CVE-2017-9788 in macOS
Summary
by MITRE
In Apache httpd before 2.2.34 and 2.4.x before 2.4.27, the value placeholder in [Proxy-]Authorization headers of type 'Digest' was not initialized or reset before or between successive key=value assignments by mod_auth_digest. Providing an initial key with no '=' assignment could reflect the stale value of uninitialized pool memory used by the prior request, leading to leakage of potentially confidential information, and a segfault in other cases resulting in denial of service.
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Analysis
by VulDB Data Team • 01/21/2021
The vulnerability identified as CVE-2017-9788 represents a critical security flaw in Apache httpd versions prior to 2.2.34 and 2.4.27, specifically affecting the mod_auth_digest module responsible for handling digest authentication. This issue stems from improper memory management within the authorization header processing mechanism where value placeholders for key=value pairs are not adequately initialized or reset between successive authentication requests. The flaw manifests when an attacker crafts a malicious Proxy-Authorization or Authorization header containing a key without an equals sign, which then triggers the retrieval of uninitialized memory values from the previous request's pool memory allocation.
The technical implementation of this vulnerability involves the mod_auth_digest module's failure to properly manage memory allocation for digest authentication parameters, creating a scenario where stale data from previous authentication attempts can be inadvertently exposed through the response headers. When the module processes a digest authentication header with an initial key that lacks an equals sign, it fails to clear the memory location designated for that key's value, allowing the system to return potentially sensitive information from the uninitialized memory space. This memory leakage can expose confidential data from previous requests, including authentication tokens, session identifiers, or other sensitive information that may have resided in the memory pool.
The operational impact of this vulnerability extends beyond simple information disclosure to encompass potential denial of service conditions through segmentation faults that occur when the uninitialized memory contains invalid data structures. The vulnerability affects the core authentication processing functionality of Apache httpd, making it particularly dangerous for web applications that rely heavily on digest authentication mechanisms. Attackers can exploit this weakness by crafting specifically formatted authorization headers that trigger the memory leakage, potentially gaining access to sensitive information from other concurrent users or causing service disruption through deliberate segmentation faults.
This vulnerability maps to CWE-125: Uninitialized Memory Read and CWE-476: NULL Pointer Dereference within the Common Weakness Enumeration framework, demonstrating how improper memory initialization can lead to both information disclosure and system instability. The attack pattern aligns with techniques described in the MITRE ATT&CK framework under T1078: Valid Accounts and T1499: Endpoint Denial of Service, as attackers can leverage this weakness to either extract confidential data or disrupt service availability. Organizations using affected Apache httpd versions face significant risk as this vulnerability can be exploited remotely without requiring authentication, making it particularly dangerous in production environments where digest authentication is actively used.
The recommended mitigation strategy involves immediate deployment of patched Apache httpd versions, specifically 2.2.34 for the 2.2.x series and 2.4.27 for the 2.4.x series, which address the memory initialization issues in the mod_auth_digest module. System administrators should also implement network monitoring to detect anomalous authentication header patterns that may indicate exploitation attempts, and consider disabling digest authentication where possible in favor of more secure authentication mechanisms such as bearer tokens or certificate-based authentication. Additionally, organizations should conduct thorough security assessments of their Apache httpd configurations to identify any potential exposure through other modules that might share similar memory management patterns, ensuring comprehensive protection against similar vulnerabilities that could arise from improper memory handling in authentication systems.