CVE-2015-5638 in H2O
Summary
by MITRE
Directory traversal vulnerability in H2O before 1.4.5 and 1.5.x before 1.5.0-beta2, when the file.dir directive is enabled, allows remote attackers to read arbitrary files via a crafted URL.
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Analysis
by VulDB Data Team • 03/11/2018
The vulnerability identified as CVE-2015-5638 represents a critical directory traversal flaw in the H2O web server software, affecting versions prior to 1.4.5 and 1.5.x before 1.5.0-beta2. This weakness specifically manifests when the file.dir directive is enabled, creating a pathway for remote attackers to access arbitrary files on the server filesystem through carefully crafted URL requests. The vulnerability stems from inadequate input validation and path handling mechanisms within the web server's file serving functionality, allowing malicious users to manipulate file access requests beyond intended boundaries. The issue falls under the Common Weakness Enumeration category CWE-22, which specifically addresses improper limitation of a pathname to a restricted directory, commonly known as path traversal or directory traversal attacks.
The technical exploitation of this vulnerability occurs when an attacker constructs a malicious URL containing directory traversal sequences such as ../ or ..\ that bypass the intended file access restrictions. When the H2O server processes these requests with the file.dir directive enabled, it fails to properly sanitize the requested paths, enabling access to files outside the designated document root directory. This flaw allows attackers to read sensitive files including configuration files, database files, system logs, and potentially confidential application data. The vulnerability's impact is particularly severe because it can be exploited remotely without requiring authentication, making it an attractive target for automated attacks and reconnaissance activities.
From an operational standpoint, this vulnerability poses significant risks to organizations deploying H2O web servers, as it could lead to complete system compromise through information disclosure. Attackers could potentially extract database connection strings, application secrets, user credentials stored in configuration files, or system-level information that could facilitate further attacks. The vulnerability aligns with ATT&CK technique T1213.002, which involves data from information repositories, and represents a classic example of how path traversal vulnerabilities can be leveraged for reconnaissance and privilege escalation. Organizations using affected versions of H2O face potential exposure of sensitive data, regulatory compliance violations, and reputational damage if exploited successfully.
Mitigation strategies for CVE-2015-5638 primarily involve upgrading to patched versions of H2O, specifically versions 1.4.5 or 1.5.0-beta2 and later. Administrators should also consider disabling the file.dir directive when it is not essential for their deployment, as this effectively eliminates the attack surface for this particular vulnerability. Additional defensive measures include implementing proper input validation at the application level, deploying web application firewalls to detect and block suspicious path traversal patterns, and conducting regular security assessments to identify other potential weaknesses in the web server configuration. Organizations should also establish monitoring procedures to detect unusual file access patterns that might indicate exploitation attempts, particularly focusing on requests that contain directory traversal sequences or access to system directories. The vulnerability serves as a reminder of the critical importance of proper input validation and secure coding practices in web server implementations, particularly when handling user-supplied data that influences file system operations.