CVE-2026-73636 in HTTP Server
Summary
by MITRE • 10/01/2026
Authentication bypass by capture-replay in mod_auth_digest in Apache Software Foundation Apache HTTP Server 2.4.x on all platforms allows a man-in-the-middle (MITM) attacker to replay captured digest authentication credentials via crafted requests that trigger garbage collection of the client's shared memory entry when AuthDigestNonceLifetime is set to 0.
Users are recommended to upgrade to version 2.4.69, which fixes this issue.
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Analysis
by VulDB Data Team • 10/03/2026
The vulnerability identified in Apache HTTP Server versions 2.4.x prior to 2.4.69 represents a critical authentication bypass mechanism within the mod_auth_digest module. This flaw specifically exploits the interaction between digest authentication protocols and the server's shared memory nonce management system when configured with AuthDigestNonceLifetime set to zero. In standard operation, digest authentication relies on nonces that are typically valid for a limited duration to prevent replay attacks. However, under specific configuration conditions where the lifetime is effectively infinite or managed via shared memory without strict expiration checks in certain code paths, an attacker can exploit this behavior. The core technical flaw lies in how the server handles client entries within its shared memory structure during garbage collection processes triggered by crafted requests.
A man-in-the-middle attacker positioned between the client and the Apache HTTP Server can intercept valid digest authentication credentials transmitted over the network. By manipulating traffic or exploiting race conditions, the attacker can trigger a scenario where the server's garbage collector prematurely removes or invalidates the shared memory entry associated with the legitimate client session. This action effectively resets the state of the nonce validation for that specific context without requiring new nonces to be generated and exchanged in real-time. Consequently, the captured authentication credentials remain valid because the server fails to properly enforce temporal constraints on previously issued nonces due to this internal state corruption or mismanagement during cleanup operations.
The operational impact of this vulnerability is severe as it allows unauthorized access to resources protected by digest authentication without needing to crack passwords or break cryptographic hashes. An attacker who has successfully performed a man-in-the-middle attack can replay the captured credentials at any later time, bypassing security controls entirely. This undermines the integrity and confidentiality guarantees provided by HTTP Digest Authentication, potentially leading to data exfiltration, privilege escalation if administrative endpoints are accessible via digest auth, or further lateral movement within protected networks. The risk is particularly acute in environments where shared memory backends for nonce storage are utilized with infinite lifetime settings, as this configuration amplifies the window of opportunity for such replay attacks.
To mitigate this vulnerability, organizations must upgrade Apache HTTP Server to version 2.4.69 or later, which contains patches addressing the logic errors within mod_auth_digest that allow this state manipulation. Additionally, administrators should review their authentication configurations and avoid setting AuthDigestNonceLifetime to zero unless absolutely necessary for specific legacy compatibility requirements, opting instead for finite lifetimes that enforce strict expiration of nonces regardless of shared memory garbage collection behavior. Implementing network-level protections such as TLS with mutual authentication or certificate pinning can also help prevent man-in-the-middle attacks from occurring in the first place, thereby neutralizing the prerequisite condition required to exploit this flaw.
From a classification perspective, this vulnerability aligns with CWE-287 Improper Authentication and CWE-345 Insufficient Verification of Data Authenticity within the context of session management. It also maps to MITRE ATT&CK technique T1076 Remote Desktop Protocol Abuse if applicable in remote access scenarios or more broadly to T1190 Exploit Public-Facing Application, specifically leveraging replay attacks against authentication mechanisms. The exploitation vector relies on network proximity for interception but the persistence of valid credentials post-interception highlights a failure in stateful validation logic within the web server software itself.