CVE-2019-0217 in HTTP Server
Summary
by MITRE
In Apache HTTP Server 2.4 release 2.4.38 and prior, a race condition in mod_auth_digest when running in a threaded server could allow a user with valid credentials to authenticate using another username, bypassing configured access control restrictions.
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Analysis
by VulDB Data Team • 08/27/2023
The vulnerability identified as CVE-2019-0217 represents a critical race condition flaw within the Apache HTTP Server 2.4.38 and earlier versions that specifically affects the mod_auth_digest module. This security issue arises in threaded server configurations where multiple threads handle concurrent requests, creating an environment where timing dependencies can be exploited to bypass authentication mechanisms. The flaw exists in the digest authentication implementation that fails to properly synchronize access to shared authentication state variables during concurrent thread execution.
The technical root cause of this vulnerability stems from inadequate thread synchronization within the mod_auth_digest module's handling of authentication challenges and responses. When multiple threads process authentication requests simultaneously, the race condition allows one thread to read or modify authentication state data while another thread is in the process of updating it. This timing discrepancy enables an authenticated user to potentially impersonate another user by manipulating the authentication flow during the critical window when credentials are being validated. The vulnerability specifically impacts servers configured with threaded MPMs such as worker or event MPMs, where thread-level concurrency is enabled.
The operational impact of CVE-2019-0217 is significant as it fundamentally undermines the integrity of the authentication system by allowing privilege escalation through user impersonation. An attacker with valid credentials can exploit this vulnerability to gain unauthorized access to resources that should be restricted to specific users or groups, effectively bypassing configured access control lists and authorization rules. This flaw can be particularly dangerous in environments where sensitive data is protected by digest authentication, as it allows malicious users to access restricted content without proper authorization. The vulnerability affects the core security model of the web server, making it a critical concern for organizations relying on Apache HTTP Server for web applications.
Mitigation strategies for CVE-2019-0217 should focus on immediate patching of affected Apache HTTP Server versions to 2.4.39 or later, which contains the necessary fixes for the race condition. Organizations should also consider implementing additional security controls such as disabling mod_auth_digest when not required, switching to more secure authentication mechanisms like mod_auth_basic with TLS, or implementing additional network-level access controls. The fix addresses the underlying synchronization issues by ensuring proper locking mechanisms are implemented around authentication state variables, preventing concurrent access conflicts that could lead to credential impersonation. Security teams should also review their monitoring and logging configurations to detect potential exploitation attempts and implement proper access control auditing to identify unauthorized access patterns.
This vulnerability aligns with CWE-362, which describes "Concurrent Execution using Shared Resource with Improper Synchronization," and maps to ATT&CK technique T1078.002 for valid accounts and T1566 for social engineering, as the exploitation requires valid credentials to initially establish a foothold. The vulnerability demonstrates how thread safety issues in web server components can create significant security implications that extend beyond simple privilege escalation to include complete bypass of access control mechanisms. Organizations should prioritize updating their Apache installations and conducting security assessments to ensure no other similar race conditions exist in their web server configurations or related modules.