CVE-2026-33489: CoreDNS transfer plugin subzone ACL bypass via lexicographic zone comparison

Summary CoreDNS' transfer plugin can select the wrong ACL stanza when both a parent zone and a more-specific subzone are configured. A permissive parent-zone transfer rule can override a restrictive subzone rule (name-dependent), allowing an unauthorized client to perform AXFR/IXFR for the subzone and retrieve its zone contents.

Details In plugin/transfer/transfer.go, stanza selection is implemented by longestMatch(), which is documented as "longest zone match wins", but it actually chooses the winner via a lexicographic string comparison: - zone := "" // longest zone match wins (plugin/transfer/transfer.go) - if z > zone { zone = z; x = xfr } (plugin/transfer/transfer.go)

So, a parent zone like example.org. can beat a child zone like a.example.org. purely due to lexicographic ordering ("example.org." > "a.example.org."), even though the child zone is the longer/more specific suffix match. The bypass is data-dependent (some child labels will win, some will lose), making it operationally non-intuitive.

PoC 1. Adjust COREDNSBIN in the PoC to point at right path (see the top-level const definitions for tunables as well) 2. Run python3 ./acl-repro.py 3. Expected output: Baseline (only subzone transfer rule) axfr a.example.org.: rcode=5 ancount=0 (expected REFUSED=5)

Candidate (add permissive parent transfer rule) axfr a.example.org.: rcode=0 ancount=5 (expected NOERROR=0 with ancount>0)

OK Subzone transfer ACL bypass reproduced: adding a permissive parent-zone stanza can override a stricter child-zone stanza due to lexicographic zone selection.

Impact Unauthorized zone transfer can expose full zone contents to a remote network client that was intended to be denied by a subzone-specific transfer policy.