CoreDNS has TSIG authentication bypass on DoT, DoH, DoH3, DoQ, and gRPC

Summary

CoreDNS' tsig plugin can be bypassed on non-plain-DNS transports because it trusts the transport writer's TsigStatus() instead of performing verification itself. In the attached PoC, plain DNS/TCP correctly rejects an invalid TSIG (NOTAUTH), while the same invalid-TSIG request is accepted over DoT (tls://) and DoH (https://), allowing a client without the shared secret to satisfy require all. The same bug class affects DoH3, DoQ, and gRPC.

Details

The tsig plugin decides whether an incoming TSIG was valid by consulting w.TsigStatus(): tsigStatus := w.TsigStatus(); if tsigStatus != nil { ... NOTAUTH ... } (plugin/tsig/tsig.go)

Two affected transports are shown directly in the PoC:

• DoH: DoHWriter.TsigStatus() always returns nil (core/dnsserver/https.go), and the HTTP server passes unpacked DNS messages directly into the plugin chain.
• DoT: the TLS server builds a dns.Server without setting TsigSecret (core/dnsserver/server_tls.go), unlike plain DNS/TCP/UDP which sets TsigSecret: s.tsigSecret (core/dnsserver/server.go).

The same transport-family bug pattern also appears on other transports:

• DoH3 reuses the DoH writer path (core/dnsserver/server_https3.go -> core/dnsserver/https.go), so it inherits the same TsigStatus() == nil behavior.
• DoQ uses DoQWriter.TsigStatus() error { return nil } (core/dnsserver/quic.go).
• gRPC uses gRPCresponse.TsigStatus() error { return nil } (core/dnsserver/server_grpc.go).

The attached PoC was kept deliberately small (baseline TCP+DoT+DoH only) for convenience.

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 ./tsig-repro.py
3. Expected output: *** Start CoreDNS *** Corefile: /tmp/vh-f001-tsig-doh-dot-bypass/Corefile Log: /tmp/vh-f001-tsig-doh-dot-bypass/coredns.log

*** Baseline (plain TCP) *** no_tsig rcode=5 (expected REFUSED=5) invalid_tsig rcode=9 (expected NOTAUTH=9)

*** Candidate (DoT) *** no_tsig rcode=5 (expected REFUSED=5) invalid_tsig rcode=0 ancount=1 (expected NOERROR=0 and ancount>0)

*** Candidate (DoH) *** no_tsig http=200 rcode=5 (expected REFUSED=5) invalid_tsig http=200 rcode=0 ancount=1 (expected NOERROR=0 and ancount>0)

*** OK *** TSIG bypass reproduced: plain TCP rejects invalid TSIG, while DoT and DoH accept it. Results: /tmp/vh-f001-tsig-doh-dot-bypass/results.json

Impact

Unauthenticated remote clients can bypass TSIG-based authentication/authorization on first-class encrypted transports, enabling access to whatever the deployment intended to restrict behind tsig { require all } (e.g., zone data/privileged queries, etc.).