CVE-2018-8011 in HTTP Server
Summary
by MITRE
By specially crafting HTTP requests, the mod_md challenge handler would dereference a NULL pointer and cause the child process to segfault. This could be used to DoS the server. Fixed in Apache HTTP Server 2.4.34 (Affected 2.4.33).
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Analysis
by VulDB Data Team • 04/18/2023
The vulnerability described in CVE-2018-8011 represents a critical null pointer dereference flaw within the mod_md module of the Apache HTTP Server. This module handles Let's Encrypt certificate management and automatic certificate provisioning for Apache web servers. The issue manifests when the mod_md challenge handler processes specially crafted HTTP requests that trigger a condition where a pointer variable remains uninitialized or explicitly set to NULL. When the application attempts to dereference this NULL pointer during the certificate challenge validation process, it results in an immediate segmentation fault that terminates the child process handling the request. This behavior constitutes a denial of service vulnerability as legitimate requests cannot be processed while the affected processes crash and restart.
The technical implementation of this vulnerability occurs within the mod_md module's HTTP challenge response handling mechanism. When Apache receives certificate challenge requests through the ACME protocol for Let's Encrypt certificates, the mod_md module processes these requests to validate domain ownership. The flaw emerges during the parsing or validation of specific HTTP headers or request parameters that are carefully constructed to force the module into a state where it attempts to access memory through a NULL pointer reference. This particular pointer dereference typically occurs in the challenge response handler code path where the module expects certain configuration values or request data to be properly initialized but encounters NULL values instead. The vulnerability affects Apache HTTP Server versions 2.4.33 and earlier, with the issue being resolved in version 2.4.34 through proper null pointer validation and initialization checks.
The operational impact of CVE-2018-8011 extends beyond simple service disruption as it provides attackers with a reliable method to perform denial of service attacks against Apache web servers configured with mod_md. Since the vulnerability can be triggered through HTTP requests alone, attackers do not require any special privileges or authentication to exploit it. The segmentation fault causes child processes to terminate abruptly, forcing the Apache master process to spawn new worker processes to handle incoming requests. This process restart cycle can consume significant system resources and may lead to complete service unavailability if the attack is sustained or if the server lacks sufficient resources to handle the process creation overhead. The vulnerability is particularly concerning for servers that automatically configure Let's Encrypt certificates using mod_md, as these systems are more likely to be targeted and the attack can be executed by simply sending malformed HTTP requests to the server.
Mitigation strategies for CVE-2018-8011 primarily focus on upgrading to Apache HTTP Server version 2.4.34 or later where the null pointer dereference has been patched. Organizations should prioritize updating their Apache installations as this vulnerability can be exploited remotely without authentication. System administrators should also implement additional monitoring to detect unusual process restart patterns or segmentation fault occurrences that may indicate exploitation attempts. Network-level protections such as intrusion detection systems can be configured to identify and block HTTP requests that contain patterns associated with the vulnerable request structure. The fix implemented by Apache developers addresses the root cause by ensuring proper initialization of all pointer variables before dereferencing and adding null checks in the mod_md challenge handler code. This vulnerability aligns with CWE-476 which specifically addresses null pointer dereference conditions, and may be mapped to ATT&CK technique T1499.004 for network denial of service attacks. Organizations should also consider implementing rate limiting and request validation mechanisms to reduce the effectiveness of potential exploitation attempts while awaiting the complete deployment of security patches across all affected systems.