GHSA-xgx4-4h9w-53pv AdGuard Home: DoQ-to-UDP State Reduction and Source-Port Oracle
SummaryThis report covers the client-triggered DoQ forwarding path in:`dnsproxy` `v0.81.2` (`adguard/dnsproxy:v0.81.2`)`AdGuard Home` `v0.107.74` (`adguard/adguardhome:latest`, image version label `v0.107.74`)The issue was reproduced on `2026-04-25` with the products configured through
their documented DoQ listener and plain UDP upstream surfaces. The scope is the
internal backend UDP hop created when a DoQ query is forwarded to a `udp://`
upstream.On that path, the backend DNS `ID` is not preserved as an independent source of
entropy. For both products, the backend observer saw `dns_id=0` for every
sampled client-triggered query on the tested path. Repeated reruns then showed
the same `txid=0` behavior and the same positive source-port oracle on every
sampled run. A separate quoted-port ICMP oracle distinguished the correct
backend UDP source port from a wrong one with a stable, client-visible behavior
change.Attached evidence:`dnsproxy` oracle path on `v0.81.2`: [attachments/artifacts/g03/20260425T141500Z-g03-v0812/summary.txt](attachments/artifacts/g03/20260425T141500Z-g03-v0812/summary.txt)`dnsproxy` `v0.81.2` repeatability: [attachments/artifacts/g03/repeatability-v0812.md](attachments/artifacts/g03/repeatability-v0812.md)`dnsproxy` steering follow-up on `v0.81.2`: [attachments/artifacts/g04/20260425T141900Z-g04-v0812/summary.txt](attachments/artifacts/g04/20260425T141900Z-g04-v0812/summary.txt)`AdGuard Home` oracle path: [attachments/artifacts/g05/20260425T113000Z-g05/summary.txt](attachments/artifacts/g05/20260425T113000Z-g05/summary.txt)Root Cause AnalysisThe observable behavior is consistent across both products:A DoQ client query is accepted on the frontend listener.The query is forwarded over a backend UDP leg.On that backend leg, the forwarded DNS `ID` collapses to `0` on the
client-triggered path instead of remaining a fresh per-query variable.The backend UDP source port is still allocated per query.When an ICMP error quotes the actual backend source port, the forwarding path
flips behavior in a way that does not occur for a wrong quoted port.That combination removes `txid` from the backend tuple on the tested path and
leaves the UDP source port as the main remaining variable. In practical terms,
the backend hop stops behaving like a fresh `(txid, source-port)` pair per
forwarded query and instead becomes a one-variable state exposure.For `dnsproxy`, the correct quoted port does more than produce a failure signal:
it can push resolution away from the primary UDP upstream and into the fallback
upstream. For `AdGuard Home`, the same condition produces a fast `SERVFAIL`.ReproducePrerequisites:Docker and Docker ComposeOpenSSLbuild the lab helper image used by the attached harness and observerThe attached reproducer bundle contains only the files needed for this report:scripts: `attachments/scripts/`helper image build files: `attachments/docker/unbound-doq-attacker/`compose files: `attachments/docker-compose.g03.yml`,
`attachments/docker-compose.g04.yml`, `attachments/docker-compose.g05.yml`shipped evidence: `attachments/artifacts/...`Build the helper image first:`cd attachments``docker build -t unbound-doq-attacker:latest -f docker/unbound-doq-attacker/Dockerfile docker/unbound-doq-attacker`To rerun `dnsproxy`:`cd attachments``bash scripts/repro-g03-dnsproxy-oracle.sh`Inspect `artifacts/g03/<RUN_ID>/summary.txt`Inspect `artifacts/g03/<RUN_ID>/entropy-backend.jsonl`,
`txid_correct-backend.jsonl`, and `port_correct-backend.jsonl`To rerun the `dnsproxy` fallback-steering case:`cd attachments``bash scripts/repro-g04-dnsproxy-steering.sh`Inspect `artifacts/g04/<RUN_ID>/summary.txt`Inspect `steering_correct-main.jsonl` and `steering_correct-fallback.jsonl`To rerun `AdGuard Home`:`cd attachments``bash scripts/repro-g05-adguardhome-oracle.sh`Inspect `artifacts/g05/<RUN_ID>/summary.txt`Inspect `entropy-backend.jsonl`, `txid_correct-backend.jsonl`, and
`port_correct-backend.jsonl`The attached evidence includes fresh `dnsproxy v0.81.2` reruns, one official-
profile `AdGuard Home` run, and the minimal reproducer bundle used by both.ImpactFor both products, the tested DoQ-to-UDP path is no longer a full
`(txid, source-port)` search surface:`dnsproxy`: four of four sampled runs showed `txid=0` on the backend hop and
a positive source-port oracle on `v0.81.2`. The remaining unknown is
`port_only`. Median wrong/correct port latency was `327.99 ms / 40.93 ms`.`AdGuard Home`: four of four sampled runs showed `txid=0` on the backend hop
and a positive source-port oracle. The aggregate again classifies the
remaining unknown as `port_only`. Median wrong/correct port latency was
`319.14 ms / 37.02 ms`.Product-specific effects:`dnsproxy`: a correct port guess produced an empty client-visible answer on
the base oracle path, and in the fallback profile it steered all eight tested
queries away from the main upstream and into the fallback upstream.`AdGuard Home`: a correct port guess produced fast `SERVFAIL` and an extra
backend query.This is the security-relevant point. On the tested official profiles, the
backend hop no longer forces an off-path attacker to deal with two fresh random
fields per forwarded DNS race. The DNS ID is already known: it is
deterministically `0` on the client-triggered DoQ-to-UDP path. The only
remaining backend tuple variable is the UDP source port, and the attached
evidence shows a repeatable oracle for that remaining variable.That places the path in the same threat-model class as oracle-assisted DNS
forgery work such as SAD DNS and TUdoor: the attack first uses an oracle to
learn or validate the tuple state that protects an off-path response race, and
only then attempts the forged response. This report stops short of a forgery
demo, but the evidence already shows the crucial precondition on the tested
backend hop: the tuple is not high-entropy anymore. It has been reduced from
`(txid, source-port)` to `source-port` only.**Attachments**
[attachments.zip](https://github.com/user-attachments/files/27227054/attachments.zip)
CVSS v4.0 score: 6.9