CoreDNS' transfer stanza selection uses lexicographic compare (subzone ACL bypass) in github.com/coredns/coredns
CoreDNS' transfer stanza selection uses lexicographic compare (subzone ACL bypass) in github.com/coredns/coredns
02 / AFFECTED SOFTWARE
Affected packages
79 explicit affected versions
03 / CONNECTIONS
Connected vulnerabilities
04 / EVIDENCE
Source records
### 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 COREDNS_BIN 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.
CoreDNS' transfer stanza selection uses lexicographic compare (subzone ACL bypass) in github.com/coredns/coredns
CoreDNS is a DNS server that chains plugins. In versions prior to 1.14.3, the transfer plugin can select the wrong ACL stanza when both a parent zone and a more-specific subzone are configured. The longestMatch() function in plugin/transfer/transfer.go uses a lexicographic string comparison instead of an actual longest-suffix match to select the winning zone. As a result, a permissive parent-zone transfer rule can override a restrictive subzone rule depending on zone name ordering (e.g., "example.org." > "a.example.org." lexicographically). This allows an unauthorized remote client to perform AXFR/IXFR for the subzone and retrieve its full zone contents. This issue has been fixed in version 1.14.3.
05 / REFERENCES