CVE-2006-3652 in ISA Server
Summary
by MITRE
Microsoft Internet Security and Acceleration (ISA) Server 2004 allows remote attackers to bypass file extension filters via a request with a trailing "#" character. NOTE: as of 20060715, this could not be reproduced by third parties.
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Analysis
by VulDB Data Team • 09/17/2017
The vulnerability described in CVE-2006-3652 represents a significant security flaw in Microsoft Internet Security and Acceleration ISA Server 2004 that could potentially allow remote attackers to circumvent file extension filtering mechanisms. This issue specifically targets the server's content filtering capabilities, which are designed to prevent access to certain file types that may pose security risks or violate organizational policies. The vulnerability arises from how the ISA Server processes incoming requests that contain trailing hash characters, creating a potential bypass mechanism for restricted file access.
The technical flaw manifests when a malicious user crafts a request to the ISA Server that includes a file extension followed by a trailing "#" character. This particular syntax appears to confuse the server's filtering logic, causing it to ignore the file extension restrictions that would normally block access to the requested content. The trailing hash character effectively terminates the file extension parsing process, allowing the server to process the request as if the file extension were not present or as if it were a legitimate request. This behavior demonstrates a classic input validation weakness where the server fails to properly sanitize or normalize the request before applying security policies.
From an operational perspective, this vulnerability could enable attackers to access restricted file types that the organization's security policies are designed to prevent. The potential impact extends beyond simple content access, as attackers might exploit this bypass to download executable files, scripts, or other potentially harmful content that would normally be blocked by the ISA Server's filtering rules. The vulnerability's existence creates a window of opportunity for malicious actors to perform unauthorized access to sensitive resources, potentially leading to data breaches, system compromise, or other security incidents. This type of bypass mechanism represents a serious concern for organizations relying on ISA Server for network security enforcement.
The security implications of this vulnerability align with CWE-20, which describes improper input validation, and could be categorized under ATT&CK technique T1071.004 for application layer protocol. Organizations implementing ISA Server 2004 should consider immediate mitigation strategies including applying Microsoft security patches, implementing additional network monitoring, and reviewing existing filtering rules to ensure comprehensive protection. The fact that third-party reproduction was not possible at the time of reporting suggests this vulnerability might have been environment-specific or required particular configuration conditions to exploit effectively, but the potential risk remains significant for affected systems. Network administrators should also consider implementing additional security controls such as deep packet inspection, content filtering at multiple layers, and regular security assessments to identify and remediate similar vulnerabilities in their infrastructure.