CVE-2017-9798 in Communications Diameter Signaling Router
Summary
by MITRE
Apache httpd allows remote attackers to read secret data from process memory if the Limit directive can be set in a user's .htaccess file, or if httpd.conf has certain misconfigurations, aka Optionsbleed. This affects the Apache HTTP Server through 2.2.34 and 2.4.x through 2.4.27. The attacker sends an unauthenticated OPTIONS HTTP request when attempting to read secret data. This is a use-after-free issue and thus secret data is not always sent, and the specific data depends on many factors including configuration. Exploitation with .htaccess can be blocked with a patch to the ap_limit_section function in server/core.c.
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Analysis
by VulDB Data Team • 11/04/2025
The CVE-2017-9798 vulnerability represents a critical information disclosure flaw in the Apache HTTP Server that stems from improper handling of the Limit directive within user configuration files. This vulnerability is categorized under CWE-200 as it involves improper information exposure, and it aligns with ATT&CK technique T1005 for data from local system. The flaw specifically affects Apache httpd versions through 2.2.34 and 2.4.x through 2.4.27, creating a significant security risk for web server administrators who may have inadvertently configured their systems to allow user-controlled configuration directives. The vulnerability manifests when the Limit directive can be processed within .htaccess files or when the main httpd.conf configuration contains specific misconfigurations that permit user-controlled access to sensitive server operations.
The technical implementation of this vulnerability exploits a use-after-free condition within the Apache server's processing logic, making it particularly challenging to predict and reproduce consistently. When an attacker sends an unauthenticated OPTIONS HTTP request to the vulnerable server, the system attempts to process the Limit directive in a manner that leaves memory in an inconsistent state. This memory state may contain sensitive data from other processes or previously freed memory segments, which can be inadvertently exposed to the attacker. The specific data that becomes accessible varies significantly based on multiple factors including the server's runtime environment, memory allocation patterns, and the exact configuration of the vulnerable system. The use-after-free nature means that the vulnerability is not deterministic, and successful exploitation depends on the precise timing and memory state when the OPTIONS request is processed.
The operational impact of CVE-2017-9798 extends beyond simple information disclosure, as it can potentially expose sensitive data including authentication tokens, session identifiers, database credentials, and other confidential information that may be stored in memory during the server's operation. This vulnerability particularly affects systems where user access to .htaccess files is permitted, as these files can contain configuration directives that influence server behavior. The attack vector requires no authentication, making it particularly dangerous as it can be exploited by any remote attacker without prior access credentials. Organizations using Apache httpd in environments where user accounts might have write access to web directories are especially at risk, as these users could craft malicious .htaccess files to trigger the vulnerability. The exposure of secret data through this mechanism represents a significant compromise of system security, potentially enabling further attacks such as credential theft, session hijacking, or privilege escalation attacks.
Mitigation strategies for CVE-2017-9798 focus on both immediate patching and configuration hardening measures. The primary recommended fix involves applying the official Apache security patch that modifies the ap_limit_section function in server/core.c to prevent unauthorized access to sensitive configuration directives. Organizations should also implement strict configuration controls that prevent user access to .htaccess files when the Limit directive is enabled, or ensure that the Limit directive is completely disabled in user-accessible configuration contexts. Network-level mitigations include implementing firewall rules that restrict OPTIONS request processing, though this approach may impact legitimate functionality. Additionally, system administrators should conduct comprehensive audits of their Apache configurations to identify any instances where user-controlled .htaccess files might permit execution of the Limit directive. The vulnerability highlights the importance of following the principle of least privilege in web server configuration management and demonstrates how seemingly benign configuration options can create significant security risks when improperly controlled. Security monitoring should be enhanced to detect unusual OPTIONS request patterns that might indicate exploitation attempts, and regular security assessments should verify that all Apache installations are running patched versions to prevent this information disclosure vulnerability from being exploited.