CVE-2026-32136 in AdGuardHome
Summary
by MITRE • 03/12/2026
AdGuard Home is a network-wide software for blocking ads and tracking. Prior to 0.107.73, 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. This vulnerability is fixed in 0.107.73.
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Analysis
by VulDB Data Team • 03/15/2026
The vulnerability identified as CVE-2026-32136 represents a critical authentication bypass flaw in AdGuard Home, a widely deployed network-level ad and tracking blocker software. This issue affects versions prior to 0.107.73 and demonstrates a sophisticated attack vector that exploits the HTTP protocol upgrade mechanism to circumvent security controls. The vulnerability specifically targets the application's handling of HTTP/1.1 to HTTP/2 cleartext (h2c) connection upgrades, creating a pathway for unauthenticated attackers to gain elevated privileges within the system.
The technical exploitation mechanism leverages the HTTP/2 protocol's ability to operate over cleartext connections without requiring TLS encryption. When an unauthenticated attacker sends a properly crafted HTTP/1.1 request that includes the upgrade header to HTTP/2, the AdGuard Home server accepts this upgrade. The subsequent HTTP/2 connection is then processed by the application's internal multiplexer or mux component, which lacks proper authentication middleware. This architectural flaw means that all subsequent requests on the upgraded HTTP/2 connection are treated as authenticated regardless of the absence of valid credentials, effectively allowing attackers to perform administrative actions without proper authorization.
From a security perspective, this vulnerability maps to CWE-287, which addresses improper authentication issues in software systems. The flaw demonstrates how protocol-level features can be misused to bypass application-level security controls, representing a classic case of insufficient authorization checks. The impact extends beyond simple privilege escalation as it allows attackers to manipulate network filtering rules, potentially enabling them to block legitimate traffic or redirect requests to malicious endpoints. This type of vulnerability is particularly dangerous in network-level security tools where the attacker can gain control over traffic routing and content filtering decisions.
The operational impact of this vulnerability is significant for organizations relying on AdGuard Home for network security. Attackers could potentially modify DNS blocking rules, disable security features, or even redirect traffic through compromised endpoints. This vulnerability aligns with ATT&CK technique T1078.002, which covers legitimate credentials usage, as it allows unauthorized access through the authentication bypass mechanism. The attack requires minimal sophistication and can be executed remotely, making it particularly attractive to threat actors targeting network infrastructure. Organizations with AdGuard Home deployments must urgently upgrade to version 0.107.73 or later to mitigate this risk.
Mitigation strategies should focus on immediate software updates and network monitoring for suspicious upgrade requests. System administrators should implement network segmentation to limit access to AdGuard Home interfaces and establish monitoring for unusual HTTP protocol behavior. The fix in version 0.107.73 addresses the core issue by ensuring proper authentication middleware is applied to all connections regardless of protocol version or upgrade status. Additionally, organizations should conduct security reviews of other network-level tools to identify similar protocol upgrade vulnerabilities that might create similar authentication bypass scenarios.