Skip to content

AdGuard Home: DoQ-to-UDP State Reduction and Source-Port Oracle

Moderate severity GitHub Reviewed Published May 19, 2026 in AdguardTeam/AdGuardHome • Updated Jul 20, 2026

Package

gomod github.com/AdguardTeam/AdGuardHome (Go)

Affected versions

<= 0.107.74

Patched versions

0.107.75
gomod github.com/AdguardTeam/dnsproxy (Go)
< 0.81.3
0.81.3

Description

Summary

This 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:

Root Cause Analysis

The observable behavior is consistent across both products:

  1. A DoQ client query is accepted on the frontend listener.
  2. The query is forwarded over a backend UDP leg.
  3. On that backend leg, the forwarded DNS ID collapses to 0 on the
    client-triggered path instead of remaining a fresh per-query variable.
  4. The backend UDP source port is still allocated per query.
  5. 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.

Reproduce

Prerequisites:

  • Docker and Docker Compose
  • OpenSSL
  • build the lab helper image used by the attached harness and observer

The 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:

  1. cd attachments
  2. docker build -t unbound-doq-attacker:latest -f docker/unbound-doq-attacker/Dockerfile docker/unbound-doq-attacker

To rerun dnsproxy:

  1. cd attachments
  2. bash scripts/repro-g03-dnsproxy-oracle.sh
  3. Inspect artifacts/g03/<RUN_ID>/summary.txt
  4. Inspect artifacts/g03/<RUN_ID>/entropy-backend.jsonl,
    txid_correct-backend.jsonl, and port_correct-backend.jsonl

To rerun the dnsproxy fallback-steering case:

  1. cd attachments
  2. bash scripts/repro-g04-dnsproxy-steering.sh
  3. Inspect artifacts/g04/<RUN_ID>/summary.txt
  4. Inspect steering_correct-main.jsonl and steering_correct-fallback.jsonl

To rerun AdGuard Home:

  1. cd attachments
  2. bash scripts/repro-g05-adguardhome-oracle.sh
  3. Inspect artifacts/g05/<RUN_ID>/summary.txt
  4. 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.

Impact

For 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

References

@ainar-g ainar-g published to AdguardTeam/AdGuardHome May 19, 2026
Published to the GitHub Advisory Database Jun 4, 2026
Reviewed Jun 4, 2026
Published by the National Vulnerability Database Jul 15, 2026
Last updated Jul 20, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability Low
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(2nd percentile)

Weaknesses

Use of Insufficiently Random Values

The product uses insufficiently random numbers or values in a security context that depends on unpredictable numbers. Learn more on MITRE.

Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently. Learn more on MITRE.

Improper Synchronization

The product utilizes multiple threads or processes to allow temporary access to a shared resource that can only be exclusive to one process at a time, but it does not properly synchronize these actions, which might cause simultaneous accesses of this resource by multiple threads or processes. Learn more on MITRE.

CVE ID

CVE-2026-47703

GHSA ID

GHSA-xgx4-4h9w-53pv

Source code

Credits

Loading Checking history
See something to contribute? Suggest improvements for this vulnerability.