IPv4 destinations embedded through the RFC 6052 prefix
- loopback, link-local, private, CGNAT, multicast, unspecified, benchmark, and
TEST-NET rejection
- network-specific NAT64 prefixes remaining undecoded
- deterministic scholarly provider tests without live DNS dependence
- redirect query parameter isolation
- relative redirect resolution
- HTTP error fallback behavior
- editor draft GET and DELETE behavior remaining unaffected
R09, R11, and R01 were independently audited and otherwise left unchanged.
Alexandre Teixeira
11ad4e2739